NiCd Batteries Charging Strategies
Simple NiCd chargers have been around for a long time: these simply charge the battery at a low constant current. It is common to find 1-Ah AA-size cells or similar in devices such as electric pepper grinders or handheld vacuum cleaners with a plug-in charger that initially charges at 0.1 C, falling back to a continuous charge at perhaps 50 mA. The NiCd chemistry can withstand this overcharging for quite a while, but eventually the cells will be damaged. For this reason, and to improve charging time, a technique known as ‘delta-V’ charging is used. This technique exploits the fact that a fully-charged battery will turn excess charging power into heat, and the increase in cell temperature leads to a small drop in the cell voltage: see Figure 9. The charging circuit thus simply has to detect the point at which the voltage starts to drop.
This type of charger is also capable of fast charging. A charge current of 0.5 C or higher can easily be used with cells that are designed to support it. Note also that NiCd cells do not like to be deeply discharged, and should be recharged when their terminal voltage reaches 0.9 V.
One particular aspect of NiCd chemistry is the memory effect mentioned above. If a NiCd cell is repeatedly partially discharged and then fully recharged it starts to notice that only a fraction of its capacity is being used and then its maximum usable capacity starts to fall: this is a result of the formation of cadmium microcrystals. It is possible to reverse this process by repeatedly discharging the cell to below 0.9 V. The better microprocessor-controlled chargers take this into account and start each charging cycle by completely discharging the cell.