Handle with care

To ensure maximum performance, both in terms of battery capacity and expected life, correct charging is essential. 

In general, a battery may be charged at any rate in amperes that will not produce excessive gassing or result in electrolyte temperatures above 45°C. A discharged battery can accept high rates of charge initially but, as the gassing stage is approached (2.35 volts per cell), the current must be reduced to avoid excessive gassing and high temperatures which can permanently damage the plates and
separators.

A battery should always receive the ‘correct’ amount of charge – sufficient to fully charge it and/or maintain it in that condition, but no more. In other words, undercharging or overcharging should be avoided to the extent that it is practical under conditions of usage. An insufficient charge, even to the smallest degree but continuously, will cause gradual sulphation of the plates, with eventual loss of capacity and a reduction of battery life.

Excessive charging will tend to ‘form up’, or corrode, the positive spines into lead dioxide, weakening them physically and increasing their electrical resistance. If overcharging is at comparatively high rates, gassing will also be excessive and this tends to “wash” the active material from the plates. All of these factors reduce the capacity and shorten the life of the battery.

With any type of battery operation there are reasonably simple checks to determine whether or not the amount of charge is correct. If the proper amount of charging is being given, the specific gravity will reach its fully charged value at the end of recharge and will remain at that value. In addition, the amount of water required by the cells will be minimal.

If the specific gravity does not reach full charge value and/or tends to show a continuing decline it is evident that the battery is not receiving sufficient charge.

On the other hand, if the specific gravity reaches or remains at full charge level and an excessive amount of water is required, the battery is receiving an excessive charge and the rate should be reduced.

To keep the battery in good condition it is necessary to use a charging regime that takes into consideration the operational function of the battery. To provide the correct charge regime the current and voltage of the charger must be correctly adjusted and the duration of the different charging stages carefully monitored. Volt and ammeters should be calibrated at regular (six-to-twelve monthly) intervals.

Charging regimes must comply with the following requirements:

▪The duration must be acceptable for the duty cycle

▪Ampere hour and Watt hour efficiencies must attain maximum values to ensure total transformation of energy

Discharge rate

The open circuit voltage of a fully charged cell is approximately 2.13 volts. When on charge, the voltage will rise from approximately 2.15 volt to a final value approaching 2.70 volts. The latter voltage is reached when the charge current has dropped to approximately 3% of the cell capacity. This is called the equalising current and serves to ensure all cells in the battery have reached top of charge.

Cell capacity is dependent upon the discharge rate. Motive power batteries are rated at the 5 hour discharge rate. This means that a 500 ampere hour (Ah) battery can be discharged for 5 hours at 100 ampere to a minimum voltage of 1.7 volt per cell. Beyond this point, known as the knee of the voltage curve, the cell voltage will drop off rapidly. Higher or lower rates of discharge have lower and higher voltage cut off points respectively.

Battery life is dependent on many factors, of which temperature, quality of topping up water, proper charging and regular maintenance are of major importance.

High cell temperatures reduce life dramatically. Unnecessarily high charge rates and high ambient temperatures are prime reasons for high cell temperatures.

Poor quality topping up water containing harmful substances will chemically reduce the ability of the cell to deliver to its rated capacity.

Incomplete charging will lead to irreversible sulphation of cells, and out of step batteries. Overcharging will result in damage to the plates and high cell temperatures.

Regular and proper maintenance is important to identify and correct such problems before permanent harm is done to a battery.