Nickel-metal hydride batteries, often called NiMH or Ni-MH, are one of the main types of rechargeable batteries. But lately they have had trouble competing with the cheaper Ni-Cad batteries and the superior Lithium-ion batteries. Here is a short nickel-metal hydride battery overview.

Most of the early development of nickel-metal hydride batteries was done in Europe, mainly in Germany and Switzerland. It would take quite some time before NiMH batteries were commercially viable. A lot of technical problems had to be solved before NiMH batteries were good enough for consumer products. The first NiMH batteries were introduced in 1989.

Compared with the older nickel-cadmium (NiCad) batteries, NiMH batteries have a number of advantages. One of the main reasons for replacing NiCad batteries with NiMH is that they are more environmentally friendly. Cadmium is an environmental hazard. NIMH batteries also have a higher energy density than many other type of batteries. One exception is lithium-ion batteries which have significantly higher energy density.

But NiMH batteries also have a number of shortcomings. Like NiCad batteries they suffer from the so-called memory effect. The batteries lose capacity after being repeatedly recharged before being completely discharged. Lithium-ion batteries have no memory effect. Due to the memory effect, many recommend that you run down NiMH batteries completely at least once a month.

Charging NiMH batteries can cause a number of issues. A complete discharge can in worst case cause irreversible damage to the battery. Some, but far from all, devices using NiMH batteries can detect when the voltage is reaching dangerous levels and shut down.

Overcharging can damage the battery so many chargers either have a fuse or use trickle charging to prevent overcharging. Trickle charging means that a very low current is used. In any case, it is highly recommended that you don’t leave the battery in the charger once it has been fully charged.

NIMH batteries have higher self-discharge than most other types of rechargeable batteries. Self-discharge is not a problem with primary batteries, that is batteries which can’t be recharged, but rechargeable batteries suffer from significant self-discharge. The term refers to the fact that the batteries lose capacity when not used. NiMH batteries can lose as much 30% of their capacity per month. This makes the batteries unsuitable for clocks and other devices that use little electricity but are supposed to run for a long time. New NiMH batteries with lower self-discharge but also lower capacity have been introduced.

NiMH batteries have been replaced in many high-end devices by the superior but more expensive lithium-ion batteries. Outside EU, the cheaper NiCad batteries are often used in low-end devices. But in EU, NiCad batteries have been replaced by NiMH batteries in most consumer devices due to the Battery Directive which banned NiCad batteries in the EU. NiMH batteries are also used in some electric cars.