The charge algorithm for NiMH is similar to NiCd with the exception that NiMH is more complex. The NDV method to measure full charge shows only a faint voltage drop, especially when charging at less than 0.5C. A hot battery or one with mismatched cells works against the already minute voltage drop.
The NDV in a NiMH charger must respond to a voltage drop of 5mV per cell. To assure reliable charging, NiMH chargers must include electronic filtering to compensate for noise and voltage fluctuations induced by the battery and the charger. Modern chargers achieve this by combining NDV, voltage plateau, delta temperature (dT/dt), temperature threshold and time-out timers into the full-charge detection algorithm. These “or-gates” utilize whatever comes first depending on battery condition. Many chargers include a 30-minute topping charge of 0.1C to add a few percentage points of extra charge.
Some advanced chargers apply an initial fast charge of 1C. When reaching a certain voltage threshold, a rest of a few minutes is added, allowing the battery to cool down. The charge continues at a lower current and applies further current reductions as the charge progresses. This scheme continues until the battery is fully charged. Known asthe “step-differential charge,” this method works well for all nickel-based batteries.
Chargers utilizing the step-differential or other aggressive charge methods achieve a capacity gain of about six percent over a more basic charger, an increase that is not possible without stressful overcharge. Although a higher capacity is desirable, filling the battery to the brim has a negative effect in that it will shorten the overall battery life. Rather than achieving the expected 350 to 400 service cycles, the aggressive charger might exhaust the pack after 300 cycles.
NiMH cannot absorb overcharge well and the trickle charge is set to around 0.05C. In comparison, the older NiCd charger trickle charges at 0.1C, double that rate. Differences in trickle charge and the need for a more sensitive full-charge detection render the original NiCd charger unsuitable for NiMH batteries. A NiMH in a NiCd charger would overheat, but a NiCd in a NiMH charger does well because the lower trickle charge is also sufficient for NiCd.
It is difficult, if not impossible, to slow-charge a NiMH battery. At a C‑rate of 0.1 to 0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full-charge state accurately and the charger must depend on a timer. Harmful overcharge will occur if a fixed timer controls the charge. This is especially apparent when charging partially or fully charged batteries.
The same scenario occurs if the battery has lost capacity due to aging and can only hold half the capacity. In essence, this battery has electrically shrunk to half size while the fixed timer is programmed to apply a 100 percent charge without regard for the battery condition. In most cases an overcharge will heat the battery, but this is not always the case. A poorly designed NiMH charger is capable of overcharging a battery without heat buildup. At a sufficiently low charge rate, NiMH can remain totally cool and yet suffer from overcharge.
Battery users are often dissatisfied with shorter than expected service life of a battery. Let’s not blame the battery manufacturer, because the fault might lie in the charger. Low-priced consumer chargers are especially prone to incorrectly charging. If you use such a charger and want to improve battery performance, estimate the battery state-of-charge and capacity and set the charge time accordingly. Remove the batteries when you think they are full. If your charger charges at a high charge rate, do a temperature touch. Lukewarm indicates that the batteries may be full enough for removal. It is far better to remove the batteries and then recharge before use than to leave them in the charger for eventual use.
Note that some nickel-based batteries heat up during charge and this is in part due to high internal resistance. Such a battery may be warm to the touch even though it is only partially charged. Another battery might be fully charged and remain cool. Charging NiCd and NiMH batteries on an inexpensive charger is guesswork, and it is best not to charge them longer than necessary. Remove the batteries when perceived full and give them a quick charge before use.