Delta peak charging
Delta peak charging is a method for determining when NiCd and NiMH batteries are fully charged.
As the charger adds energy to the battery the Voltage it generates rises. As the battery reaches its fully charged level, the energy the charger pushes into the battery is dissipated by the battery as heat, causing the capacity of the battery to drop slightly, lowering the capacity and hence voltage of the battery. The charger can look for this drop in voltage and stop charging.
NiCd cells work well with delta peak voltages (the size of the voltage drop when fully charged) of 5mV. NiMH are much more sensitive to overcharging than NiCd cells so it is probably best to use 'zero delta peak' (no voltage drop, just a 'levelling off').
To use delta peak charging, the charge rate must be high enough to cause the cell to heat slightly at the end of the charge. One advantage of using delta peak charging is that the voltage drop can be observed even in a large pack of cells, allowing the charging to be stopped as soon as the first cell reaches its limit. Usually a charge rate of 1C works well.
Delta peak charging must never be used to charge Lipo cells, else permanent damage and, likely, pyrotechnics will result.
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