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.

From April 2016 through to May 2016 Fiamm have been in the process refreshing the FLB, FIT and SLA series of VRLA standby batteries, which has included a change in box colour, product codes and updating the battery label. The following offers a description of the changes for each type.

Battery boxes for this type have now changed from solid grey box and lid to a dark blue box and grey lid. In physical size, terminal type, build standard and operational terms the battery is equivalent and completely compatible with previous models. To identify the new models the suffix of ‘P’ has now been included into the product code, for example the ‘12FLB350’ has now been rebranded as the ‘12FLB350P’.

Please see the following image showing the old and new FLB battery.

The product label has also been refreshed to offer greater brand and range visibility together with additional technical information such as recommended float voltage and watts per cell rates at 15 minutes. Connection torque is also included on the label, which will assist in ensuring correct installation and on going maintenance.

New products have also been added as part of this programme offering a greater choice for high Ah mono blocs. These include the 12FLB700P 12V 710Ah and 12FLB800P 12V 792Ah and the 6FLB800P 6V 792Ah, which have been developed and designed for suitability in high discharge applications such as UPS, Telecommunications, Rail applications and similar.

Roll out of the new models have been scheduled as follows –

Once the roll out is complete, previous ‘solid grey’ models will no longer be available.

As one of the most long-standing and successful series of the Fiamm battery catalogue, the SLA has also been subject to revision in order to suit today’s DC power market and common OEM demands.

Whilst retaining the traditional solid blue colour, some models have been redesigned to meet with universal box sizes found in todays industry.

It should be noted that box sizes for some models will differ from historical types and this should be accounted for when replacing like for like product codes in pre existing cabinets and racking systems. Products affected by changes in physical design feature prominently within the 26Ah to 180Ah spectrum, we will be advising of these changes on each specific SLA product page, showing both old and new data sheet information to ensure compatibility. As always, should you require advice regarding a specific project then please do contact us directly to discuss.

An example of the SLA product modernisation is shown in the image below, it includes the upgraded labelling offering clear product information.

Expected later in 2016, the evolution of the FIT products continues with the introduction of new power ratings, with particular reference to the new 12FIT151 with compact dimensions designed to suit the needs of clients requiring a high energy density within 19 inch telecoms cabinets. Additional launches will also include the 12FIT201, which will offer a 12V 200Ah battery solution within the same physical dimensions of the 12FIT180.

As seen with the FLB series, the FIT battery box will be blue with a grey lid and the label will follow suit with the FLB and SLA in providing good brand and product information with technical specification and performance description relevant to each battery.

This article discusses the different type of connectors used for batteries in float standby applications. It does not consider traction batteries or those used for cycling applications but some of the practices can be translated to all battery types. The document discusses inter monobloc, inter cell, inter row, inter tier and connectors from the last cell to the transition box, fuse box, or main switch. It does not consider connections from the transition box etc to the load. However, it does make reference to these connections in respect of volt drop.

Irrespective of the voltage of the battery, all connectors should be insulated. For batteries above 60 cells it is a requirement to insulate the complete battery including the connectors to prevent direct contact. Full details may be found in the latest edition of EN 50272 - Safety Requirements for Secondary Batteries and Battery Installations.

In the most part, inter bloc and inter cell connectors are a solid copper connector usually tinned or similar and very occasionally lead plated. These are generally insulated by a clip on plastic cover to prevent direct contact. Some connectors have a “shrink” heated sleeve and occasionally a cover is provided that will completely cover the top of the bloc or cell. Solid connectors have advantages over other types because no end lug is required and they are bolted direct to the monobloc or cell terminal.

These require some special thought because of the complexity they offer. The connector should be as short as practically possible to keep volt drop to a minimum, the cable lugs will require an insulating cover and the method of attaching the cable lug to the cable must be controlled.

Different types of cable may be used from multi strand flexible “welding cable” type to large diameter strands making the cable very stiff and more like a solid copper connector. In all cases, the correct crimping machine and die must be used to connect the lug to the cable. A different die will be require for a multi strand welding cable of 70mm2 using strands of 0.3mm diameter when compared with a 70mm2 having strands of 0.7mm diameter. During manufacture, a quality plan needs to be implemented to ensure the correct crimp is achieved. Poor crimping may lead to a high resistance joint, excessive voltage drop, overheating and the possibility of a fire.

Some battery manufacturer’s offer braided connectors. These have advantages over cable and solid connectors because they are more flexible and offer a tolerance in three dimensions. As with solid connectors they may be insulated with a “heat shrink” cover or complete monobloc or cell cover. Because braided connectors are flexible in three dimensions they are often used where a seismic resistance is required.

The flame retardant properties of connectors must be considered. If the battery has to comply with the appropriate EN standard for flame retardant levels, this must include the connectors. Both cable and braised connectors and end lug covers are available with different flame retardant levels. The flammability of containers, lids and covers are discussed in the appropriate EN where the manufacturer must state V0, V1 or V2 and this requirement also applies to the connectors.

The current rating of connectors does not follow conventional standards for current carrying capabilities. The connectors are an integral part of the battery and manufacturer’s data considers the standard supplied connector volt drop and heat gain. In this respect, if an alternative connector is used there may be consequences in respect of voltage drop and temperature. However, in the majority of cases the connector cross sectional area (CSA) for battery connectors will be lower than that recommended for normal continuous rated cables. This is because the current will not be continuous but will have a finite run time depending on the battery size and load.

An example of the typical connector cross sectional area CSA with the typical tri rated cable current ratings is given below.

The CSA of the inter connectors used by most battery manufacturers do not follow BS or International recommendations / regulations for cables and conductors. The Standards will normally consider continuous running and for a battery this is not the case. The higher the current being drawn from the battery and the shorter the discharge time will be For example, a battery on discharge at the 1 minute rate may have a current of 500A but the 1 minute duration is insufficient to cause any heat problems providing the manufacturers connector is used. However, in many cases, the connector size chosen by the manufacturer is larger in CSA that most Standards recommend. This is to keep the voltage drop within tight limitations specifically for high currents and ensure sufficient contact area is available between the connector and cell terminal pillar. In many cases, even for low current applications, the manufacturer’s standard connector should be used. Some manufacturers will stipulate a larger connector for discharge rates shorter than 1 hour or in some cases 15 minutes. It is always recommended to advise the manufacturer of the discharge rates that the battery will be subjected to.

It is essential that the correct torque value is used for the connectors. Over tightening can damage the threads or even break the stud for male connector posts. Under tightening will inevitable result in a higher connector to pillar resistance which in the extreme may lead to a fire from overheating. The author has seen many examples where the pillar has melted as the result of poor connections cause by under tightening or cross threaded nuts.

The subject of applying a corrosion inhibiting gel or “grease” to the terminals remains in debate. Whilst it is important to ensure the mating surfaces are clean some argue that a layer of “grease” similar to petroleum jelly is beneficial. There is no hard and fast rule for this and advice should be sought from the battery manufacturer. However, it is the author’s opinion that vented cell connections and connectors should have some protection. Vented cells on boost charge can give off a fine acid mist and unless the connection and connectors are protected, corrosion will follow.

Manufacturer’s data will normally consider the use of the standard connector and battery calculations consider the voltage at the last cell terminal; this is usually referred to as “the battery terminals”. From a practical point of view, the length of cable from “the battery terminal” to the load may be considerable. Consideration needs to be applied to ensure the voltage at the equipment does not fall below the minimum required as a result of excessive voltage drop. In many cases, the cable between the battery terminal and load will have a larger CSA than the inter cell or inter bloc connectors.

Battery layouts requiring a large number of flexible connectors may result in a high voltage drop. It may be necessary to increase the battery ampere hour capacity to compensate. Some complicated layouts, particularly those involving battery enclosures, may have a connector length more than 10 times that when using the standard connector. This is often overlooked by the battery enclosure designer.

• Always use connectors supplied by the battery manufacturer or those approved by them.

• Advise the manufacturer of the duty requirement to ensure the correct connectors are supplied.

• Ensure connectors are correctly fitted and the correct torque is used.

• Where flexible connectors are used ensure the correct cable lug is fitted and crimped using the specified die.

We hope our continued guide to lead acid batteries proves useful, we will continue to add more articles for information and discussion. Should you wish to address a specific subject with us please contact Blue Box Batteries today.

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.

The history of the VRLA battery goes back to the early 1930’s. However, the term VRLA was not introduced until the 1980’s. Originally they were known as “sealed” batteries but following a number of serious incidents the more appropriate term of Valve Regulated Lead Acid (VRLA) battery took over.

Today we have two distinct types of VRLA batteries; those with gelled electrolyte (GEL) and those with absorbent glass mat (AGM) separators, which hold most of the acid.

The first VRLA batteries were in fact 2-volt cells and nothing like the multi-cell monoblocs or large capacity single cells we see today. They were “laboratory” types used for portable radio applications. By mixing fine silica with dilute sulphuric acid a thixotropic gel electrolyte was produced. Over time this partially dries out producing fisher cracks that allows oxygen to pass to the negative plates and gas recombination to take place. These original types had poor gas recombination efficiency and were designed to be spill resistant. They were not regarded as gas recombination products. There is evidence that the earliest “mat” batteries appeared about the same time and used separators made from felt, which held the acid. These were “sealed” with simple plugs when discharged and the plugs were removed when charging. Again, these were regarded as non-spill rather than recombination types. They were used for aircraft application when conventional “wet” batteries were not acceptable in aerobatic activities.

It can be reasonably argued that Otto Jache who was working for the German battery manufacturer Sonnenschein developed the modern GEL battery in 1957. (left) The GEL product was considered to be better for cycling applications and more vibration tolerant than the wet product equivalent. Later, Otto Jache and Heinz Schroeder developed the design for high capacity cells and multi cell monoblocs. GEL cells and monoblocs use a conventional separator similar to those used in “wet” cells. These early products had a simple valve to prevent leaks.

As the GEL product became more popular and as cell technology developed it was realized that the water usage was very low and cells did not dry out very quickly. With a one-way valve set at about 2psi good gas recombination was achieved. This was in the very early days when gas recombination was not fully understood. It was discovered that at the end of the charge cycle when oxygen is being produced at the positive electrode, it is capable of passing through the gel, which had develop fisher cracks as it dried out slightly. Oxygen is rapidly attracted to the negative electrode, which is highly charged spongy lead. Upon contact with the negative electrode, oxygen changes the spongy lead to lead-oxide, and being in contact with the electrolyte water is produced.

Today, GEL cells are available in small capacity monoblocs of only a few Ah up to large capacity single cells of over 4000Ah. The largest cells may be mounted vertically or horizontally. There resistance to vibration makes then inherently good where a seismic resistant product is preferred.

The early “mat” batteries were few and none were regarded as very good. Investigations moved towards a sealed nickel-cadmium product but the real objective was to produce a “maintenance free” lead-acid battery at a lower cost. It was not until the early 1960’s that real studies into a viable maintenance free mat separator battery took place.

It was well understood that oxygen would recombine with the negative electrode in nickel-cadmium cells and also in the gelled electrolyte products. However, the problem was to develop a separator that would allow sufficient acid to be present within the cell and would also allow gas to pass through it to the negative electrode where gas recombination would occur.

In 1967, John Devitt and Donald McClelland began researching small cylindrical lead-acid cells with spirally wound electrodes. At that time, they were working for Gates Energy Products, a USA company, who were the largest manufacturer of rubber belts and hoses in the world and wanted to go into the battery business. By the end of the year they showed the first “Gates D Cell” to the Gates board of directors. In 1971, the newly formed company Gates Energy Products was offering the D cell and the X cell recombination products to the world market.

These cells were the first to use a microfiber glass separator and the term we know today as “absorbent glass mat” (AGM) was born.

The secret was in the separator, which held sufficient electrolyte for the electro-chemical reactions to work but would also give sufficient “open area” for the transportation of oxygen to the negative electrode.

Development of the product continued with a change to a more corrosion resistant grid alloy, higher plate to separator compression ratios and a better understanding of the internal gas pressure on recombination efficiency.

One of the most interesting demonstrations of the characteristics of the early “D” cell was to connect a paper clip between the positive and negative terminals. The short circuit current would quickly result in the paper clip glowing red-hot before melting. The current flowing during short circuit was over an order of magnitude greater than that from an alkaline product and this was already substantially higher than the best high discharge current available from the GEL or “wet” product. This high current superiority is one of the advantages of the AGM when compared with the GEL product.

The product was known then, as it is now, as the Cyclon battery.

Development continued and manufacturing spread. A relatively small Company called Varley Batteries in the UK was manufacturing aircraft batteries and Gates wanted part of the business. Varley adopted the Cyclon technology and offered a prismatic version of the AGM product to several aircraft manufacturers. Notably BAe 125 and 146 business jets used the product along with the Harrier the AV8B and some F16 variants as alternatives to the NiCd batteries previously used.

To increase high performance and durability, rolled-punched grid technology is often used in the production of VRLA AGM product. Plates can be less than 1mm thick with correspondingly thin separators. The high rate currents from the batteries made today surpass that available from any other type of lead acid battery of comparable volume or capacity. The vibration resistance is also superior to conventional “wet” batteries and similar to the GEL battery.

Development of both GEL and AGM batteries continues. The specific gravity has been increased in mat batteries to enhance the voltage profile and offer better long rate capacity whilst additions of carbon to the plate structure of GEL batteries means that the high current discharge characteristics are substantially better than only a few years ago.

Undoubtedly, development of the VRLA battery will continue because lead-acid batteries are more cost effective when compared to any other electrical storage system for industrial applications.

Today there are many battery manufacturers around the world producing GEL and AGM products. Some are low cost types built down to a price whilst others are state of the art built to exacting international standards.