A nickel metal hydride cell is a type of rechargeable battery. This battery has a negative electrode made of an alloy that absorbs hydrogen. The chemical reaction uses nickel oxide hydroxide, which is similar to that of the nickel–cadmium cell. The positive electrode contains a hydrogen-absorbing alloy. During the recharge cycle, the negative electrode will be filled with water and the positive electrode will be charged with a hydrogen-absorbing alloy.
The negative electrode can also absorb some of the hydrogen gas, but the majority is removed from the cell by the positive electrode. The hydrogen remains in the cell and is removed from the battery by the negative electrode. The negative electrode has more active material than the positive electrode. The negative electrode depletes itself of its active material and is left with a dead cell. This process leads to permanent capacity loss in the battery. Moreover, the battery’s life can be reduced as a result of overcharging.
The negative electrode of NiMH cells has a much higher capacity than the positive electrode. This increases its efficiency by up to 30%. The energy density is 60Wh/kg. As a result, it can last up to 300 times. A NiMH cell can be recharged four times and drained as many as 300 times. Its slim geometric shape makes it more suitable for small appliances, while the long life and energy density make it a popular choice.