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’).

pic1 Lead acid Batteries Charging Strategies

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.

30. May 2019 by sam
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