This article discusses test discharging of VRLA batteries. The principal is almost identical to the procedure that should be followed when testing vented Lead-Acid batteries but some important changes are necessary to obtain the correct results.

The article covers the broad spectrum of testing and some manufactures and users may prefer to test batteries in a slightly different way. Never the less, this article follows the UK accepted practice as given in BS 6290 Part 4 1997. It also discusses the International Standard IEC 60896 -

The article does not discuss “fault finding” which can be found in another article.

1.1) BS 6290 Part 4 principally considers the 3h to n x 1.80Vpc at 20°C rate as the preferred discharge rate when capacity testing. IEC 60896 suggests 3h to n x 1.70Vpc at 20°C or 25°C. Within the BS additional discharge rates are listed for discharge tests between 1h and 10h. Also listed are 4 discharge rates of 1m, 5m, 15m, and 30m. There is a recommended final voltage for all discharge rates.

1.2) Frequently, the user will want to test at a discharge rate other than those recommended in the BS or IEC and this can normally be accommodated. For example, the 10m is frequently specified for UPS batteries to a final voltage of 1.70Vpc. Any test can be carried out but it is important to agree these before commencement and agree the pass / fail points.

2.1) Before any test is carried out, the battery must be commission charged in accordance with the manufacturers procedures. For VRLA AGM batteries, this can normally be achieved by applying a voltage to the battery terminals averaging either typically 2.40Vpc for 48 hours or at a voltage of typically 2.27Vpc for 6 days. Other voltages and times may be used but the manufacturer must approve any method. For VRLA GEL product, the voltage may be increased to typically 2.45Vpc for the 48h charge and reduced to typically 2.25Vpc for the 6-day charge.

2.2) AGM 2.40Vpc and 2.45Vpc GEL for 48 Hours Method.

For all commissioning charge procedures, the battery must be located in a well ventilated environment at an average voltage of 2.40Vpc / 2.45Vpc with a minimum current equivalent to 5% of the A·h capacity expressed as C3 A and a maximum current of 10% C3 A·h capacity expressed as A. The battery and battery room must be at a temperature of between +10°C and +35°C. If the battery and room are outside the range 15ºC and 25ºC the charging voltage should be adjusted to compensate. For the correct applied voltage the manufacturer must be consulted.

2.3) AGM 2.27Vpc and 2.25Vpc GEL for 6 Days Method.

Charge the battery in a well-ventilated environment at a voltage recommended by the manufacturer. This voltage is normally the voltage at which the battery will be charged at when in normal service. As with the 2.40Vpc / 2.45Vpc for 48h method, the current available must be not less than 5% of the C3 A. However, the maximum current is often not limited and frequently, 100% C3 A is acceptable. The battery and battery room must be at a temperature of between +15°C and +25°C. If outside this range, the manufacturer must be consulted to establish the correct charging voltage.

2.4) At the end of the 48 hours, 6 days or other approved method, measure and record the typical cell or monoblocs temperature, ambient temperature, charge current total battery voltage and individual unit voltages.

2.5) If any individual unit voltage differs from the average significantly, the battery manufacturer should be consulted before the test carrying out the discharge test. For guidance, the following values may apply as limits.

For 2V cells: maximum difference from the average: 0.10V For 4V units: maximum difference from the average: 0.14V For 6V units: maximum difference from the average: 0.22V For 12V units: maximum difference from the average: 0.44V For 16V units: maximum difference from the average: 0.55V

2.6) No matter which charging method is used, the manufacturer must approve it and a record of the charge must be made.

3.1) Typically and to comply with the requirements if BS 6290 Part 4, the battery must be held on open circuit for not less than 1h and not more than 24 hours before commencing the discharge test. However, in a real live situation when the mains power fails, the battery will go from float to discharge immediately. Therefore, many users stipulate that the test shall be performed “direct from float”. It is rare that this procedure is a handicap to the battery and in most cases; it is to the batteries advantage.

3.2) Before starting the test, record the surface temperature of several units. This temperature shall be used in the final analysis for the temperature corrected discharge time. This is the only temperature that is used to “temperature compensate” the discharge time. For vented products the temperature is recorded at intervals and the average is used to compensate the discharge time. Also measure and record the ambient temperature near the battery. The unit and ambient temperature must be within ±5ºC. If they are not, do not start the test and contact the Company for advice.

3.2.1 The battery may be discharged at any agreed discharge rate including constant current, constant power or constant resistance. In all cases, the parameter should be maintained within ±5%, for the whole of the discharge.

3.2.2 During the discharge test, measure and record at regular intervals, the parameters below. Typically, at least 10 measurements should be taken at regular intervals.

3.2.3 The individual unit voltages, including one inter unit connector. 3.2.4 The overall battery voltage, at the battery terminals. 3.2.5 The discharge current or power if the discharge is at constant power.

3.3 When the overall battery voltage, when measured at the battery terminals, falls to the minimum accepted, record the time, and all individual unit voltages. Stop the discharge test.

3.4 The battery should be recharged as soon as possible. For high current discharged such as 5-minute constant power, the battery may be elevated above the ambient by several degrees. In spite of this, the battery may be recharged immediately. If the recharge cannot be started immediately, the manufacturer usually accepts a stand period of up to 12 hours. However, this should be confirmed.

3.5 If during the discharge test, the voltage of any individual 2V cell falls to below 0.5V, it is recommended that the test be stopped. Similarly for individual voltage of 6V units falling below 3V, 12V units falling below 6V, and 16V units falling below 8V, stop the test.

4.1 The test shall be deemed passed providing the discharge time corrected for temperature meets the requirement

4.2 The battery manufacturer should be consulted for the actual temperature correction values for the type of battery and discharge that it has been tested at.

4.3 The British Standard allows for individual cell or monobloc voltages to be below the average but the battery, as a complete system must meet the requirements. For example, BS 6290 Part 4 states that as a complete battery the terminal voltage shall be not less than n x 1.80Vpc at the end of a standard 3h test and all cells shall be not less than 1.75Vpc after 2h 30m. It will be apparent that if an individual cell has marginally exceed the 1.75Vpc at 2h 30m then there is still a likelihood that the battery as a complete system will fail or the individual cell voltage will be lower than the suggested 0.5V (see the last paragraph in section 3.4 above. This principal may be extended to cover different discharge rates. It is suggested that a minimum voltage of 0.05Vpc below the minimum average accepted as a pass at the end of discharge time at 83% of the discharge time. For example, if the discharge test has to run for 10m with a minimum voltage of 1.65Vpc, then every cell shall have a voltage of not less than 1.60Vpc after 8m 20s.

4.4 The graph below illustrates a battery tested at the 10m rate where the total battery voltage exceeded the requirement but one cell collapsed after about 4m. This battery test should be regarded as failed. In these circumstances, the faulty cell should be replaced. Although the customer may insist on a complete battery re-test it is often acceptable to test a single cell separately and insert it into the battery system. This avoids a complete battery re-test. This battery system consisted of 333 single cells of 950Ah each. In a subsequent teardown analysis of the battery the fault was discovered as an internal short caused by a bent end negative plate shorting to an adjacent positive plate

The open circuit voltage of lead acid cells can give us useful information providing it is used correctly.

This article shows the relationship between the open circuit voltage and state of charge of lead-acid cells and how to interpret the values. The values are always considered as that of a single cell and monobloc voltages must be averaged according to the number of cells. This method was developed primarily for VRLA cells where the specific gravity cannot be measured. Measurement of the specific gravity will give a good indication of the state of charge. For measurement of either specific gravity or open circuit voltage, the fully charged and fully discharged specific gravity must be known. Used correctly, the method described in this article will give reasonable results to the state of charge of cells on open circuit.

Some specific rules apply when using this method to determine the state of charge as follows: -

a) The cell or monobloc must have been on open circuit for at least 12 hours and preferably more than 24 hours.

b) The cell or monobloc temperature should be between +10°C and +30°C for best accuracy but any cell or monobloc temperature between 0°C and +40°C will give reasonable results.

c) The measuring instrument must be accurate to within ±1% and capable or recording voltages from 0.5V to 2.2V to at least 2 decimal places.

The open circuit voltage of vented cells such as Planté, tubular and flat plate pasted types can be used in a similar way to the method used for VRLA AGM and VRLA GEL cells. However, when the electrolyte specific gravity can be measured, this will give a more accurate result. Never the less, open circuit voltage measurement can provide sufficient accuracy in most cases and the ease of measurement means the process is quick. A graph below gives the typical parameters. Similar graphs for VRLA AGM an VRLA GEL products may also be found below.

It can be seen that the relationship between the open circuit voltage and state of charge follows a straight line. This, in the most port is correct within the limits required for lead-acid batteries. The mathematical equation is: -

Open Circuit Voltage = Specific Gravity + 0.84.

Example: OCV = 1.15s.g. + 0.84 constant = 1.99V. Note: this is close to 2V which is why the measuring instrument needs to be reasonably accurate.

If we know the fully charged and fully discharged specific gravity, we can draw the straight line graph.

Typically for VRLA AGM cells, the fully charged specific gravity is 1.310s.g. at 20°C and the fully discharged value is 1.10s.g. These reflect an open circuit voltage of typically 2.15V when fully charge and 1.94V when fully discharged. It should be noted that even when the cell is fully discharged the voltage will be close to 2V again illustrating that the measurement accuracy is important. When it comes to monobloc voltages, the total voltage needs to be divided by the number of cells to find an average.

We can make use of this characteristic in several ways but the most practical is to establish the state of charge of cells that have been in storage for some time. Typically, for VRLA AGM cells, manufacturers will recommend a freshening charge after 6 months in storage. This will be shorter for higher temperatures and longer for lower temperatures. For example, 6 months at 20°C will typically become 3 months when the storage temperature is 30°C. Although the storage time may be extended if the temperature is lower, this is not recommended beyond the manufacturer’s recommendations because irreversible sulphate may result and special charging techniques may be required to fully recover the product. This technique of measuring the open circuit voltage can be used to identify low voltage cells prior to installation. For monoblocs this is more difficult but can be used to identify low cells within a monobloc that may be the result of a soft short resulting from a manufacturing fault.

For example if a battery contains 55 VRLA AGM cells it would be expected that the voltages will all be reasonably the same such as within 0.02V of each other; e.g. 2.10Vpc to 2.12Vpc. If any cell is outside the 0.02V limit, this should be questioned. Where soft shorts are concerned, the voltage differential is likely to be more like 0.05V for cells that have been on open circuit for some time.

Although the principal can be used for multi cell monoblocs, it is not as easy to identify rouge product. Even soft shorts which may result in one cell having a voltage 0.05V lower than the average are not very easy to identify. For example, if we consider a 12V 6 cell monobloc having a voltage of 12.70V this could be made up of 6 cells each 2.117V or 5 cells each 2.12V and one cell of 2.10V (total 2.12V x 5 = 10.6V + 2.10V = 12.7V). Under normal circumstances this would not be rejected and the rouge bloc may be missed.

For monoblocs, the voltage deviation from bloc to be needs to be wider otherwise good product may be rejected. For 6V 3 cell blocs a limit of 0.1V of each other is reasonable to question the unit. For 12V 6 cell blocs a variation of 0.15V is reasonable. These limits may seem close but in the context of monoblocs, all the cells would have been stored in exactly the same conditions and this cannot be said for individual cells. In fact, individual cells may have come from different batches and it is possible that a group of cells may be different from another group of cells. The technique must be used with some knowledge and expertise and if in doubt, it would always be wise to contact the supplier or manufacturer.

Looking in isolation at bloc voltages may not show up any rouge units but if a battery comprised 33 blocs and 32 are 12.70V and one bloc is 12.60V, a question has to be raised. The installation process could be completed but a note should be made and if the bloc voltage does not recover on float charge after 6 months and perhaps deteriorates or the differential between other blocs increases, then there is a reasonable case for exchanging the rouge bloc.

A final comment about open circuit voltages is to mention that for NiCd cells you can not apply this principal because the s.g. of the electrolyte does not change with state of charge. It shall also be noted that the NiCd open circuit voltage is not stable and typically varies between 1.2V and 1.3V.

This article looks at the preferred designs for battery rooms and discusses how batteries should be laid out to give a safe environment. Alternative battery stand types are discussed to illustrate accessibility of the cells or monoblocs and safety considerations.

VRLA, Vented and Nickel Cadmium battery types are included.

Fully detailed information can be found in International Standards such as BS EN 50272-2:2001. This article gives an overview only to the more important subjects.

Battery rooms can be a hazardous place and all persons entering must be aware of the dangers. All too often, inexperienced people enter a room without receiving any safety information. Visitors who may have never been in a battery room previously are particularly vulnerable and must be give a short overview of the hazards. Topics discussed should include emergency evacuation procedure although this may be covered immediately after entering the room when emergency exit routs can be pointed out. High voltage and the need not to touch any part of the battery or stand should be explained. Battery acid and lead compounds and the risk of explosion due to the build up of explosive gasses should be discussed. The hazards with nickel cadmium batteries, which contain highly corrosive potassium hydroxide and give off hydrogen, should be discussed.

No persons should be allowed to enter a battery room without the correct clothing. Man made synthetic clothing may generate a static charge which can result in a spark which in turn may ignite the explosive gasses given off by all lead-acid and nickel cadmium battery types. This is less critical when considering VRLA batteries. For people not actually working on the battery or touching it, such as visitors, every day clothing is acceptable. The owner of the battery has a duty of care towards all persons entering a battery room.

A fully documented Method Statement and Risk Assessment must be covered for any person working on any part of a battery system. Even visitors may be required to sign the documents to confirm they have understood them and will comply with a simple Method Statement, even if they are not directly involved in the work. Lone working is possible when working in a battery room providing the activity is clearly understood and emergency procedures are in place.

As a final overview, all doors to the battery room must be anti-panic and open outwards.

Because battery rooms are a hazardous place, appropriate signage must be applied to the door. Doors should be locked to ensure only authorised persons can enter. A list of typical signs for lead acid batteries is given below. These signs are self-explanatory.

The examples given below are not exhaustive but do give the reader an appreciation of the hazards that may be encountered within a battery room. Where nickel cadmium batteries are installed, the appropriate warning for the potassium hydroxide, nickel and cadmium should be posted.

Batteries may be mounted on racks or in cabinets. When installed on racks, these may be of wood or steel and both may be insulated from earth. Generally speaking, battery stands are not earthed but isolated from earth. Some steel stands are earthed and consideration should be given to the method of earthling each component, such as rails and frames, to the next component as well as to “ground”. While steel stands are normally isolated from ground, steel enclosures are normally earthed. There is no Standard that covers earthling and the decision is left to the user.

Induced voltages may be generated into an insulated stand by ripple current in the battery circuit. Caution has to be applied before touching any metal stand rail or frame. Older UPS systems where notorious for inducing a high voltage and while modern systems generally have very low ripple current, caution should also be applied until it is established that no high induced voltage exists and there is no risk.

These should be designed and installed to provide good access. Multi-tier steel stands are very popular for UPS batteries. These may be up to 6 tiers high and measuring over 2m. Not very popular but still evident is some applications are very high stands with vented lead-acid cells or vented nickel-cadmium cells. Working at height on these batteries is not easy and special safety measures must be used. Very occasionally, even higher stands may be used. High stands present a high risk factor. Working at height has its own hazards and when working on live batteries extreme care must be taken. It is worth remembering that a battery cannot be isolated in the conventional way and will always be live, even when fully discharged. It only takes a few milliamperes to kill a person.

Stands that are two rows deep are generally easy to service but those of three or four or more rows deep may be difficult to service. Stretching over the front row(s) can result in electrocution or shorting of cells or monoblocs unless they are fully insulated. Insulating covers may be accidentally knocked off if care is not taken. A better design would be one having a total height where cells can be serviced without steps or ladders and where the depth of the stand is restricted so that the cells at the back of the stand may be reached easily. Tier pitching should also be considered at the design stage. Again, good spacing allows easy access and makes servicing easier.

Batteries housed in enclosures are notorious for having poor access. The writer has seen examples of enclosures, which are over 1m deep with less that 50mm between the top of cells and the underside of the shelf above. When the manufacturer was asked how the cells were installed, it was admitted that they were built up from the bottom and shelves added as required. This was a special design where this was the only way the required power could be provided in the volume available. Servicing the battery was impossible. Even long probes could not reach the terminals of the back monoblocs.

To cater for emergency exit situations the distance between stands or enclosures should be considered. It is reasonable to consider a “gang way” of not less than 600mm. However, if the “gang way” is partially closed such as during servicing, a larger gang way should be considered. It is not acceptable to use 600mm “gang-ways” and then obstruct them at any time.

Below are two examples of stand arrangement for 55 High Performance Planté cells. One offers good access whilst the other is much more compact making servicing difficult.

Typical 2 tier x 2-row stand for 55 x Planté cells – This is a good design giving good access.

Typical 4 tier x 3-row stand for 55 x Planté cells – This has a smaller footprint but is much higher, has less tier spacing and has restricted access for servicing the cells.

Ventilation recommendations are given in National and International Standards and this section is included to give the reader an appreciation of the risks involved when entering or working in a battery room.

Ventilation must be provided and be adequate to remove hydrogen from the room to a concentration of less than 4% which is the safe lower limit to prevent an explosion. It is preferable to use natural ventilation because forced ventilation systems can fail.

Many battery rooms do not have adequate ventilation and it is particularly important that when entering any battery room a Risk Assessment is carried out. It may be prudent to open battery room doors and allow any gasses to disperse before entering.

When batteries have been on boost charge such as constant current for vented cells, it is fundamentally important to consider a period of time for gasses to disperse.

Many users use hydrogen detection equipment to measure the concentration and ensure it is at a safe level before entering. The detection equipment may then be kept in the room whilst the technicians are working. The types that will give an audible alarm when the hydrogen level reaches a dangerous level are preferred. The alarm level should be set considerably higher than 4% and a normal suggested value is 10%.

Having discussed ventilation to remove explosive gasses, we need to consider heat generated by the battery both on charge and off charge. Looking specifically at cell or monobloc spacing to allow a flow of air between the units, it is reasonable to allow a minimum gap between VRLA cells and monoblocs of 5mm. It would also be prudent to increase this distance where the battery is housed in an enclosure. Batteries in enclosures are best mounted on rails rather than a solid shelf. Good designs use perforated shelves and increase the spacing to 15mm between cells or monoblocs. It is not unknown for batteries to go into thermal runaway for no other reason that the spacing between cells or monoblocs has been insufficient to cater for the natural heat generated whilst on float charge. Vented cells are less prone to these thermal problems but spacing for these should also be considered at the design stage.

Several factors need to be considered when designing a battery room floor. For VRLA batteries the simplest of protection is normally acceptable but rooms housing vented battery types need to be impermeable for battery acid or alkaline for nickel cadmium types. An alternative to having a complete floor treated is a bunded floor lip, which prevents any spillage from spreading beyond the immediate vicinity of the battery. As a further alternative, drip trays may be used.

For any battery type, the floor must be capable of withstanding the point loading of the stands. Good battery stand manufacturers are capable of providing the point loading details and advising on designs suitable for spreading the load. Whilst point-loading issues can normally be overcome by load spreading plates, in some instances the floor may not be capable of withstanding the battery weight as a whole. If this is suspected, a structural survey will need to be carried out.

For vented batteries, room lighting should be intrinsically safe to avoid explosions caused by sparks igniting hydrogen gas. For VRLA batteries normal lighting may be used. This does assume that the correct battery charging characteristics are used and correct ventilation has been provided.

Lighting should be sufficient for maintenance technicians to be able to see the complete battery without difficulty. With some enclosure design with restricted access almost any type of fixed lighting will not provide the level of illumination required for the service technician. Hand held torches are fine providing they are fully insulated and for vented battery types the unit must be intrinsically

It is easy to assume that battery rooms need not be anything special. However, this article, which is only intended to give the reader “food for thought”, illustrates the need to be cautious when designing a battery room, stand or enclosure. Reference to National and International standards must be made for more detailed information on this subject.

This article discusses the effects of storing VRLA AGM & GEL types and also vented Planté, Pasted Plate and Tubular types.

The article covers the effects of time, temperature, humidity and light and also discusses sulphation; lead dendrite growth and corrosion of the internal lead parts as well as corrosion to the pillars and UV aging of the plastic containers. The relationship between open circuit voltage and state of charge is also discussed.

The article is intended to give the reader a better understanding to the reasons why battery manufacturers make specific recommendations for storing batteries which should not be ignored. It is not intended to be a definitive article and the battery manufacturer’s instructions should be followed at all times.

Battery manufacturers quote very different maximum storage times for their batteries and this can be confusing for the installer and user. One manufacturer may quote 3 months whilst another may quote 2 years. In the first instance we need to look at the storage temperature because the discrepancy in time may be explained by different temperatures. It is also important to confirm if the storage time is from the date of manufacture or first charge which is normally printed on the product, or from the date of dispatch from the manufacturing plant. Never the less, the storage time for “like to like” circumstances may be significantly different depending on many factors including design, materials and manufacturing differences.

Product quality in terms of material purity affects the storage time as does the minimum state of charge that the manufacturer is happy that the product may be successfully commission charged and what the commissioning charge recommendations are.

It is true to say that the longer the product has been stored; the more attention to commission charging is required.

For product supplied filled and nominally charged, the product will “self discharge” and lead sulphate will form on both positive and negative plates; and the specific gravity of the electrolyte will fall. The longer the storage time and the more difficult it is to convert this lead sulphate back to active materials. Where the product has been stored for a long period of time, some lead sulphate may never be re-converted and a permanent loss of capacity will result.   Product that is supplied “dry charged” is more easily dealt with but this condition is reserved for vented Planté, vented Pasted Plate and vented Tubular types. Although the “dry charge” will be lost over the storage time, the condition can be recovered because the electrolyte is added after the storage time and only a limited amount of sulphate can be formed during storage. However, most manufacturers stipulate a storage time based on an 80% state of charge being achieved after filling with dilute sulphuric acid. A long storage time may result in no real capacity being seen after filling which can normally be recovered providing the correct charging regime is followed. This charging regime may be different than that required for a storage time within the manufacturer’s limits.

Fig 1 below shows the typical loss of capacity with time. This is an example only and should not be used to calculate the state of charge for any specific battery type. It can be seen that the capacity loss is more pronounced in the first months of storage before a more gradual fall off occurs.

The correct storage temperature is arguably the most important aspect to consider. The higher the temperature and the quicker the capacity will fall and the shorter the recommended storage time.

Typically, manufacturers consider a storage temperature of 20ºC when quoting storage times. It is unwise to store a battery at above 40ºC for two main reasons: a) the storage time will be very short and b) at this temperature aging of the plastic components is accelerated. Whilst low temperatures will reduce the open circuit losses and give an extended storage time, it is normally recommended not to go below +5ºC. However, storage temperatures as low as -40ºC, and even below this for some types, are possible without problems providing the product is allowed to “thaw out” over several days until a more reasonable temperature above +5ºC is achieved before charging the battery. Very low temperatures will make plastic containers more brittle and special conditions may apply if the product is transported at these extreme temperatures.

In Fig 2 below the storage time for different storage temperatures can be found. It can be seen that if the storage temperature is 20ºC, the storage time is shown as 100%. If the storage temperature is 30ºC, the graph shows 50% and if the manufacturer quotes a storage time of 18 months at 20°C the storage time at 30ºC becomes 9 months.. Similarly, if the storage temperature is 10ºC, the storage time will be 200%, i.e. 36 months. This graph is in the most part correct but it would be unwise to consider a storage time of 400% of the nominal for the reasons illustrated in the above section, “EFFECTS OF TIME”. Similarly, the maximum storage temperature should be considered as 40 ºC for the reasons given in this section above.

Storage humidity can have detrimental effects on the product. Terminal pillars are typically manufactured from lead alloys or brass and both will oxidize over time but this is accelerated in high humid environments. This corrosion should be removed prior to making the electrical connections. Failure to do this may lead to a high resistance joint and could, in extreme circumstances, lead to dramatic failures. The author of this document has firsthand experience of a battery fire developing as the direct result of severely oxidized terminals of a Planté battery where the terminals were not cleaned prior to installation. The storage time was less than 4 months but the humidity was extremely high.

Internal corrosion is completely unseen in VRLA product but can be considerable. In vented product assembled into transparent containers, “flaking” of the positive group bars and pillars inside the cells can become extensive. This may be severe enough to cause early battery failure. The “flakes” are not normally seen in storage but develop rapidly during the commissioning charge process. Typically, this is not noticed until the battery is several years old when failure due to internal shorts develops. To the experienced battery technician this type of failure can be clearly identified as a storage issue and not one of manufacture, over charging or undercharging. The structure of this corrosion is quite unique depending on how it was formed.

Corrosion to the plates is more subtle. Corrosion to the “weld” area between plates and group bars and group bars to pillars are the first to be affected. In some grid designs, the nodes between horizontal and vertical wires are affected. It is stressed that this corrosion is extremely rare and is insignificant if the product is stored correctly.

When batteries are discharged the lead oxide (PbO2) of the positive plates and lead (Pb) of the negative plates are converted to lead sulphate (PbSO4). The extent that this occurs depends on the depth of discharge. The more rapid the discharge and the faster sulphate will be formed. When batteries are stored the self discharge is low but may be significant over time and this will be higher at elevated temperatures. Ultimately, lead sulphate will be formed and if the state of charge is very low, the specific gravity of the electrolyte will also be low and may be as low as 1.050 s.g. Under these circumstances, the lead sulphate (PbSO4) will react with the electrolyte (H2SO4) and some lead may go into solution. When the battery is eventually charged following storage, lead dendrites may be formed resulting in shorts between positive and negative plates.

Lead dendrites are rare because manufacturers often include chemicals to reduce their formation. Also, these lead dendrites only occur in VRLA AGM cells. Lead dendrites are not seen in vented products and VRLA GEL cells.

Battery containers can be affected by UV light. The effect is commonly known as UV degradation and sunlight contains a significant amount of UV light. The result of UV light can often be seen as cracking of the containers. Occasionally crazing of the container corners can be seen and a loss of colour definition can occur. Significantly many manufacturers include additive in the manufacture of containers to make them UV stable. However, this is very difficult where transparent containers are used such as with Planté and other vented products where clear containers are used.

It should be remembered that UV will affect the product once installed and direct sunlight on the battery should be eliminated.

Within reasonable limits, there is a direct relationship between the open circuit voltage of a lead acid cell and the specific gravity of the electrolyte. For this relationship to be accurate, the cell must have been on open circuit for at least 12 hours and preferably 24 hours. It therefore follows that it will be reasonably accurate for product that has been in store for several days or more. The temperature limits for this technique are reasonably wide but for best accuracy this should be taken as +10°C to +30°C.

If we know the fully charged and fully discharged specific gravity of the cell then we can calculate the state of charge. This is illustrated below in Fig 3 for a typical VRLA AGM product.

The mathematical relationship is: - OCV – 0.84 = the s.g. of the electrolyte.

Example 1: OCV = 2.15V S.G. = 2.15 – 0.84 = 1.31 s.g..

Example 2: OCV = 1.95V S.G. = 1.95 – 0.84 = 1.11 s.g.

Note: Even at an open circuit of 2.00V, the cell will only be about 25% charged or, more to the point, 75% discharged.

A typical average rate of losses is 2% per month which equates to 3 years for a 75% discharged cell. Recovery from this very low state of charge is unlikely without a considerable capacity loss. A reasonable minimum open circuit voltage is typically about 2.10V for VRLA AGM cells. The product should be recharged if this voltage is reached because anything lower than this is likely to result in a permanent capacity loss. The 2.10Vpc represents about 75% charged.

The mathematical formula is relevant to all lead acid cells, not only VRLA types. However, the fully charged and fully discharged s.g. will be different for different battery types; particularly vented types compared with VRLA.

The fully charged specific gravity of Planté cells is typically in the order of 1.215, for Vented Pasted Plate types a value of about 1.250 is typical and for Vented Tubular Plate about 1.280. For vented cells it is more accurate to use a hydrometer or density meter to measure the specific gravity and from this the state of charge can be determined.

To make best use of this technique it is important that the fully charged and fully discharged specific gravity is known and at what reference temperature. For VRLA AGM cells where the voltage is measured and cross referenced to the state of charge the technique is reasonably accurate. This is because there is a large difference between the fully charged and fully discharged s.g.; typically 1.310 when fully charged to 1.110 when fully discharged.

The storage conditions can interact and cause serious problems. In particular, whilst the store time may be extended at lower temperatures, this time frame must not be extended beyond the recommended limit as advised by the manufacturer.

Where high storage temperatures such as above 30°C are experienced, consideration to the humidity is very important. High temperatures with high humidity will cause rapid corrosion to terminals and if this is not addressed, high resistance electrical connections may result.

In conclusion, always follow the manufactures instructions.

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.

A frequently asked question is, should I use a GEL or AGM battery?

To answer this question we need to understand the differences between VRLA GEL and VRLA AGM and we must understand what a VRLA battery is. Simply, a VRLA battery is one which would not normally be topped up with water at periodic intervals which is what has to be carried out on a “vented” battery. Because of this characteristic they have often been miss-quoted as sealed. They are not sealed but have a one way valve to release excessive internal pressure. The design of the valve varies but they all have a pre-set value and when this is reached releases and maintains the pressure within design limits. This pressure is usually low and in the order of 0.5 BAR. Both AGM and GEL are VRLA batteries and have this valve arrangement.

VRLA batteries, both AGM and GEL, are often referred to as Gas Recombination Batteries because they do not need topping up and a large percentage of the gas generated on overcharge is recombined into water and in this respect they are maintenance. However, it must be remembered that when being overcharged both types will give off explosive hydrogen and oxygen and they must never be installed in sealed enclosures or where ventilation is restricted.

Looking at the different characteristics between AGM and GEL batteries shows that they both have some advantages over the other and the choice is not easy. Looking at the designs, in an AGM battery the electrolyte is held in an absorbent glass mat which also acts as a separator between the positive and negative plates. In a GEL battery the electrolyte is “immobilised” in a GEL and a separate separator similar to that used in “vented” batteries has to be fitted. Both types may be considered as “spill proof” but the manufacturer should be consulted if they are to be operated in anything other than a conventional orientation.

Most AGM batteries use high quality very absorbent glass matt separators which in many respects look feels and have the same “mopping up” characteristics as blotting paper. However, it is very different in characteristics when used in a battery. In a GEL battery the electrolyte is “immobilised” by mixing the filling acid with fine silica which creates a gel. In the first instance this looks like thick milk or “runny” cream. It is thixotropic and has to be continually stirred to stay as a liquid, once inside the battery it “sets” into a “wet” GEL format.

In an AGM battery, typically 10% of the electrolyte is in the plates and 90% in the separators. The GEL battery has a greater volume in the plates because they are usually thicker and also have a larger volume of electrolyte in the gel. Consequently, the GEL battery can have twice the volume of electrolyte than the AGM type. This higher quantity of electrolyte in the GEL battery can help the characteristics in many ways but also has drawbacks. The electrolyte specific gravity (s.g.) in the AGM battery is generally higher at typically 1.310s.g. at 20ºC compared with 1.250s.g. for GEL. The higher s.g. results in a higher initial on load voltage for the AGM battery. Because the GEL battery has a greater electrolyte volume it has a better voltage profile towards the end of the discharge and it does not drop sharply as with the AGM battery. The AGM battery has often been describes as “acid limited”. Again we have to put this in context because the fall off in voltage as the result of acid depletion is only relevant for long duration discharges and is rarely noticed for discharges shorter than 1 hour.   The construction of AGM batteries with its thinner plates and no requirement for a conventional type of separator means that a much more compact design can be achieved. The space between the plates (plate pitch) is much smaller in AGM batteries which reduce the internal impedance resulting in a higher on load voltage for the same current drawn. Overall, this improves the power to volume and weight ratio and gives considerably better high current characteristics. AGM batteries are usually smaller in volume for the same ampere-hour performance.

GEL batteries are generally more tolerant to the influences of the surrounding ambient temperature. Experiments have shown that the recombination reaction generates internal heat and this is more easily dissipated into the surrounding air with GEL batteries. It is argued that this is because of the greater volume of electrolyte in the GEL battery and because this is in contact with 5 sides of the “cube” design they have a better heat dissipation. For the AGM battery, frequently none of the electrolyte is in contact with any of the sides which then forms a “double glazing” effect and retains the heat. Excessive heat within batteries can cause thermal runaway and although this is not a problem with either type when installed correctly, in very high ambient conditions, typically over 40ºC, the GEL battery is better at coping with this abuse condition.

Some GEL batteries have tubular positive plates which makes them better for cycling applications. The AGM design will not work with tubular plates, therefore thicker more durable flat plate are used by some manufacturers which come some way to matching the tubular plate GEL battery characteristics. Tubular plate or thick flat plate GEL designs do not have the high rate performance of the thinner plate AGM battery. Plates having a thickness of less than 1mm are now available from some AGM battery manufacturers and with modern lead alloys for the plates, corrosion is low and the life is long for float charge standby applications. In some designs it has been shown that the life of thin plate AGM batteries is as good as or even better than tubular plate or thick flat plate designs in standby applications.

It has often been stated that VRLA batteries, both GEL and AGM, do not cycle well. This is not true. It has been shown that VRLA AGM batteries of thin plate designs can easily achieve over 750 deep cycles when operated with the correct charging equipment. Some GEL batteries have achieved twice this for true cycling applications with sophisticated chargers which reduce overcharge to a minimum.

In the world of float charge standby batteries, the charging source is usually a constant voltage type. This will give a constant voltage at the charger design maximum current up to a preset voltage which is usually in the order of 2.23Vpc to 2.28Vpc, depending on the VRLA battery design. These chargers are very efficient in providing power to the battery following a discharge but will inevitably result in taking several days before the battery is truly fully charged. A “nominal” 100% recharge can usually be achieved within 72h even following a 100% depth of discharge. All too often, cycling applications require the battery to be recharged in a short time such as 8 or 12 hours and this is insufficient time when using a standby system battery charger. For this type of industrial application it is essential to oversize the battery by at least 20% and to have a higher recharge voltage available. It is also desirable to have a higher recharge current. Typically after adding 20% to the required capacity to carry out the discharge, a voltage of typically 2.35Vpc and a current of 10% C10 amperes is desirable. Some charger designs have multiple voltage steps and consider battery temperature to achieve the shortest recharge time with minimal overcharge.

'Off grid' applications using solar and / or wing generators often have a generator back up to cater for the inevitable shortcoming of the “renewable” charging source. The GEL battery is generally the better choice for this application. The user should always seek professional advice from the battery manufacturer. If the design is flawed, the battery will fail prematurely.

For mobility scooter and golf cart applications the first choice is usually a GEL battery because it offers a better whole life cost. Battery failure due to abuse is frequently seen for these applications. The battery is often left it in a discharged condition for many days or even weeks. The GEL battery is much more tolerant to this abuse but it is not immune. Users should be aware of this because there is no point in buying a higher cost GEL battery and then destroying it by abusing it. For this application, the scooter or golf cart will have its own charger and it is essential to use this if a satisfactory life is to be expected. Interestingly, lithium batteries are now finding their way into this application but the cost is high. When we understand the characteristics of AGM and GEL, we are in a better position to make our choice for the application. However, we often find that the initial cost is a major factor and this will make the choice simple. If initial cost is not a deciding factor then whole life costs may make the choice easier. The lowest initial cost very often turns out to be the highest cost over even a 5 years life. Space considerations will also influence the type purchased and principally, the AGM battery will be a better choice where this is important. Where high power outputs are required from the battery such as 15 minute UPS applications, the AGM battery is the first choice. They offer very good power to volume and power to weigh ratios and they offer a lower cost both initially and as a total life cost. There are very few UPS applications having a required run time of less than 1 hour where the GEL battery has been chosen.

For telecommunication applications, the choice is much harder to make. Small batteries are generally AGM but in high ambient conditions the GEL battery has advantages. The temperature in street side cabinet can be alarmingly high in the summer months and the GEL battery is generally better suited to this application. Where batteries are in buildings with good temperature control, the AGM is usually installed. There are large ampere-hour batteries in both GEL and AGM design and the costs are often very similar. The discharge voltage profile can favour the AGM battery in some cases but the more stable profile of the GEL battery can also be of benefit. Each application should be considered on its own requirements to ensure the correct choice is made.

Many factors will influence the choice between GEL and AGM and sometimes no rational logic is used in the decision.