Know how to apply the correct charge to moderate heat and prevent overcharge.
The charge algorithm for NiMH is similar to NiCd with the exception that NiMH is more complex. The NDV to detect full charge is faint, especially when charging at less than 0.5C. A mismatched or hot pack reduces the symptoms further.
The NDV in a NiMH charger must respond to a voltage drop of 5mV per cell. This requires electronic filtering to compensate for noise and voltage fluctuations induced by the battery and the charger. Well-designed NiMH chargers include 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. 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 as the “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 dislike overcharge and the trickle charge is set to around 0.05C. In comparison, NiCd is better able to absorbed overcharge and can take 0.1C. 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 functions well. The lower trickle charge for NiMH is 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 and 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 presumed 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 better to remove the batteries and then recharge before use than to leave them in the charger for eventual use.
Last updated 2015-03-30
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