BATTERY CAPACITY

The capacity of the battery is determined by the capacity of the individual series connected cells. Parallel connection of cells can be made to increase capacity. In the past this practice was discouraged because a weak or defective cell in one of the batteries means that this battery on discharge does not carry its share of the total load. Also, on charge the battery with a defective cell tends to accept a greater share of the available charging current to the detriment of healthy cells in parallel with it. However, provision of parallel strings provides continuity of service, albeit with reduced capacity, in the event of one cell failing in opencircuit mode – something not detected by the simpler battery voltage monitoring alarms. Selection of approach must depend on the manufacturer’s data as to failure mode probability, and the required installation reliability.

Capacity is expressed in ampere-hours (Ah) and is a measure of the electricity that the battery is able to deliver. The following factors affect its capacity:

1. The rate of discharge. If a lead acid battery has a capacity of 100 Ah at a 10 hour discharge rate it can deliver 10 A for 10 hours while maintaining the load voltage above a certain value. Rapid discharge over a 1 hour period will reduce its capacity to typically 50 Ah, i.e. a constant current of 50 A for 1 hour. This effect is not so severe with NiCad batteries. 126 Substation Auxiliary Power Supplies

2. The output voltage reduces as the battery is discharged. It is therefore necessary to specify required current delivery over a given period within voltage limits. In particular the required ‘end voltage’ at the end of the discharge period must be detailed when specifying battery capacity.

3. Battery capacity varies with temperature. The maximum and minimum temperature range at which the battery will be expected to supply the required capacity must be specified. A battery with 100 Ah capacity at 15°C might have a capacity of 95 Ah at 10°C. Typically the variation in capacity with temperature is as follows:

NiCad batteries 0.6% increase per °C from 0°C to _30°C

1.5% decrease per °C from 0°C to _20°C

Lead acid batteries 1.0% increase per °C from 0°C to _60°C

1.5% decrease per °C from 0°C to _10°C

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