When reaching a certain voltage threshold, a rest of a few minutes is added, allowing the battery to cool down. Some advanced chargers apply an initial fast charge of 1C. Many chargers include a 30-minute topping charge of 0.1C to boost the capacity by a few percentage points. These “or-gates” utilize whatever comes first. Well-designed NiMH chargers include NDV, voltage plateau, delta temperature (dT/dt), temperature threshold and time-out timers into the full-charge detection algorithm. This requires electronic filtering to compensate for noise and voltage fluctuations induced by the battery and the charger. NDV in a NiMH charger should respond to a voltage drop of 5mV per cell or less.
A mismatched or hot pack reduces the symptoms further. Negative Delta V to detect full charge is faint, especially when charging at less than 0.5C. The charge algorithm for NiMH is similar to NiCd with the exception that NiMH is more complex.
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