A nickel metal hydride (or NiMH) battery is a type of rechargeable battery similar to a nickel-cadmium (NiCd) battery but which does not contain expensive (and environmentally risky) cadmium. This is why they are sometimes called the most environmentally friendly battery type. NiMH batteries tend to have a higher capacity than NiCds and suffer far less from memory effect. However, when compared with Li-Ion batteries they have a lower energy density and a higher self-discharge rate.