The so-called Nickel Metal Hydride batteries belong to the group of secondary or rechargeable batteries. They use an anode of nickel oxidroxide (NiOOH) and their cathode is made of an alloy of metal hydride. They are capable of storing up to 30% more power (80 Wh/kg) than a NiCd battery. These types of batteries have no memory effect, but present some problems if no special care is taken in the charge. The self-discharge effect is also greater than in Ni-CD batteries.