A nickel metal hydride (or NiMH) battery is a type of rechargeable battery similar to a nickel-cadmium battery (NiCd or NiCad) 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 NiCads, suffer far less from voltage depression as they empty, but also have higher internal resistance than NiCds and suffer from larger voltage droops when supplying higher currents. This is difference in internal resistance is easily corrected by adding an extra cell to boost the voltage back to NiCd levels (along with the increased capacity of NiMh). However, when compared with lithium polymer batteries they have a lower energy density and a higher self-