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 battery 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 18650 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. 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 rechargeable battery 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.