Lead-acid Batteries Charging Strategies
In the case of sealed gel lead-acid batteries as well as the so-called ‘maintenance-free’ starter batteries it is important to ensure that they are not overcharged, as otherwise they can outgas. This can lead to a loss of electrolyte, which it is not a straightforward job to replace, and consequently to a shortening of service life and a reduction in capacity. In non-sealed lead-acid batteries, as used, for example, in fork-lift trucks, this is less of a problem as they can be topped up with distilled water. Lead-acid batteries should also not be subjected to deep discharge: the cell voltage should not be allowed to fall below 1.75 V. Relevant for storage is the self-discharge rate of 2% per month or more: for example, a starter battery on a petrol lawnmower may well not make it through the winter without some attention in January. The normal charging procedure is to begin with a constant current until the cell voltage reaches 2.35 V and then switch to constant voltage charging until a minimum current threshold is reached. Unless indicated otherwise the charge current should be at most 0.1 C (that is, 10% of the rated capacity in Ah/h). The current threshold when charging stops is typically 0.01 C.
In the case of a typical 12 V starter battery rated at 60 Ah this means that charging begins at a constant current of at most 6 A, and then, when the battery voltage reaches 14.1 V, the charging voltage should be kept constant.
The current will then fall and when it reaches 0.6 A charging can stop. Continuing to charge at this point (‘trickle charging’) does not cause any problems. Figure 8 shows a typical charging curve for this type of battery using the CC/CV strategy (‘constant current/constant voltage’).
A feature of lead-acid batteries is that over time they can lose capacity owing to a process called sulfation. To mitigate this special chargers are available that as well as providing a maintenance charging current, also provide regular millisecond-long pulses of current at over 100 A. This acts to prevent the formation of crystals in the battery and even help to break them down.