This article discusses the manufacturing principals and processes in the manufacture of VRLA batteries. The article discusses the manufacturing processes and how deviations from the ideal can seriously affect the finished product.

The article assumes the reader has a reasonable knowledge of the construction of both AGM and GEL types. The process described here is very much simplified but will give the reader an understanding of the processes involved in manufacturing VRLA batteries.

It has been said that it is very easy to manufacture a bad VRLA battery and quite difficult to manufacture a good performing product having a durable long life. It is the writer’s belief that this is true.

The following notes will help those not familiar with the manufacturing process to understand this article.

a) Lead (Pb): the main ingredient of the negative plate. b) Lead dioxide (PbO2): the main ingredient of the positive plate. c) Dilute sulphuric acid (H2SO4): the third component to make the battery work. d) Lead grids: the essential component which the positive or negative active materials are applied. The grids are the main current carrying component responsible for delivering power from the active materials to the terminals. Lead as Pb active material is a reasonable conductor of power but lead dioxide as PbO2 is a poor conductor and good grid design is essential. e) Plate lug: the upper most part of the grid which is “welded” to the group bar. In battery language, the welding process is called “burning”. f) Plate wires: the vertical and horizontal wires of the plate grid. g) Group burning process: this is the term used in the process where the plates are welded to the group bar or where the group bar is welded to the pillars. h) Flag: the section of the group bar which protrudes above the plate group which is used for the inter cell weld.

All lead acid batteries have three active ingredients; lead (Pb), lead dioxide (PbO2) and dilute sulphuric acid (H2SO4). The principals have been discussed in a previous article titled “HOW DOES A LEAD-ACID BATTERY WORK”.

In addition to the active materials, pure lead or lead alloy grids are required to “hold” the lead and lead dioxide. Separators are required to keep the plates apart and AGM batteries have an absorbent separator to hold acid whilst GEL batteries have special jellified electrolyte.

It is easy to say the active materials are lead (Pb), lead dioxide (PbO2) and dilute sulphuric acid (H2SO4), but, the purity of these materials is critical to performance and life of the finished product. Impurities in the dilute sulphuric acid can affect the initial performance or the life of the product, in some cases resulting in unexpected catastrophic failure. At first sight the purity of the lead used in battery manufacture seems to be fantastic with values of 99.97% pure being quoted. However, 99.99% pure lead is not only superior but desirable... A purity of 99.97% and even lower may be used but the impurities such as iron, manganese, copper and other harmful elements must be closely controlled. Interestingly, it has been proven that trace elements of gold in the lead will enhance both performance and life. Unfortunately, most of the gold today is extracted from lead before the battery manufacturer takes receipt.

In principal the manufacture of the lead oxide which then becomes lead for the negative plates and lead dioxide for the positive plates should be a simple process. However, the process has to be controlled finitely. Several methods are used to manufacture lead oxide but the most popular are the Barton Pot and Ball Mill. In both, lead oxide is produced in a vessel where fine particles carried away by “extraction” fans are then used to produce the active material. The particle size is very small but the superficial volume is high making the particles act more like dust than lead.

Ball Mill Active Material Manufacturing Machine

The particles are critically controlled for size, density and “free lead”. Variations in the oxide characteristics will affect the battery performance and life. For example, high “free lead” content will reduce the overall efficiency but can improve the overall life of the product. The finished product characteristics can be “manipulated” by varying the lead oxide characteristics within the Barton Pot or Ball Mill manufacturing process.

The next stage of manufacture involves mixing the lead oxide powder with acid, water, and proprietary additives to produce a paste have a consistency similar to thick toothpaste. This is then pasted onto the lead grids. For VRLA GEL cells using Tubular Plates, the process is quite different and the active material is “inserted” into the tubes as slurry or a dry powder.

Tubular plate cells and increasingly pasted plate types have a rather special lead compound added; triplumbic tetroxide (Pb3O4). This material enhances the plate formation acting as a catalyst to produce a more even and consistent active material in the finished product. It is normally produced by a specialist manufacturing plant rather than the battery manufacturer. Relatively speaking it is several times more expensive to produce when compared with lead oxide (PbO2).

It is normal to add a colour element to differentiate between positive and negative paste. Carbon black is often added which arguably improved conductivity. This is a good identifier.

Areas open to error are:- a) Poor quality control of impurities in the lead, lead alloys and dilute sulphuric acid. b) Poor control of particle size and free lead content. c) Poor control of paste density.

Corrosion of the grids is at the front of the battery manufacturers mind and many different alloys and manufacturing methods have been tried to minimise this corrosion. It does not always follow that thick grids will last longer than thin grids. Thick grids will undoubtedly last longer than thin grids, all other aspects remaining the same. Development has taken place and today thin grids with exceptional corrosion resistance are now common place.

The alloys used are normally pure lead, tin, calcium or cadmium. Some exotic alloys are also used but these are very special and not commonly available and in most instances are regarded as “experimental”.

As with the active material, the purity of the lead and alloying element play an important part in achieving the design life. The design of the grids also affects the performance and life of the product. Radial grids are popular with automotive batteries and are increasing in popularity for VRLA industrial products. Inboard lugs, tapered group bars, large inter cell connectors etc all play an important part in the overall design which ultimately will affect the internal resistance and terminal voltage on discharge.

Thin grids have, for many years, been frowned upon but with the availability of very pure lead and by using the correct additives such as tin, superior products have been developed. It is interesting to note that the very first VRLA AGM cells used pure lead grids. The level of tin in the plates varies but typically this is between 2% and 3%. The “hardening agent” such as calcium and cadmium are introduced at much lower levels and it is true to say that the lower the better. Typically 0.05% to 0.10% calcium is added. Some batteries do not contain any “hardening agent” making them delicate to handle in manufacture but resulting in an exceptional product with a very long life.

The lead grids are usually manufactured using a gravity casting process. Grid weight is controlled by the operator attending to release agents and mould “maintenance” which is ongoing throughout the casting process. Grids are weighed at regular intervals to ensure compliance with the specification. An under-weight grid will result is a lower performance and a reduced life while a grid heavier than the design specification will cost more in material and profitability will suffer.

Casting Machines

Grids are also manufactured using a rolled and punching method. This latter method results in a very stable grain structure similar to forged steel when compared with case iron. With a more stable grain structure corrosion resistance is enhanced. When a lead tin alloy is used for rolled and punched grids they can be very thin with a finished grid thickness as low as 0.6mm. By using robotic machinery to assemble finished plates into the containers, thinner plates can be accommodated which results in better active material utilisation and superior high rate performance. Unfortunately there is a cost to this process because thin plates are more difficult to handle in a volume production cycle. For many years finished plates were assembled into the containers by hand.

Before pasting the grids with the active material, they are allowed to age harden for several days and sometimes even weeks. The grids become more robust which makes the next stage of pasting easier.

Areas open to error are:-

a) Poor quality control of impurities in the lead. b) Poor control of additive concentrations. c) Poor mixing of the lead and alloying agents. d) Poor control of the casting temperature e) Poor control of the grid thickness.

Battery Grid Plates

The paste has to be added to the grids and automatic machinery is used for this process. Grids are fed, horizontally, under the head of the pasting machine which normally consists of counter rotating “screws” which force the paste into the grids. The underside of the grid is said to be “flush” while the upper side has a small amount of “over paste”, typically about 0.1mm. At this stage we can call the product “plates”. These plates are fed into a “flash drying oven”. After passing through the flash dryer, plates are sufficiently dry to allow stacking. The flash dryer is a critical area where a good or bad product may be manufactured. If the flash drying temperature is too high a “skin” can form which inhibits plate formation. Both performance and life will be adversely affected. If flash drying is inadequate, plates cannot be stacked separately because they will stick together. With plates not separated correctly the curing process will be affected.

Grids Being Fed In To A Pasting Machine

Plates After Pasting

Before flash drying, some special battery types are returned to the start of the pasting process and turned over before being pasted again. This results in the grid wires being embedded within the plate active material which enhances life. The down side is the additional cost with questionable benefit.

Flash Dryer

For Tubular plates the term pasting is inaccurate because the tubes are filled with positive active material in a power of slurry form. However, this only applies to the positive plate. Negative plates follow the same process for all lead acid battery types.

Areas open to error are:-

a) Inaccurate blending of the dry lead oxide with acid and water. b) Incorrect quantity of additives and / or ineffective mixing. c) Insufficient quantity of active material inserted into the tubes for Tubular plate cells. d) Under pasting resulting in insufficient weight and the consequential loss of performance. e) Missing pellets, i.e. parts of the grid not pasted or sections missing. f) Bent plates as a result of machine settings not correct.

After pasting and flash drying the plates are cured. There are several methods for curing plates but the most popular is to use large curing ovens where the oxygen content, temperature and humidity are controlled. It is possible to cure plates without going into a controlled environment but a radically different manufacturing process has to be used later in the manufacturing cycle. The curing process takes time and even when using curing ovens the process takes many hours.

Plates coming out of the ovens should be completely dry.

Areas open to error are:-

a) Poor control of the oven temperature, humidity and oxygen content. b) Curing time too short. c) Stacking in the oven too tight leading to uneven or inadequate drying.

Plates Being Loaded In To Curing Oven

Curing Ovens

Before the plates can be assembled into the cells, a finishing process is required to remove paste from the plate lugs, and edges. Loose material must also be removed and this is particularly important for AGM product. A plate having a piece of dried active material on the surface will eventually puncture the separator and a short will be inevitable. Many manufacturers over paste the grids and then plane them down to the correct thickness.

Areas open to error are:-

a) Inadequate cleaning leading to dry paste debris not being removed. b) When plates are planed to the correct thickness, this may not be to the desired design leading to under or over compression in AGM cells.

Plate stacking is where the positive plates, negative plates and separators come together to form a “group” or “stack”. This is more critical in AGM cells compared with GEL types.

Stacking can be either by hand or machine. For AGM cells the separator is more often than not wrapped round one plate or both plates. This may be the positive or negative plate. Occasionally, wrapping is not carried out and a single sheet is inserted between the plates. A very popular process is to wrap both positive and negative plates because two thin layers of separator are better than one thick layer.

For GEL cells a conventional separator is used and the plate stacking process is very much simplified. Double separation is often used which generally is in the form of two different types of separator “bonded” together to form one piece which makes plate stacking very simple. Generally, this process is dome automatically be machine.

Areas open to error are:-

a) Plates can be damaged which may ultimately find their way into finished cells. b) Separator damage may occur.

#GROUP BURNING

The term “burning” is used to describe the “welding” process when the plates are connected to the group bar and pillars.

This process involves loading the plate group (see plate stacking above) into a jig ready for group “burning”. The burning process may be by hand or machine. In the case of monoblocs, several cells are normally assembled into a multi cell jig.

Finished Battery Plate Ready To Be Assembled In To The Burning Jig

For large cells typically over 500Ah, group burning is generally done by hand involving skilled operators burning (welding) the plate lugs to group bars. The group bar may already be an integral part of the cell pillar or may be formed with molten lead by the operator. The principal is simple to describe but great skill is required to obtain a grain structure which is corrosion resistant. Too much local heat will result is a coarse grain structure at the plate lug / group bar interface which will be liable to rapid corrosion. If too little heat is used, a poor electrical connection will result.

Typical Hand Burning Process   Smaller capacity cells and monoblocs tend to be assembled using a “cast on process”. This involves loading the plate groups into a jig with the plate lugs uppermost; the jig is then inverted with the plate lugs facing vertically down. The lugs of the complete group or groups are then cleaned and flux is applied... The plate lugs are then dipped into molten lead which effectively welds them together. The molten lead is in a cavity which then forms the group bar and pillar or inter cell “flag”.

Areas open to error are:-

a) Plates not burned (welded) to the group bar adequately. This may lead to local corrosion and premature failure. This may be caused by inadequate lug cleaning, too much or too little flux being applied, plate lugs being misaligned. b) Too much heat applied during the burning process resulting in a coarse grain structure susceptible to corrosion. c) Poor choice of lead alloy leading to early corrosion of the group bar / pillar.

This is the mechanical process of inserting the plate group into the container. It is purely mechanical but can lead to early failure of the product if not controlled.

Exploded View Of A Cell Group

VRLA AGM cells have a microfiber separator which relies on being compressed in the finished product. As a plate group it will the larger than the cell container and must be compressed and “shoe horned” into the container. The procedure may involve a machine to ram the group through a tapered “shoe horn” or for small cells a hand insertion method may be used.

VRLA Battery Assembly Line

VRLA GEL cells are easier to assemble because the group is not a compression fit into the container. Simple machines or a hand insertion method may be used.

Areas open to error are:-

a) Over compression at the “shoe horn” stage leading to fractures in the micro fibre. This may ultimately lead to loss of compression and failure of the product. b) Incorrect alignment of the “shoe horn” to the container leading to snagged plates which can lead to internal shorts.

For monoblocs, an inert cell connection must be made. Very occasionally this is done externally in a similar way to connecting single cells in series. However, most monoblocs have an inter cell weld. This is normally achieved by an “extrusion fusion” method which, in mechanical terms is a spot weld. The process has to be controlled very closely because the weld is “through” the inter cell wall and it has to be acid tight as well as being electrically correct.

The process involves applying pressure to the group flags which will force lead through the cell wall. With pressure still applied, a high current is applied which melts the lead at the electrodes. With pressure still applied, the current flow is stopped to allow the lead to solidify and finally, the pressure applied by the electrodes is removed.

Areas open to error are:-

a) Poor alignment of the electrodes. b) Inadequate electrode pressure c) Bad current control. d) Worn electrodes

Typical Cast On Strap Machine

Cast On Strap Machine Being Loaded

Having inserted the plate group into the cell, and completed the inter cell welds for monobloc types, the lid must be fitted. The lids are normally sealed to the container by either; a) heat seal method or b) adhesive bonding. Some small cell lids are ultrasonic sealed but this method is the quite rare for industrial batteries above 25Ah.

As the term implies, heat sealing involves applying heat and bonding the lid to the container. It is normal for a hot plate to heat the mating surfaces of both lid and container and while the mating surfaces are still in a semi-molten state the two components are brought together. The method is well proven and very reliable.

Adhesive bonding is very popular because a “hot plate” does not have to be manufactured for every combination of lid and container. Adhesives normally come in the form of the two-part mixed at the point of application. Large cells and those with low volume production runs are more than often adhesive bonded. Modern adhesives are extremely strong and reliable but some polymers cannot be easily bonded and heat sealing is the only solution.

It is normal to pressure test cells after lid sealing. Often very high critically important product, every cell may be tested using helium which is very searching.

Areas open to error are:-

a) Poor heat bonding caused by inaccurate machine setup. Too hot, to cold, wrong cycle time and misalignment of tooling contribute or cause poor heat bonding. b) Adhesive bonding problems can occur due to inadequate adhesive being applied, or a poor mixing of the two parts.

Some products have plates formed prior to cell assembly in a process known as “tank formation”. The alternative formation process is known as “container formation” where cells are filled with acid and the formation process is carried out.

Arguably, “tank formation” is more reliable because the temperature can be more closely controlled and the acid circulation is achieved. The down side is that the process is considerably more expensive than “container formation”.

Cells going through the “container formation” process must first be filled with acid which normally ends up as the finished cell electrolyte. However, some large GEL cells are formed with a disposable liquid acid which is then dumped before the cell is filled with the finished acid mix. Inn GEL cells the acid mix is formed by adding fine silica, and other proprietary ingredients to dilute sulphuric acid. This produces a milky acid mix which is thixotropic. The mixture is poured into the cells and soon sets into a GEL forming the electrolyte.

The process for VRLA AGM cells following the container formation process, normally involves filling with cold acid typically of about +5C. As soon as the acid reaches the plates which at this stage are in the lead oxide (PbO) form, considerable heat is generated and this must be controlled. Cells or monoblocs are normally formed in water filled baths to control the temperature. Filling the cells is quite complex because the micro-fibre separator does not allow instant filling. Typically, a vacuum will be pulled before acid is allowed to flow into the cell. Most processes involve several pulses of pulling a vacuum and acid flow.

The actual formation process involves putting typically, 7 to 10 times the nominal Ah capacity of the cell into the cells. For example, a nominal 100Ah cell will have 700Ah to 1000Ah input before it may be considered fully formed.

The process normally involves a controlled discharge part way through to check the capacity and “exercise” the active materials.

The formation process is critical the performance and life of the finished product.  

Areas open to error are:-

a) Inadequate acid filling. This is more likely to occur on AGM cells and large tubular plate GEL cells. b) Poor mixing of the silica and acid for GEL cells. c) Poor temperature control during the formation process. d) Inadequate capacity input.

Most products are checked for voltage and some type of conductance, impedance or performance testing is carried out prior to despatch.

This is the “final” QA check that the product normally must pass before packing and shipment.

Areas open to error are:-

a) Inadequate checks allowing defective product through and into the supply chain.

Machine For Final Testing Of Voltage & Impedance

This document covers the more obvious and more critical processes used to manufacture a VRLA battery.

Because batteries are not regarded as “exciting” and are a “low tech” product, the complexity of manufacture is not interesting to the lay man. This article is written to illustrate that the processes are far from simple and extremely complex.

The article only “skims” over the subject of metals used but these are critical if a high quality; long life product is to be manufactured.

This article discusses charging of lead-acid industrial standby batteries on float charge and “off mains” locations. It does not include charging of batteries on full cycling applications such as daily traction duties or similar.

Charging of parallel battery strings, inrush currents, ripple voltage and ripple current are also discussed in outline.

The majority of industrial standby batteries are charged using a Full Float Charging system but Pulse Charging is becoming popular particularly for UPS systems. In the main, the chargers are of the thyristor or switch mode type. Both types have some advantages over the other and for a detailed evaluation of each type it is advisable to contact the charger manufacturer direct and discuss the options.

In Full Float Charging it is intended to continuously charge the battery 24 hours a day, 365 days a year and the only exception will be when a mains power failure occurs or during testing and servicing.

Generally, battery manufacturers prefer this method of charging because it is fully automatic in providing the current that the battery requires, irrespective of the state of charge. The battery and charger are in parallel. This system is a full “no break” system. See the Diagram 1 below.

Diagram 1.

When voltage is applied to the battery terminals, current will flow. It has to be remembered that voltage does not charge the battery; it is the resultant current that will maintain the battery in a constant state of charge or recharge following a discharge. The value of the current going into the battery will depend on its state of charge, battery temperature and type of battery. Typically, but depending on the battery type; the applied voltage will be between 2.23Vpc and 2.28Vpc for a battery temperature of 20oC. Different applied voltages are used for different battery temperatures. The maximum resultant current will depend on the charger current output rating. When the battery is fully charged, the current will be very low and may be as small as 10mA per 100Ah of battery capacity. However, International Standards give guide lines of 1mA per Ah to 8mA per Ah depending on battery type. These values are published as being the typical maximum and have a large “safety margin” in them. Never the less, the current will be very small and in most cases, it cannot be measure without the correct current measuring instruments. A simple “clamp on” ammeter is unlikely to measure the current or the displayed value may be widely inaccurate. It is impossible to accurately measure the current in a fully charged battery with a 200A or even a 20A “clamp on” ammeter when the actual current in a 100Ah battery will normally be in the order of 25mA.

Chargers are normally current rated to supply the full load plus the minimum required for the battery which is typically between 5% and 10% C10 amperes depending on the battery type and manufacturers recommendations. If the system is for a short standby time an interesting scenario may arise. For example, if the load is 250A but is only required for 5 minutes, then the battery capacity will be low and in the order of 75Ah depending on the operating temperature and minimum voltage. If we consider a recharge current of 10% C10A i.e. 7.5A for the battery then the charger will need to have a current rating of about 250A to cater for the standing load of 250A plus 7.5A to recharge the battery, i.e. 257.5A. If a power failure occurs the battery will discharge into the load at the 250A required until the battery is exhausted, or until the mains power is restored, or until the load is disconnected. Either way, the battery will not be fully charged when the charging source is restored. If the load is disconnected from the battery for the recharge, the full charger current capability will be available to recharge the battery.

Under these circumstances the initial recharge current flowing in the battery will be considerable but because the voltage is limited this is not normally an issue. The back EMF of the battery being charged will rise very fast and will meet the float voltage quickly. In some instances, the time to the float voltage may be only a few seconds. However, there is the possibility for this current to flow for many seconds or even longer and there is a danger that any fuses or OCB protection in the battery circuit may be affected. For this reason, battery chargers are often designed in two sections, one for the load and one for battery recharging. Batteries themselves are rarely affected by this high initial recharge current and the effects can largely be ignored for Full Float Charge systems. To avoid any problems with fuses or OCB protection, this is often omitted between the battery and charger.

For float recharge systems, the time to fully charged does not change significantly for recharge currents between 3% and 10% and even at higher currents the time to fully charged still remains more or less the same at typically 3 days. However, the time to a state of charge of about 80% or less does change. The value of 80% is typically the state of charge when the float voltage is reached and this is more or less irrespective of the current available to the battery. This is illustrated in Fig 1 below for recharge currents between 3% and 10% C10A. Up to an 80% state of charge the Ah efficiency is close to 100%. Beyond this point, the efficiency gradually falls away until the battery is fully charged when the efficiency can be said to be 0%. Once the battery is fully charged the energy is lost in heat, battery corrosion electrolyte losses etc.

Fig 1.

The relationship between recharge current and voltage is illustrated in Fig 2 below.

It can be seen that the charging current falls rapidly once the float voltage is reached and in this case this is after approximately 11 hours of charging. Referring to Fig 1, it can be seen that a charge condition of 80% is achieved after approximately 11 hours when using a current limit of 7% C10A.

Fig 2.

Pulse Charging can be in several forms but as the term suggests, the charge is delivered in a series of pulses.

Pulses of charge may be delivered to the battery at time intervals dependant on the charger settings. The interval between charging may be as short as 1 second or many minutes. Some systems charge for 1 second, then rest for 1 second and this is continuous. Where the pulse intervals are short, the control voltage will be close to the normal Full Float charge value. Some systems charge at the Full Float voltage until the current has fallen to a pre-determined level or for a set time at which point the battery is put onto open circuit. Over several hours the voltage will gradually fall and at a pre-determined value charging recommences. Some systems charge a higher voltage such as 2.35Vpc before returning to the normal 2.23Vpc to 2.28Vpc value. For these systems, the battery is not connected directly to the load but only to the charger. A fast acting static switch is employed to reconnect the battery to the load in the event of a mains power failure. This system is illustrated in Diagram 2 below in outline for a UPS system.

Diagram 2.

This method of charging is normally reserved for commission charging vented batteries such as Planté, Tubular and Pasted plate types. Constant Current Charging should not be used with VRLA cells without specialist information from the battery manufacturer.

The best way to charge by constant current is to have a charger where both the voltage and current are adjustable. Initially, both voltage and current control should be set to a minimum. The voltage and current should be adjusted slowly until current starts to flow. Typically, once the current has started to flow, the voltage should be adjusted to the maximum available for the charger and the current control adjusted to the correct value. During this process, the actual voltage seen across the battery will not change because the current controller limits the current flowing and hence the voltage across the battery. At this point the current can be adjusted to the required value. Charging will then continue at a constant current and as the battery reaches about 80% charged, the voltage will be seen to increase. However, if the maximum voltage of the charger cannot be adjusted to about 3.00Vpc, then the current may not be constant for the complete charging process. For example, if the maximum voltage of the charger is 2.50Vpc, the current will remain constant until this voltage is reached and then the current will taper off and the voltage will remain steady at 2.50Vpc: this is not constant current charging.

Figure 3 below shows the typical voltage characteristics when charging a vented cell from fully discharged at a constant current of 10%

More information on charging with a voltage limit of 2.50Vpc can be seen below in the section titled Limited Voltage Charging.

Fig 3.

Limited voltage charging is similar to Full Float Charging but instead of using the float voltage which is typically in the order of 2.23Vpc to 2.28Vpc, a higher voltage is used. This type of charging system is often found where the voltage of the charger has an upper limit which is below the natural cell voltage maximum which is in the order of 2.80Vpc to 3.00Vpc. Voltage limits above 2.40Vpc should only be used under guidance from the battery manufacturer.

Limits to the maximum charger voltage are often imposed to protect the load from a voltage that may cause damage. A voltage of 2.40Vpc is often referred to as an equalizing charge. When fully charged, the current flowing in a VRLA cell at 2.40Vpc is typically 8 times that flowing at the float voltage of 2.23Vpc to 2.28Vpc, i.e., about 8mA per Ah. For Vented products such as Planté, Tubular or Pasted Plate, the current will be typically 20mA per Ah.

The higher the voltage limit, the shorter the recharge time will be. However, once the cell voltage reaches the voltage limit of the charger, the current diminishes at a rapid rate. This is illustrated below in Fig 4.

Fig 4

This charging regime is often misinterpreted as constant current because, in some cases the current is seen to be constant for some time before the voltage limit is reached and because it is not a float charge mode. Frequently, chargers have two settings “float” and “equalising”.

The time to fully charged from fully discharged is shorter than using the Full Float method but is considerably longer than using Constant Current. Some manufacturers will not accept this method for commission charging vented products such as Planté, Tubular and Pasted Plate types.

Table 1 below gives an indication of the time taken to reach the voltage limit and the time to fully charged. The current available is taken as 7% C10A.

Table 1

Off Mains Charging is often used in rural locations where no a.c. mains supply is available. Examples are country houses and telecommunication systems in remote locations. In the vast majority of applications a generator is provided but to avoid having to run this continuously a battery is used. The battery may be charged by a wind generator or solar system, or both, in addition to a generator. In these applications, the battery will at some time need to be brought back to a near fully charged condition if a satisfactory life is to be obtained. Continual undercharging will result is sulphation of the battery plates and premature failure. For systems with wind or solar power, a full recharge may only be required after many months and in some cases this may extend to a full year. In all cases, a full recharge once per year is normally recommended by the battery manufacturer. Typically a constant current recharge is used but a recharge at a value of about 2.50Vpc may prove sufficient.

For county houses the system usually consists of a d.c. generator and battery only. The d.c. power is supplied through an inverter to the normal 240V a.c. house load. To avoid unnecessary noise, the generator may be run through the day and turned off at night. Some systems may run on the battery for several days before recharging is necessary. Again, a full recharge is advisable at least once a year.

To reduce the battery size and if no other charging source is available other than by generator, an automatic system may be used. Country houses often use this system. To obtain a satisfactory life the battery may be charged daily up to a voltage of typically 2.35Vpc to 2.45Vpc and then reduced to a more value typical of a full float system. In this way, the battery will receive a good charge without being overcharged.

It may be argued that a full float system may be more appropriate to avoid overcharging but there is a real risk of undercharging if a value in the order of 2.23Vpc to 2.28Vpc is used. The Full Float Charging is intended for batteries which will only be discharged rarely and typically no more than 4 times a year. The Full Float Charger will not adequately recharge a battery on an Off Mains installation which is subject to regular discharges.

Providing all the strings in the battery are in good condition, no issues should be experienced. However, charging of parallel strings can present some interesting characteristics. For example, as on discharge, the recharge current in each string may be marginally different and providing they are within 10% of each other it is reasonable to consider as normal. However, if the currents are more than 10% difference it would be prudent to investigate. The cause may be they are a different state of charge and then the question arises – why? If the battery is new, the differences will stabilize after a good charge.

Occasionally, battery systems are made up on multiple strings but test discharged as individual strings. In these circumstances, it is important to recharge each string independently following a discharge test and before reconnecting to the other string or strings. If this is not carried out and the discharged battery is connected to the fully charged strings, two unwanted characteristics will occur. First, there will be a very large inrush current into the discharged string from the charged string(s). Protection may be affected. Second, considering a two string system, the second string will not be fully charged because of the energy transferred to the first string. Testing of the second string will not be correct because it will not be fully charged unless it is allowed to remain on float charge for several days. The system is further complicated when more than two strings are involved.

The inrush current into a fully discharged battery can be very high and in the order of 3 or 4 times the Ah capacity of the battery. This is not normally a problem because the back EMF of the battery will rise very quickly, within seconds in some cases, to meet the applied charging voltage and the current will rapidly fall away.

Under normal conditions, the charging source will have a current limit and the maximum inrush current will be the maximum of the charging source.

#RIPPLE CURRENT AND RIPPLE VOLTAGE

International standards discuss this but in all cases; the manufacturer’s instructions should be followed.

Excessive ripple voltage will cause a higher than desirable ripple current and batteries have been known to fail after a very short time when the charging source has excessive ripple.

Corrosion due to cycling by the ripple or heat will reduce the battery life.

In Planté batteries, cases have been recorded where catastrophic failure has occurred within a few hours. Rapid corrosion of the positive plates takes place which is normally seen as a white deposit in the base of the cells. This is easily identified because the containers are transparent. In VRLA cells with opaque containers, it is not possible to identify the problem until it is too late. VRLA cells are affected very differently to Planté or other vented products. Because the state of charge of the negative plates in a VRLA cell are inevitable in a less than 100% state of charge, it is these where the problem is seen. Corrosion of the negative grid wires takes place and the state of charge is very low. Ripple will cause heating and the life may be reduced without abnormal corrosion characteristics. However, the occurrence of “a.c” corrosion is rare in VRLA cells and the effects are not fully proven.

At first, the charging of industrial standby batteries seems simple but to obtain a satisfactory life and performance it should not be taken lightly.

The manufacturer’s instructions should be followed without deviation and any suggestion of using an alternative method of charging must be approved by the manufacturer.

Manufacturers are increasingly concerned about the effects of pulse charging which is being promoted as an energy saving feature. However, there is no evidence that pulse charging does save energy and it has been argued that pulsing can cause additional heat within the cells and reduce the life. Long term “field testing” continues.

Battery back up is essential in todays world, particularly when utilised in applications such as UPS systems and emergency lighting, but how does a lead acid battery produce the power needed to keep critical building services online?

There is a lot of information available on the internet which gives detailed information of the electrochemical reactions of the lead-acid battery. It can be seen that three effective components are require; lead, lead dioxide and dilute sulphuric acid. Looking at some of the basic information shows that Faraday discovered the theoretical amounts required to produce 1 ampere-hour (Ah) of electricity are; 3.87g of spongy lead (Pb), 4.46g of lead dioxide (PbO2) and 3.66g of dilute sulphuric acid (H2SO4). However, in practice many more time the theoretical value is required to produce an effective battery.

When a battery discharges it “uses” lead, lead dioxide and sulphuric acid. Eventually the battery will run out of one of these active materials and the voltage will collapse. It is not straightforward to say that all lead-acid batteries will run out of the same active material. Some will run out of positive active material, other negative and others acid.

This article will discuss how the three active materials work together to produce electrical energy. Basic battery designs are discussed along with the need to “design for manufacture”.

For this article we are only interested in industrial standby batteries on continuous float charge applications. However, some of the basic information is relevant to all lead-acid batteries.

The positive active material is lead dioxide and this can be found in three basic plate designs; Planté, Pasted Plate and Tubular Plate. Some special designs are available but in reality all lead-acid batteries will have plates as described above.

In the Planté battery the plate consists of a lead casting upon which the active material is electro chemically formed. This type of battery is generally regarded as the most expensive to manufacture but it has a unique characteristic where the performance will improve by typically 10% over the first years in service before reaching a plateau. End of life is determined when the battery fails to achieve its nominal declared performance. This is typically after 25 years of float service. There are many examples of this battery type still in service after 40 years in service. For other battery types, the end of life is generally considered to be when 80% performance is reached.

Pasted Plate types are by far the most popular and can be found in VRLA batteries of the AGM and GEL construction as well as many vented types. The manufacturing process starts with lead oxide (PbO) as powder which is mixed with additives and dilute sulphuric acid to form a paste. This paste is applied to the lead grids.

Negative plates are manufactured in a similar way; starting with a lead powder, mixing with additives and dilute acid and applying as a paste to the lead grid. The negatives of Planté and Tubular Plate batteries are of the pasted plate type.

The plates are then formed either after assembly into the container or in large vats before being dried and then assembled into the battery.

For Tubular Positive Plates, the positive material it is often poured under pressure as slurry down the tubes. The material often includes triplumbic tetroxide (Pb3O4). This additive acts as a catalyst to assist the formation of the lead dioxide positive active material.

The performance and efficiency of the battery not only depends on the type (Planté, Pasted Plate or Tubular Plate) but also on the specific makeup of the material. Lead dioxide can be course where there is a lead (Pb) central core to the particles. In other types, the particle may be 100% lead oxide. Clearly, the more PbO2 and the more capacity can be expected – all other things being equal. Active material manufactured with triplumbic tetroxide additions is regarded as very efficient but has a manufacturing cost penalty.

Manufacturers have different ways of producing active material and it cannot be concluded that the battery having the heaviest weight will have the greater ampere-hour capacity.

The negative active material is a much simpler material than the positive. In all battery types, the negative plates are of the Pasted Plate type. The material is spongy lead (Pb) which chemically is basically the same as the base material; lead.

Particle size has little effect on the overall capacity of the battery and because it is much easier to form spongy lead when compared with the lead dioxide of the positive plate, the cost is lower. It is unusual that a battery performance is limited by the negative material.

This is often ignored but without acid, the battery will not work.

The specific gravity plays an important part in the overall performance of the battery. Different types of battery use different concentration ranging typically from 1.210sg to 1.315sg.

Planté batteries have the greatest quantity of acid excepting special application batteries such as those designed with an extended topping up period. These special types start with a “high reserve” of acid which is generally of a quite low specific gravity (sg) such as 1.200sg. With time, the volume of electrolyte will reduce due to electrolysis and evaporation and the specific gravity will increase. Electrolysis consumes water and not acid. The volume of “acid” remains the same.

The strength of the electrolyte as sulphuric acid and water must be balanced to attain the correct strength for the type of battery. For example, if we start with 1.30sg acid and add water to reduce the strength to 1.20sg, the volume of “effective acid”, as determined by Faraday, remains the same. This is why we should refer to the electrolyte as dilute sulphuric acid and quote the specific gravity. It follows that if the available space in each cell is restricted, we must compensate by using a higher density (sg) electrolyte, i.e. we need more acid to balance the electrochemistry. We still need to balance the lead dioxide, lead and acid in the battery for it to work effectively.

The graph below gives an approximation between strength of acid and specific gravity (sg).

Because of the tight assembly of Tubular Plate batteries, there is very little space for the electrolyte and consequently the sg has to be higher; this is typically about 1.280 sg. Similarly VRLA AGM batteries have a higher sg, typically in the order of 1.315sg. These battery types generally run out of acid before they consume all the positive or negative active materials. They can be described as “electrolyte” starved. It may be argued that all we need to do is increase the strength of the electrolyte and stop these types from running out of acid. Unfortunately it is not that simple because very strong acid will destroy the negative active material.

VRLA GEL products generally use a lower density electrolyte than VRLA AGM because they have more space between the plates and room above the plates for electrolyte. On the down side GEL products do not have the same high current capability as AGM because the space between the plates is smaller. The reduction in space, known as plate pitch, means the resistance is lower and therefore the high current capability is better.  

The positive grids corrode as batteries age and this is often the cause of failure at the end of life. Several factors influence the corrosion rate of the positive grids including the grid alloy, design, thickness and method of manufacture.

In the past it has been argued that the thicker the grid, the longer it will last. This is no longer the case. Grids manufactured using pure lead of better than 99.99% purity, with life enhancing additives such as tin, silver and even gold, have been shown to last longer than grids many times thicker.

The method of manufacture is also critical. Expanded metal grids will corrode faster because of the “stress zones” where the metal is expanded. Arguably, rolled and punched grids will have the longest life. By rolling the grid metal the stress lines will lie in a uniform direction which makes them very corrosion resistance. This can be compared with cast iron and forges steel.

However, there is a cost penalty. Rolled and punched grids will cost more to manufacture when compared with cast grids or expanded metal grids. Similarly, very pure lead with expensive additives such as tin will again have a higher manufacturing cost.

Because the negative grids do not corrode in normal operation, the life of the battery is generally not cause by negative grid failure.

Designs are more simple, spongy lead is quite good at conducting the current and therefore the cost is much lower than that for the positive grid.

The high rate performance of a battery will be superior if a larger number of plates are used. Thinking of a loaf of bread, the thick slice “toast” bread may have 25 slices whereas the thin sliced loaf may have 35 slices, but the overall size will be the same. If this is translated to a battery technology, the more plates in the same volume will result is a greater high current capability. There is a down side to more plates. If the plates are thin, they are more difficult to handle during manufacture and therefore they will inevitably have a higher cost. Coupled with the number of plates is the plate pitch. i.e. the distance from the positive plate to the negative plate. The thinner the better but separators which are very thin are more difficult to handle and are more expensive.  

The positive grid and to a lesser extent the negative grid design can also contribute to the high rate performance. The positive active material (PbO2) is not a particularly good conductor of electricity when compared with lead (Pb) of the negative plate and to compensate for this, the vertical wires and top bar may have an increased cross section as they approach the lug area. Designs with “vertical” wires radiating from the plate lugs (radial grids) are often used but again there is a cost in manufacture. Designs with increased sizes of wires will cost more to manufacture because the volume of lead and therefore the weight of the grids will be higher. Lead is the most expensive active component in a lead-acid battery.

The separator material also has an effect on the high rate performance. Separators with extremely low resistance are available but again there is a cost to be paid. The separator material for VRLA AGM batteries has a very low resistance and coupled with very small plate pitch, this makes them very good for high current applications.

Comparing designs for long life, you may think that the opposite applies to designs for high rate. This is not the case.

It has been shown that thin grids manufactured from very pure lead such as better than 99.99%, as discussed earlier in this paper, will enhance battery life. This also applies to the negative plate but to a much lower extent.

The purity of the filling acid can have a dramatic effect on life. This filling acid is made up of two parts; a) sulphuric acid and b) water to give the dilute sulphuric acid of the correct strength (sg) for the battery type.

The purity of the active material will also have an effect on life. Both positive and negative active materials come from base lead. It is also argued that virgin lead should be used if the longest life is expected. The best batteries will be manufactured from 99.99% pure lead or those where the critical impurities such as iron are kept to a very low level.

How a battery works is not simply down to the electrochemistry. The design, materials and manufacturing methods play a vital part in getting the best from the base materials discussed by Faraday.

Batteries manufactured using very pure materials, state of the are designs and manufacturing processes will offer the best return for long life, high performance and best reliability.  

The market place has vented battery design such as Planté, Tubular Plate and Flat Plate designs to choose from. For VRLA types, the choice between AGM and GEL can be difficult but generally, the AGM battery will be smaller, lighter and have a better high rate performance. GEL batteries are not all the same and different life claims are seen for different designs even from the same manufacturer.

The choice of which battery to use is not easy and is even more complex when capital or whole life cost is thrown into the equation.

We need lead (Pb), lead dioxide (PbO2) and dilute sulphuric acid (H2SO4) for the battery to work but how we use these materials makes a substantial difference to the end product.

We hope our article has proved useful and informative, our next blog discussion will be ‘VRLA Battery Storage’ which will describe best practices to ensure VRLA batteries are stored correctly. In the meantime, please do contact us directly should you have any enquiries our team can assist you with.

This article considers ohmic measurements as a means of identifying rogue cells and monoblocs of VRLA AGM and GEL product installed on float standby systems. For batteries operated on cycling applications there are better ways of identifying rogue cells or the end of life for the complete battery. For vented product, the established method of visual inspection, float voltages, specific gravities and ultimately discharge testing are more reliable than predictions based on ohmic measurement.

Searching the internet will show that there are many articles discussing battery ohmic measurements. Some will be written by operators such as telecommunication or UPS companies whilst others will be from instrument manufacturers and papers written by battery manufacturers are also available. Rarely is an article written with the intent of giving an overview of the subject and guidance for the interpretation of the results obtained and actual case examples. This article will show that whilst ohmic measurements may be used to identify rogue cells or monoblocs, the results can also be very misleading and may give the operator a false sense of security, or condemn product that is still fit for purpose.

It has been shown that when ohmic values for a battery are compared over time, they can be used to predict the end of life. (See FIG 1 below). Care must be taken on drawing the wrong conclusions when comparing the results over time. Rogue cells can skew the results and it must be understood that the ohmic value of a cell will change in a none-linear way over time. Typically, the ohmic value will become better over the first years of service before falling away as the end of life approaches. This fall off may be steep and measurements taken on an annual basis may not be sufficient to identify imminent failure. It has also been shown that whilst the ohmic change is not linear with time, similarly the expected performance is also not linear with time and cannot be correlated with the change in ohmic value without detailed information of the instrument and product characteristics. If long term monitoring of cells is used to predict the end of life, the operator should consult with the equipment manufacturer and battery manufacturer to obtain the best possible predictions. Establishing the base ohmic value is essential and this should be done after an initial stabilising period of approximately six months in service.

It has been suggested by some instrument manufacturers that a value more than 30% different in the negative direction from the base number represents a failure. This may or may not be correct depending on the instrument used and the product being evaluated.

Ohmic measurements can be taken on large vented (wet) lead acid batteries but most of the instruments available are aimed at VRLA product of less than 500Ah. In this article, we look at a variety of VRLA AGM products. The principal can be extended to GEL product but it would not be practical to extend this to vented cells.  

Although it is rather academic and mainly only for the interest of Chartered Engineers, it is worth looking at what is meant by ohmic value. There are three values for us to consider as follows: -

DC RESISTANCE: R = V / I

AC IMPEDANCE: Z = √ (XL2 + R2)

AC CONDUCTANCE: S =1 / (√ (XL2 + R2))

The internal ohmic value of lead acid batteries are made up of all three components listed above and can also be described as more complex because cells also have a capacitance and inductance which interact with each other to give an overall ohmic value. Understanding the overall value and using this to determine the health of the battery is what users search for, but is not easy to draw conclusions.

There are many instruments available on the open market that claim to show the state of health and overall performance expectations. Most of these instruments will give an ohmic and will leave the interpretation to the discretion of the user. The user will often contact the battery manufacturer for “base numbers” for the battery being measured and from this some sort of conclusion can be reached. What is missing is that different instruments will give different results and some real results for different instruments are given below.

Four different instruments were used to establish the ohmic value of a 12V 90Ah monobloc and the results are given below along with the manufacturers declared internal resistance. All four instruments claimed to measure the impedance but one actually measured the DC resistance. The manufacturers internal resistance was determined by the method described in IEC 60896 21-22 and not by using a measuring instrument.

Instrument “1” - 2.72mΩ

Instrument “2” - 3.39mΩ

Instrument “3” - 3.80mΩ

Instrument “4” - 6.49mΩ

IEC 60896 21-22 Method - 5.20mΩ

The inevitable question is “which is the correct value”? The answer is that all the readings are correct. Each instrument uses a different method to determine the “impedance” and it follows that each instrument will show different impedance. It follows that unless you have a “base number” for the specific product using a specific instrument, correlation cannot be made between the base value and readings obtained for batteries in service. It has also been shown that the actual method used can also affect the results. Specifically, the point on the cell or monobloc where the measurement is taken will, in many cases affect the result. Some instruments are more sensitive than others in this respect. A measurement taken on a bolt head can be significantly different to that taken next to the bolt on the connector. All measurements should be taken in the same way. Different values can be seen if the battery is on line or off line due to charger and load induced ripple.

The most consistent values are obtained by using the IEC 60896 21-22 Method which involves discharging the cell or monobloc at two different currents and then calculating the resistance using a formula. It does not matter if the tests are carried out by an independent laboratory, user or manufacturer; the results will correlate well within experimental limits.

Using the IEC 60896 21-22 method, the Internal Resistance is found by: Ri = (Ua – Ub) / (Ib – Ia) in ohms However, it is not practical to use the IEC 60896 21-22 Method on a battery in service. As a consequence, ohmic measuring test instruments have been developed which will measure the impedance, conductance or resistance of cells and monoblocs whilst in service. Some instrument manufacturers claim that the devices they offer are immune from ripple generated by the charger or load and will detect all types of battery failure from internal shorts to end of life.

Some will argue that it is essential to have the base number for comparison purposes and this brings its own problems. The spread of ohmic values from a batch of lead acid cells or monoblocs can easily be ±10% or more within two standard deviation. With this type of spread in values, the process of identifying rouge cells or monoblocs becomes difficult. We also have to establish the value to consider for replacing a cell or monobloc. Even with the practical difficulties of establishing a base value, this should be done where ever possible. The best way of establishing a base number is on a new battery that has been in service for six months. During this time, the product will stabilise in both float voltage and ohmic value terms. The float voltage and ohmic values will have a lower spread as the product settles down. This assumes that no discharges have taken place. If the battery has been discharged, then a further 6 months stabilisation period should be applied. Then, using a selected instrument and test method, all cells or monobloc values should be measured and recorded for future reference. At that time it may be decided to consider failure as a given percentage from the mean value. Caution needs to be applied because stipulating too tight a tolerance will result in good product being rejected whilst a tolerance that is too wide may leave the installation exposed.

Battery manufacturers and users have been using ohmic measurements for some years. In many cases these have been successful in finding weak cells or monoblocs before failure occurs when the float voltage has been within acceptable limits. However, in some cases the instruments used have either failed to identify weak product or have condemned product that was still serviceable. Below are real case examples of what has been found in the real world.

For information, the typical conductance and capacity against life for a VRLA AGM cell is shown in Fig 1 below.

In a real case study 3 x 12V monoblocs from a battery of 66, were rejected by the customer. The battery had been in service for over 18 months with a hard wired impedance monitoring equipment. It had previously been established beyond reasonable doubt that the base impedance for the monobloc type was 3.00mΩ. In the 18 months no problems had been recorded and no alarms triggered. Three monoblocs triggered the high impedance alarm and the customer replaced them... The customer understood that a stabilising period was required and although the impedance was out of specification, the float voltage did not trigger an alarm. However, after 6 months, the impedance was still out of specification and impedance alarms were still being triggered.

The customer was not happy that 3 blocs failed and the replacements also triggered the high impedance alarm. The three subject blocs had impedance values of 7.8mΩ, 7.9mΩ, and 12.4mΩ, (base value 3.00 mΩ). It was not identified if these values were steady, or variable. The blocs were removed from site and evaluated under controlled laboratory conditions.

The battery was installed on a 15 minute UPS application and it was decided to test the blocs at this rate in the laboratory. They were tested without recharging to avoid any influence this may have on the test results.

The expected run time based on the bloc actual temperature of 14⁰C was 13 minutes to 10.2Vpb. The actual run time was in excess of 17 minutes for all three monoblocs.

The results are given in graphical form in Fig 2 below. This example shows that the impedance values obtained on site were not representative of poor product performance. Possible explanations being; a) poor connections between instrument and monobloc, b) different connection points on the monoblocs, c) instrument inaccuracies, or d) an installation fault.

It was decide to carry out a site visit to try and establish the cause of the discrepancy. The product had been installed correctly, the float voltage was correct and there were no apparent faults with any of the equipment that may cause the problem. Because the battery had a “hard wired” impedance monitory system, it was decided to carry out cross reference checks using a hand held instrument. This showed a problem with the hard wiring and several connections between the monitoring cabling and monobloc had a high resistance. Once the faults had been corrected, the system operated successfully with no alarms being generated. The system was monitored over the following two years and the alarm tolerance was progressively reduced until a final value of ±10% about the mean was considered to be acceptable.

In this case, faulty monitoring equipment resulted in false alarms.

A new battery of 40 x 12V 60Ah blocs installed on a UPS machine with a service contract was identified as having 1 bloc with low conductance.

Although a base number had not been establish, an estimate had been given by the instrument manufacturer of 1000mohms. This was applied to the test results.

The float voltage was low but not to the extent that it represented a failure. See bloc J7 in Table 1 below.

The bloc was removed from the battery and discharge tested at the 30m rate without being recharged. The run time was 22m 14s proving that a problem existed with this monobloc. To rule out undercharging, the bloc was recharged for 72h at an average voltage of 2.40Vpc and the test discharge was repeated. The run time reduced to 19m 46s proving that the bloc had a fault. A destructive analysis of the monobloc revealed an internal short in one cell due to a bucked plate. This was a manufacturing fault.

Although the float voltage was low in comparison to the other blocs in the battery, to the untrained eye this would not have been obvious and the fault would have been missed. Eventually the float voltage would have deteriorated to a very low value and a fault would have been obvious. However, the low conductance found the fault earlier. This is a good example where the ohmic measurement identified a problem when the float voltage did not.

The graphical results are given below in Fig 3 below.

A 6 years old battery of 4 x 33 x of 90Ah blocs installed on a UPS machine serviced annually by an independent service company was identified as having problems with 8 blocs when evaluated by a portable impedance instrument. No official base value was available therefore an estimate of 2.5mΩ was used. This was derived from historical information of similar product. All 8 blocs had a float voltage within the accepted range which was selected as 13.4Vpb to 14.1Vpb.

The suspect blocs had impedance values as follows: - 3.1mΩ, 3.0mΩ, 3.0mΩ, 3.1mΩ, 2.9mΩ, 2.9mΩ, 3.0mΩ, 2.9mΩ

All suspect blocs were tested at the 14 minute rate 10.00Vpb, direct from site, without being recharged. The actual test results are given below in Table 2 and Fig 4 below.

Although the actual discharge test results prove that a problem existed with all but one of the 8 blocs, the actual discharge performance cannot be correlated with the impedance values. One bloc having an impedance of 2.90mΩ had a run time of over 14m whilst another bloc having the same impedance had a run time of approximately 7m 20s.

It is of further interest to note that these 8 blocs had the highest impedance of all the blocs in the battery and out of the complete battery of 132 blocs (4 x 33) a total of 42 blocs had a value higher that the 2.5mΩ base value. Based on impedance values it is impossible to state if the complete battery would, or would not perform as required.

It was recommended that further on site investigations should be carried out to try and prove the battery performance. An actual “on site” test discharge was proposed. Because of the unpredictable performance and some evidence that the battery had been subjected to a higher than recommended operating temperature, the customer decided to replace the complete battery arrangement.

In this case, the poor impedance suggested a problem which turned out to be correct even though the absolute accuracy could not be verified. Float voltage measurements did not identify a problem.

The actual test results at the 14m rate can be found in Table 2 below.

The battery in this case had been installed less than 6 months before the failure was reported. The system was at a new installation and used for emergency lighting through a static UPS system. The fault had been reported as failure to meet a run time of 120m with a typical performance of less than 15m. It was not connected to a monitoring system and the no impedance measurements had been taken on site.

Following initial investigations it was clear that a major problem existed with the battery. Because of the “high profile” of this installation it was decided to replace all 80 monoblocs and send them to the laboratory for detailed evaluation.

At the time the battery failure was reported, the test team had three different ohmic measuring instruments available and these were used in the first evaluation process.

Although the measured values differed from instrument to instrument, this was expected and the results correlated well with a base value obtained for new product taken from stock. The base values used were as follows: -

Instrument “A”: 2.5mΩ

Instrument “B”: 2000mhos

Instrument “C”: 3.5mΩ

All ohmic values were similar with a total spread of less than 16% from the highest to the lowest using all three instruments. This 16% was well within the expected range for a battery of this age.

Eight monoblocs were chosen by the end user for discharge testing. Ohmic measurements of the eight blocs can be found in Table 3 below.

The product was test discharged at the 120m rate which was the design standby requirement on site. The test was carried out without recharging and the results are presented in graphical form below in Fig 5.

Because of the unexpected and strange test results, further onsite investigations followed. These investigations revealed that the battery had been cycled on a daily basis. Cycling of standby batteries operating on a float charging system can lead to a passivating layer being formed between the active material and grid structure. The passivating material has been described as PbOX. This layer results in a high voltage drop between the active material and grid structure when the cell is being discharged. Consequently, the terminal voltage is suppressed and the total battery voltage does not match what is required. This failure mechanism is relatively rare and is caused by a poorly designed operating system and it is not a battery fault. As can be seen in Fig 5 below, when the affected cells were discharged, the voltage did not reverse as is the norm with poor performance cells but remains more or less steady at zero volts. This can be clearly seen for bloc No 24. It is speculated that all 6 cells within this bloc were affected but in other blocs some cells were not affected to the same extent and held the voltage above zero for longer. Never the less, the steep fall in voltage followed by a more stable period suggests a passivating layer has been formed within some cells of the bloc.

It is suggested that ohmic measuring instruments cannot identify this phenomenon because the grids, top lead and active materials are not corroded and no plate sulphation has taken place compared to an internal short problem which would result in plate sulphation...

The following illustrates how a hand held impedance instrument successfully identified three “faulty” monoblocs. The instrument measured float voltage and impedance. All float voltages were within the expected range but three blocs had high impedance.

This battery system comprised 4 parallel strings each of 31 x 12V 133Ah monoblocs. The arrangement had been in service for 5 years without problems and the user commissioned an independent service organisation to carry out impedance checks. Previously, only float voltages had been measured and recorded; no faults had been identified up to this point in time. No “base” impedance value was available and therefore the only pass / fail criterion was to ensure that all values were reasonably consistent. Any abnormal values would be investigated further.

The three high impedance monoblocs and one other were removed and tested at the 3h rate to 10.8Vpb. The fourth monobloc, No AA1 was removed and used as a “control” monobloc. The performance of the three high impedance monoblocs was found to be marginally low with a run time of approximately 95% of the claimed performance. The “control” monobloc had a run time approximately 10% better than published. The three low performing blocs would not have caused a battery failure but eventually, if not replaced may have in the future. It can be argued that it was correct to replace the three monoblocs.

The impedance of the three identified monoblocs was more than double the typical value, but the actual run time was only approximately 15% lower. This case shows that although the instrument correctly identified three impedance monoblocs it could not be used to correlate the actual expected performance.

The test discharge results are tabulated in Table 4 below.

The voltage and impedance values are given in Figs 6, 7, 8 and 9 below.

A seven years five months old battery comprising 33 x 12V 90Ah installed on a static UPS system had many monoblocs with high impedance. As an investigation, eight monoblocs were removed for laboratory investigation. The customer chose the 8 blocs and replaced these with new product.

Because the time from removing the monoblocs from the battery and being received by the laboratory was several months, it was decided to charge them at an average cell voltage of 2.40Vpc for 72h to establish they were fully charged.

The blocs were evaluated using the same impedance instrument as used on site and the results compared with actual test discharge results. The base value had previously been established as 2.71mΩ from a large sample of known “good” blocs.

A 15m rate test was used and the run time compared with the impedance value to try and establish a correlation. This would then allow a better on site evaluation of the complete battery to be carried out without having to carry out an actual test discharge.

Monobloc No 5 having an impedance of 114.8% had a very poor performance of 2m 47s whilst monobloc No 4 with a similar impedance of 112.9% had a run time of 10m 44s. The results are scattered with no reasonable correlation between impedance and performance. However, any bloc with impedance above the base value of 2.71mΩ must be suspect. The results show that it is not reliable to use impedance values as a guide to actual discharge performance.

The design life of the product was declared by the manufacturer as 10 years when tested in accordance with BS 6290 and because this battery was seven years five months old at the time of investigation, the failure may be the result of aging and not a manufacturing fault. Detailed destructive analysis of the product was not carried out.

Based on the assumption that these 8 monoblocs were representative of the complete battery, it was recommended that further on site investigations should be carried out. Following these investigations, the customer decided to replace the complete battery. Part of this decision was based on the product being seven years five months old battery

In this case, although the impedance measurements were slightly above the base value, they were very variable and when the product was discharge tested it was found to be below requirements.

Impedance and charging volts are given in Table 5 below.

Test discharge results are presented in Fig 10 below.

A 4 years old battery comprising 24 x 515Ah cells was reported as having several cells with low conductance values. It had previously been established that the practical conductance for the cells type was 0.20 mohms. The customer concluded that any cell with a conductance lower than 0.14mohms, i.e. more than 30% lower than the base number would be faulty. The battery manufacturer put the case that the battery was not at fault but agreed to carry out an on-site test discharge. The test was carried out at the published 180 minute rate and all cells exceeded this with the minimum run time being 181 minutes, the maximum 202 minutes and the average 192 minutes. As a complete battery the run time to the minimum voltage is accordance to International Standards was 194 minutes.

The customer was not satisfied because several cell had a conductance of less than 0.14 mohm and the manufacturer agreed to replace all 24 cells for further test work.

All 24 cells removed from site were test discharged without recharging at the 180 minute rate. The conductance and run time percentage results can be compared in Fig 11 below and for clarity, the conductance values have been presented in Fig 12 and the run time in Fig 13.

The results clearly show that there is no correlation between the conductance readings and the actual performance at the 180 minute discharge rate. All 24 cells achieved in excess of the design run time of 180 minutes.

Following the laboratory test discharge, all cells were recharged for 6 days at 2.28Vpc ±0.01Vpc at 20ºC and the conductance and discharge test was repeated.

The conductance average “direct from site” was 0.19 mohms and after the discharge and recharge it had improved to 0.20 mohms. Although the conductance improved, it was not large and in experimental terms we can say that there was no significant difference. In addition, some cells still had a conductance lower than the customer expected minimum of 0.14mohms.

The run time average “direct from site” was 192 minutes and after recharging the average run time was 213 minutes. This result can be explained by the battery being fully charged independently from the connected load and under a controlled charging regime where voltage, current and battery temperature were closely specified. In this case a full 6 days recharge at 2.28Vpc with a current of not less than 50A being available at a controlled temperature of 20ºC

The results were presented to the customer who then accepted that there is no correlation between conductance and run time and although the conductance values varied considerably across the 24 cells with some being lower than the 30% below the base value, it was accepted that this did not indicate a problem with any cells.

This case illustrates that conductance values alone can be misleading and suspicion of a fault should be investigated and if necessary an actual test discharge should be performed to confirm the battery capability.

If trending of the complete battery and detailed analysis on a cell or monobloc basis is used then the reliability of ohmic measurements can be reasonably accurate but not 100%.

Ohmic measurements may assist in identifying low performance cells or monoblocs but this is not 100% reliable.

The use of ohmic measurements is better than only using float voltages to evaluate the battery condition but they are no substitute for actual battery discharge testing. If the ohmic instrument identifies a problem this should always be followed up with further investigation work to verify the actual performance...

It has been found that whilst a 30% deviation in the negative direction is a good indicator that the cell or monobloc condition has deteriorated this cannot be considered as a pass / fail criterion. Whilst it may be prudent to replace a unit whose ohmic value is more than 30% different to the base value, it should be remembered that the failure of one unit may be the first in a series leading to failure of the complete battery within the near future or the reading may not be accurate and the battery may have no faults. In all cases, further investigation to identify the reason for the deviation from the base value should be carried out.

It is important to keep records of the ohmic measurements and any deviations from the normal should be thoroughly investigated.

In all cases, an alarm caused by a deviation in the ohmic value should be followed up by further detailed investigation.

This article discusses the float voltage characteristics of VRLA AGM and VRLA GEL cells and batteries in standby power applications when charged using commercially available battery charging systems having a constant voltage characteristic with limited current output.

Batteries in standby applications require a charging voltage that is sufficiently high to recharge following a discharge, and maintain a fully charged condition without overcharging the battery. This voltage needs to be regulated for different operating temperatures but for simplicity, this document considers operating at 20oC. The principals and overall characteristics shown in this document can be considered to apply for operating temperatures of between 0oC.and 40oC.

The applied voltage to the battery terminals will cause a current to flow and this will result in different voltages being seen across individual cells or monoblocs. It follows that there is much more to be discussed about the float voltage apart from what is applied to the battery terminals. If all standby batteries were of a nominal 2V, i.e. all cells connected in parallel, the subject and characteristics would be simple. However, we live in the real world where 2V cells are more often than not, connected in series resulting in battery voltages from typically 24V or 48V telecommunication systems up to 1000V UPS applications.

When operating at 20oC VRLA AGM product will generally require a float voltage in the order of 2.27Vpc to 2.29Vpc whereas GEL product is normally float charged at between 2.23Vpc and 2.25Vpc. This is primarily due to the different specific gravities of the electrolyte. Although the values differ, the overall reaction to the applied voltage is similar for both types. Where cells are connected in series to form a battery (as described above) the applied voltage will be the required voltage per cell times the number of cells in the battery (Vpc x n). The voltage applied to the battery terminals will cause current to flow through all the cells or monoblocs. Under steady state fully charged conditions, this current is very low and the sum of the voltages across each cell (or monobloc) in the battery string will be the total battery voltage as applied by the charger. Although the current flowing as a result of the applied voltage will be identical in all cells, because of manufacturing variations the voltage across each cell or monobloc will be different. The difference is marginal when the battery is in a fully charged stabilized condition but it is still noticeable and can lead to misunderstanding of the battery condition. It has been shown that a “snap shot” of individual voltages cannot be relied upon to measure the state of charge. There is probably only one exception and this would be when the individual voltage is very low and less than about 2.10V when in the normal fully charged float mode.

When a battery is first charged on site and a voltage of 2.28Vpc x n is applied (typical AGM battery) it is not unusual to measure individual voltages as low as 2.12Vpc and as high as 2.41Vpc. This demonstrates two things: a) the individual cells have small differences due to manufacturing tolerances and b) they will be in a different state of charge. A commissioning charge as recommended by the battery manufacturer will help to stabilize the individual voltages before the battery is put on line. Never the less, it may be several weeks before the individual voltages reach a reasonably consistent value throughout the battery string. Typically, a variation for individual cell voltages having been on uninterrupted float charge for 3 months will be better than ±0.07Vpc. i.e. between about 2.21Vpc and 2.34Vpc for this example, depending on the actual float voltage applied at the battery terminals. See Fig 1 below which illustrates the spread of float voltages of individual cells for a new VRLA AGM batter

#FIG 1

Having established a stabilized fully charged condition, voltage measurements may be used to identify suspect low cells. As an example, if the majority of the cells have a voltage within 2.21V and 2.34V and only a small number have a voltage outside this range, additional monitoring should be carried out to establish if these lower cells recover or continue to have a declining float voltage. Where cell voltages are within this range, no special action needs to be carried out other than regular monitoring and recording of the float voltages as recommended by the battery manufacture. For monoblocs a voltage equivalent to the number of cells multiplied by 2.21V and 2.34V respectively may be used providing very close monitoring is carried out. It is more difficult to identify low capacity cells within a multi cell monobloc. Comparison of float voltages over a period of time will show if the bloc has a low capacity cell. It follows that records should be kept and compared as time goes on.

It has been shown on many occasions that after a discharge, even of an extremely low magnitude, the voltage stability will be disrupted and float voltages will have an increased spread. These will stabilize but it may take several weeks depending on the depth of discharge. See Fig 2 below which illustrates the variation that can be expected and how these variations narrow over time. The values in Fig 2 represent the highest and lowest values of individual cells within the same battery.

#FIG 2

We know that the normal charge voltage at 20oC for AGM will typically be between 2.27Vpc and 2.29Vpc and for GEL product between 2.23Vpc and 2.25Vpc we may want to charge at a higher voltage levels to reduce the recharge time. For AGM product a voltage of 2.40Vpc is quoted by many manufacturers and 2.45Vpc for GEL product. Before using these elevated voltages it must be understood and considered that the higher charging current will create more heat and gas and additional ventilation may be required. The benefits of using these higher voltages are not just a shortening of the recharge time. Using the higher voltages as above will result in the fully charged current being about 10 times higher than using the normal float voltage. This has a beneficial effect in both commission charging and recharging following a discharge because it helps to convert the discharged active materials both faster and more efficiently. The higher voltages also result in better stabilized voltages and the spread of voltages is not as high when the battery is returned to the normal float voltage. It is important to provide adequate ventilation when using these higher voltages and the battery must be returned to the normal float voltage upon completion of the charge; failure to do so may dramatically reduce the battery life. Many engineers recommend an annual charge at a higher than normal float level to help stabilize the cells. The effect on total battery life is small. A 24 hour charge at 2.40Vpc, may, in theory, reduce the life by about 1 week. For good quality AGM and GEL batteries having an expected float life of over 10 years, a loss of 1 week is small.

It is not sufficient to look at the voltage alone when considering float or recharging VRLA batteries. We should also consider the current capabilities and characteristics of the charging source. Most manufacturers recommend that the current available to the battery shall be a minimum of 5% of the 10 hour ampere hour of the battery i.e. 5% C10A. Some manufacturers recommend 10% whilst other may recommend 3%. Without sufficient current the battery may never recharge but may stay in state of equilibrium. A failure will occur upon the next discharge. There have been situations where large capacity batteries have been charged by inadequate solar panels and within one year the battery has been destroyed. In one example, a 5000Ah battery had a 50A (1%) solar panel. After the failure, the system was redesigned using a smaller capacity battery with a controlled charging regime capable of raising the voltage to typically 2.40Vpc and returning the battery to float charge after a pre-programmed time period with a current capability of 10%C10A as a minimum. As an added precaution, a diesel powered generator and separate charger was installed to annually fully recharge and re-commission the system using an elevated voltage. Several years later and the total battery system continue to operate successfully.

Whilst voltage ripple can have a serious effect on battery life, modern charging equipment can easily meet battery manufacturer’s requirements and is not discussed in this document.

This article discusses the gasses given off by Industrial Batteries and how to manage these gasses. The document primarily considers Standby Batteries but the overall principals apply equally to lead acid and nickel cadmium types both vented or VRLA.

Under normal operating conditions, the gasses evolved are hydrogen (H) and oxygen (O). However, under extreme conditions other gasses may be produced such as hydrogen sulphide (H2S). Some strange gasses are also given off in very small quantities such as carbon dioxide (CO2). This document only considers the evolution of hydrogen, oxygen and hydrogen sulphide.

Hydrogen will burn in air when in concentrations of between 4% and 75%. Oxygen in itself will not burn but does support combustion. Hydrogen sulphide is flammable and will explode and is very poisonous.

Lead-acid and nickel cadmium batteries only generate gases when on overcharge. However, gases can be seen to come from vented batteries with clear containers when on discharge. This is because gas will be present on the plate surface and within the active materials and when plates expand or contract when discharging, cells can appear to gas. VRLA batteries often “vent” on discharge because of the heat generated which causes the internal pressure to rise and the vent then opens to release the excessive pressure. In a quiet environment, a faint whistle sound can sometimes be heard.

Overcharge is a normal operating condition for all industrial batteries on float charge. The level of overcharge is very low and correspondingly, the volume of gas generated is also small but importantly it cannot be ignored. Batteries must not be placed in sealed rooms or enclosures.

The volume of the gas given off is, in the most part, proportional to the overcharge current. It should be noted that it is the charging current that produces gas and not the charging voltage. Normal float charge voltages result is low overcharge currents but a small increase in float voltage can result in a dramatic increase in float current. It is recommended that all float voltage systems are fitted with “over voltage shutdown control”. For VRLA batteries, a typical “over voltage shutdown control” voltage would be 2.40Vpc. At this voltage level the volume of gas given off is typically 7 to 10 times greater than the volume at the normal float voltage.

If the battery temperature is elevated, more gas is given off because more current will flow for the same float voltage. Typically, if the temperature increases by 10oC the volume of gas will double. In the extreme, a high battery temperature can lead to thermal runaway and hydrogen sulphide can be evolved in large quantities. This condition has to be avoided under all circumstances. Fortunately, hydrogen sulphide can be detected very easily because it has a foul odour of rotten eggs. If present, extreme caution should be taken when entering the battery room. It is recommended to isolate the charging source and allow the gas to disperse before entering the battery room.

For vented batteries, the charging current may be many times higher than for VRLA types. Vented batteries are often commission charged at constant current and a value of 10% C10 amperes as a constant value is normal. Comparing this to the current for a 100Ah VRLA battery, the current may be 0.2A at the “overvoltage shutdown control” voltage; similarly for a 100Ah vented lead-acid battery being commission charged at 10% C10A the current will be 10A. Considerably more ventilation will be required.

We need to manage these gasses to avoid disastrous consequences.

International standards give guidance to the ventilation requirements and people responsible for battery installations must be fully aware of the guidelines. Below you can find the formula for maintaining the hydrogen concentration below the 4% explosive limit. This formula is directly from BS EN 50272-2 and you MUST refer to the full document for the correct use of the formula.

Q = v . q . s . n . Igas . Crt . 10-3 [m3/h]

(The author does not accept any responsibility for the miss-interpretation or use of the above formula.)

Please follow our blog for continued technical information and advice relating to a full range of battery installations regarding both management, implementation and maintenance. Should you require particular assistance with a specific project then please do not hesitate to contact us directly for assistance. Our team are available to provide the necessary support you may need.