4. Batteries

4. BATTERIES 4.1 General The storage battery is the basis of most UPS supplies. It is essential that it can be relied upon to provide the required power for a specified period and within the specified voltage limits. The choice of battery type is therefore very important. Many types of batteries exist, however three types in particular are best suited for instrument power supply purposes. These are the lead-acid battery, which comes in two main types - vented and valve-regulated (VRLA), and the Nickel-Cadmium type. In this handout these are referred to in short as Pb (v), VRLA and NiCd batteries respectively. The choice between these three types is largely dependent upon the application involved, and it is therefore necessary to discuss the characteristics of each in some detail. Initially, however, it is worth discussing the basic aspects of battery capacity, which applies to all three types of batteries, in order to facilitate the detailed discussion on each type.

4.2 Battery Capacity For most purposes a battery is a series connection of a group of cells, the precise number of which depends on the particular voltage requirements. The capacity of the battery is determined by the capacity of the individual series-connected cells. The capacity of a storage battery is normally expressed in Ampere-Hours (Ah). It is a direct measure of the electricity that the battery is able to deliver, i.e. it is the product of the current in amperes and the period for which the current can be supplied. However, when specifying a battery capacity it is necessary to take several factors into consideration: - The rate at which a battery is discharged can affect its capacity, the higher the discharge rate, the lower the capacity, e.g. if a Pb(v) battery is quoted as having a capacity of 100 Ah at the 10-hour discharge rate. This means that it can be discharged at a constant 10 A rate for 10 hours while maintaining the load voltage above a certain value. If, however, the same battery is discharged within a period of 1 hour it will only have a capacity of about 50 Ah i.e. a constant current of 50 A for 1 hour. - The voltage output of a battery on load reduces as it is discharged. In most applications certain permissible voltage limits are applied to a system and, to avoid falling below the lower limit, the capacity of the battery must be specified such that it can deliver sufficient current while maintaining the voltage limits. It is therefore necessary to consider the 'end voltage' when specifying a battery capacity, particularly because the required value as dictated by the load requirements may not be the same as the end voltage used by the manufacturer when quoting the capacity of a cell. - The capacity of a cell also varies with the temperature of the battery, such that when specifying a battery capacity it is essential to quote the minimum temperature at which the battery will be expected to supply the required capacity, e.g. a Pb(v) battery with 100 Ah capacity at 15°C might have a capacity of about 95 Ah at 10°C. The required capacity of a battery can be calculated by determining the load, which the battery will be expected to supply, the period for which the supply is required, and the voltage limits of the system.

4.3 Lead Acid (Vented) Batteries The constituents of a lead acid storage cell are two dissimilar electrodes immersed in an electrolyte. The physical construction of the electrodes, or plates, differs from one manufacturer to another but the materials used are the same: Positive active material - lead dioxide (PbO2) Negative active material - spongy lead (Pb) Electrolyte - dilute solution of sulphuric acid (H2SO4) in water

During discharge the electrolyte reacts with the plates, such that, when fully discharged, both positive and negative plates are coated in lead sulphates and the electrolyte is very much diluted with water. Pb + PbO2 + 2 H2SO4 discharge charge 2 PbSO+H20 During charging this chemical process is reversed, with oxygen and hydrogen being evolved and the original compositions of the electrodes and electrolyte being regained. Consequently, the amount of acid in the electrolyte is a measure of the state of charge of the cell. The concentration of the electrolyte is determined by the specific gravity of the electrolyte, hence a measure of the specific gravity will give a direct indication of the state of charge. The majority of Pb(v) batteries used in UPS applications are of the "Plante" positive plate type. In this type of cell construction the negative plate is normally a "pasted" plate in which the active material is applied to the plate structure in a paste form. The positive plate initially consists of pure lead, as opposed to a paste, normally formed by numerous thin vertical laminations, strengthened by a series of horizontal cross-ribs. The laminations effectively increase the surface area by up to 12 times that of a plain lead plate of similar dimensions. Plante cells have a life expectancy of approximately 25 years. Other cell constructions of the vented lead acid type include Tubular plate and Flat plate. Pb(v) batteries are generally enclosed in a transparent, non-flame propagating plastic container. Vent plugs are installed on the battery containers to permit easy access for topping up, for ventilation, and to permit removal of the gases produced during charging. The plugs are normally of the labyrinth type and also designed to prevent acid from being splashed out of the cells during transport. Other special types of vent plugs are available examples of which are: - a "gas dryer plug" which retains a large amount of the acid gas escaping from the electrolyte by means of a special granulate filling. - a "flame arresting plug" which in case of ignition of gases escaping from the cell, inhibits the flame from travelling inside the cell. - a “ceramic funnel plug” which fulfils the same function as the flame retardant plug but in addition it allows testing and refilling with distilled water by means of a tube, without having to remove the plug. The main disadvantage of Pb(v) cells is their susceptibility to corrosion of the positive plates, collector bars, or terminal pillar. Especially pillar corrosion can be a cause of concern if the pillar seal is not adequately designed to prevent escape of electrolyte to the pillar and causing it to corrode. The pillar corrosion usually takes place underneath the battery lid where the volume of the lead sulphate, produced by this corrosion, eventually leads to cracking of the battery container. A further disadvantage being their susceptibility to damage, or greatly reduced lifetime, caused by deep discharging of the battery and AC ripple. Damage will arise if the cells continue to be discharged below cell voltage of 1.6 V per cell. This may result in cell voltage reversal from which the cell cannot recover. End of battery life is signified by a loss of capacity, which is likely to occur, by structural failure of

internal plates, connectors, the cracking of battery container due to pillar corrosion, negative plate sulphation or the growth of dendrites due to AC ripple causing an internal cell short circuit.

4.4 Lead Acid (Valve Regulated) Batteries VRLA cell basic chemistry is similar to the Pb(v) cell except it utilises the principle of gas recombination. Gas recombination is defined as the process whereby, during charging, the positive plate reaches the top-of-charge before the negative plate, producing oxygen, which then diffuses rapidly to the negative plate where it recombines in a closed cycle to form water. The following sequence of reactions takes place: - H2O ----- 2H(+) + O2 + 2e - Pb + O2 ----- PbO - PbO + H2SO4 ----- PbSO4 + H2O - PbSO4 + 2H(+) + 2e ----- Pb + H2SO4 In terms of cell construction, VRLA cells rely on the use of special glass micro fibre separators to achieve efficient oxygen recombination. These materials have a large, uniform volume of fine pores and, with the separator only partially saturated with electrolyte, there is a connected path in the gas phase between the plates. This design allows the oxygen to diffuse readily from the positive to the negative plate where it is recombined. Flat pasted-plates are used for both electrodes and grid alloys with high hydrogen over-potential are used. Cell containers are built with cases and lids moulded from polymers with a high elastic modulus and high fracture toughness so that container distortion is minimal under internal pressure. The pillar seals need to be highly reliable and a non-return valve is incorporated to act as a pressure relief valve. Cell operation takes place in a pressure range from 3-5 psi, above which the relief valve is supposed to operate. The main advantages of VRLA cells, on other type of cells, are that they require less maintenance (no topping- up), no special (forced) room ventilation, and are appreciably less expensive than the others. One disadvantage of VRLA cells is that they tend to get damaged more readily by deep discharges, by overcharge and by high ambient temperatures than traditional Pb(v) battery types. This is because the cell has a limited quantity of electrolyte and when this is depleted short circuits can develop in the separator system. For that reason rapid charging for VRLA cells is not allowed and in some applications, a low voltage disconnected facility can be installed on the battery circuit so as to disconnect it from the load when discharged. The principal disadvantage of VRLA cells though is its high dependence on cell temperature and its relatively low lifetime. VLRA cells are sensitive to variations in temperature because of the cell compactness and the limited amount of electrolyte used in them. The most optimal operation temperature for VRLA is 20°C + 2°C. Outside these temperature limits the battery capacity and lifetime is reduced. Temperature dependence is also affected by the float voltage used i.e. with high cell temperatures a lower float voltage is required in order to compensate for the extra heat energy to be absorbed by the cell. High cell temperatures and high float voltages effectively reduce service lifetime but in any case the expected lifetime of a VRLA cell does not exceed 10 years which is less than half that of other `traditional battery types. Reference should be made to BS 6290 Pt 4 and VRLA cells specified as being classified as 1 H 23. This ensures longest lifetime and durability.

4.5 Nickel-Cadmium Batteries The most common type of NiCd cells used in UPS applications is the pocket-plate type. The active

material for the positive plate is trivalent nickel hydroxide (NiOH), and for the negative plates a mixture of cadmium and iron (Cd+ Fe). On battery discharge the positive plate active material is reduced to divalent nickel hydroxide and the negative plate active material to cadmium hydroxide in accordance with the following reaction: 2 NiOOH + 2 H2O + Cd discharge charge 2 Ni (OH)2 + Cd(OH)2 The electrolyte is an aqueous solution of potassium hydroxide (KOH) containing small quantities of lithium hydroxide (LiOH) or lithium fluoride (LiF) to improve cycle life and high temperature operation. The electrolyte function is to provide an adequate supply of ions and water to support the reaction of the two plates. The active materials of NiCd cells are relatively insoluble in the electrolyte and the overall charge and discharge reactions result in transfer of a relatively small amount of water

4.6 Comparison of Battery Types The choice of battery for UPS duty lies between Plante Pb(v), VRLA and pocket plate NiCd types. All three have disadvantages and the choice is dependent on a number of factors, including particular manufacturers design features and battery price. Fig. 11 a & b show, for the three battery types, respectively, a comparison of battery physical parameters and system parameters. In section 5 the maintenance aspects of those batteries will be discussed in some detail, however the main differences will be used for comparison here below. The salient points being: - Pb(v) batteries produce hydrogen during rapid/boost charging (The lower explosive limit LEL hydrogen in air is 4%). NiCd batteries produce essentially oxygen only. No significant gassing will occur during float charging for either type. For both however battery rooms with forced ventilation required to prevent accumulation of explosive gas mixtures. VRLA batteries do not produce large quantities of gas, and therefore natural room ventilation is sufficient for them. - NiCd batteries are not easily damaged by overcharging or deep discharging whilst both of the lead acid battery types are. Additionally NiCd batteries can be stored for several months, either charged or discharged without incurring permanent damage. Lead acid batteries on the other hand cannot be stored without regular charging unless the electrolyte of a Pb(v) battery is removed. In practice this means that the NiCd batteries are more tolerant to abuse or neglect than the lead acid batteries and in general therefore require less care and supervision. - The average nominal voltage of a lead-acid battery is approximately 2 V per cell whereas the corresponding average nominal voltage of a NiCd battery is 1.2 V per cell. A battery of Ni-Cd cells will therefore require a larger number of cells for a particular voltage value than with lead acid batteries. There will therefore be a larger number of interconnections causing more internal voltage drop within the battery. Also, a larger number of cells will require more maintenance. - VRLA and NiCd cells are appreciably lighter in weight than Pb(v) batteries and have a higher mass energy density. This maybe of significance in certain applications where weight is a major consideration, but remember that you need more NiCd cells for a given voltage, so the total weight could higher than VRLA and equal to Pb (v). - The voltage required to charge a battery is greater than the voltage that battery will produce during discharge. This difference is important if the load has strict voltage limits. A typical drop in voltage between charge and discharge for lead acid batteries is 11%. The corresponding drop with a NiCd battery is about 20%. Thus for dc UPS loads, with very strict voltage limits, NiCd batteries

may not be suitable. However such strict limits do not normally apply for UPS applications and the range 1.4 to 1.05 V per cell (float to final voltage) is acceptable to the standard instrumentation load. Stricter voltage limits require a higher rated NiCd capacity in order to avoid the requirement

5. MAINTENANCE REQUIREMENTS 5..1 Lead- Acid Pb(v) Batteries A satisfactory lifespan of any Pb(v) battery is largely dependent upon it receiving sufficient correct maintenance. - In general the electrodes, or plates of a cell should not require any maintenance throughout their lifetime. However because of the normally transparent construction of the battery containers it is possible to see if excess deposit is occurring indicating damage to the plates. In such cases a cell would be replaced before it actually fails in service. - The electrolyte does not need replacement during the life of the battery, however frequent topping up should take place such that the active materials on the plates are never exposed. The required frequency of topping up depends largely on the voltage used for float charging and the ambient temperature. With a voltage of 2.23 V per cell and at 20°C the frequency of topping up should be about once every year. It is important to note that topping up should only be with pure water, either distilled or deionised, but never with acid. If a battery is frequently given a rapid charge the frequency of topping up with water will be considerably increased. - The battery should not be permitted to be discharged below the value recommended by the manufacturer, typically 1.65 V per cell. Further discharge can permanently damage the plates. - The battery should not be left for long periods in a state of discharge unless the electrolyte has been removed. It is essential to maintain at least a periodic charge to ensure that the battery will be maintained in good condition and in full state of charge. - If a battery is to be stored arrangements should be made to either: i. remove the electrolyte ii. periodically charge the battery iii. maintain the battery on float-charge permanently. - The specific gravity of each cell should be between 1.200 and 1.225, typically 1.210 at 15°C. Rapid charging should take place if it drops below 1.200 at 15°C. The temperature at which the s.g. is measured is very important as it is temperature sensitive. In general, it is recommended that the batteries be inspected once a month for excess deposits on terminals, electrolyte level and general condition. Specific gravity readings should be taken once every two years. Topping up with water should take place as required but on average once in six months. Rapid charging is required to get a proper mix of the electrolyte and to equalise the charge and capacity among the battery cells. It is recommended that rapid charging be carried out once every two years.

5..2 Lead - Acid (VRLA) Batteries In general, little maintenance is required on VRLA cells; however, a number of things have to be done to ensure that the battery is in good condition and will still deliver its rated capacity or whether it needs replacement. - The general condition of the battery and its terminals should be inspected once a year. Usually one would look for noticeable damage on the cell container housing (e.g. bulging or cracking) or for corrosion of battery pillars. - Cell voltages should be measured once every two years to determine whether there are any defective cells i.e. cells with voltages outside the allowed 2.27 V per cell plus and minus 2.5% tolerance band. Modern techniques are available which are used to measure cell (or cell block voltages) on-line. - A full discharge test should be carried out once a year to determine whether the battery can still deliver its rated capacity. This can be done either off load by means of the use of a dummy load, or

by means of an on-line partial discharge facility, which would induce a battery discharge into the load and compare the resulting characteristics with those of the new battery. - Complete battery replacement would be necessary at maximum 10 year intervals, but consider seriously a shorter replacement of 5/6 years, When a 10 year battery reaches 10 years it will have only 80% capacity and from 8 years to 10 years will be climbing the bathtub curve rapidly – expect cell failures.

5.3 Nickel Cadmium (NiCd) Batteries The recommended maintenance of NiCd batteries consists primarily of topping up the level of electrolyte. Frequency should be as required but it may average once every year depending on local temperature conditions. The battery condition should be inspected regularly say once every month and its terminals kept corrosion-free. It is also advisable to check cell voltages for voltage reversals at least once every two years. In addition the following tests/restoration are required: - Specific gravity readings of the electrolyte should be taken occasionally (e.g. once every 4 years), to ensure that it is well mixed and that it is not degraded. A value of about 1.210 is normal, and a change-out is desirable when the s.g. drops to 1.190, and mandatory at 1.170. - A battery discharge test is necessary once every two years to verify its capacity. - Rapid charging should be performed about once every year to ensure full battery capacity (by reversing the voltage degradation of the negative electrode).