Nickel metal hydride (NiMH) batteries are a type of rechargeable battery similar to NiCad batteries, but made with more affordable and environmentally friendly components. They are increasingly popular, particularly for use in consumer electronics, because of their relatively long lifespan, high charge rate, and affordability. These batteries are often considered an alternative to lithium ion batteries. While lithium ion batteries may have longer life-spans, their high cost and relatively high maintenance needs make NiMH a wiser choice for many types of consumer electronics. In fact, with the exception of items such as cellular phones and laptops, NiMH batteries are usually recommended.
NiMH batteries use hydrogen ions to store energy, unlike the lithium ions of lithium ion batteries. Because of the size of hydrogen ions, the batteries are larger than comparably charged lithium ion batteries. The "metal" referred to in the name is comprised of both nickel and another metal such as titanium. The nickel is usually supplemented with manganese, aluminum, and cobalt, and higher-quality batteries also include metals such as zirconium and vanadium. The metal essentially serves to contain the released hydrogen ions to ensure they don't reach a gaseous state.
The most common size of NiMH batteries is AA, with a capacity ranging from 1200 milliamps (mA) to 3100mA. At the upper end, these batteries tend to outperform all other rechargeable types, with the exception of lithium ion. Pricing varies considerably, depending on total capacity and brand name.
Most portable battery chargers are able to handle NiMH batteries, including many speed chargers. Because of the nature of these batteries, however, chargers specifically designed for this type are recommended to ensure that speed charging will not harm the batteries. Additionally, indefinite trickle-charging of NiMH batteries will damage their future capacity and discharge rates, so chargers that automatically shut off are recommended.