Nickel metal hydride batteries are the rechargeable alternative to common primary cells with stable voltage.
NiMH batteries are widely used in the usual types of standard batteries. They can therefore generally replace the conventional alkaline batteries in household devices as a rechargeable alternative. Despite the slightly lower voltage, the NiMH battery can achieve longer run times especially under high loads, since the voltage remains very stable. Typical applications are therefore usually high-powered devices, such as digital cameras, toys, and of course cordless phones.
The other rechargeable system of nickel cadmium (NiCd) batteries can be replaced as well. Advantages over nickel-cadmium batteries which are no longer available over the counter are the lack of toxic cadmium, a higher energy density, and the non-existent memory effect. Disadvantages are the lower overcharging resistance, a higher internal resistance, and a smaller temperature range.
Nickel metal hydride cells are rechargeable cells with a rated voltage of 1.2 V. The cells / batteries are system and voltage compatible with NiCd batteries. They are robust to a certain extent, deep-discharge capable, over-chargeable, have a long shelf life even in discharged state, a good life span, good performance, and a very high specific energy. But for certain applications special cells are required, despite the mostly superior lithium ion technology.
A problem with conventional NiMH batteries is the self-discharge of five to ten percent on the first day, which then stabilizes at one-half to one percent per day at room temperature. This high rate of self-discharge prevents the use of such batteries in devices where a battery life of several months or years is desirable, such as in watches, remote controls, or even safety-related devices such as fire detectors or flashlights for emergency use. The ambient temperature strongly influences the level of discharge. Lower ambient temperatures have a lower self-discharge rate and higher ambient temperatures have a higher self discharge rate. Similarly, the capacity has an effect on the self-discharge rate: High-capacitance cells (> 2200 mAh for cells in AA-size) have the highest self-discharge rate.