NiMH stands for nickel-metal hydride, which is one of the most successful batteries in history. It uses the same nickel-hydroxide positive electrode and KOH electrolyte as the Nickel-Cadmium battery but uses hydrogen gas to replace the cadmium in the negative electrode. The nickel-hydrogen battery is mostly used in low-orbit satellites, which charge and discharge the batteries on every pass around the earth, and requires a long cycle-life battery. Nickel-hydrogen cells have a cycle life of tens of thousands of cycles.
There are a number of charging methods that can be used with nickel-metal hydride batteries, although sometimes this isn't as straightforward as other types of cell or batteries. NiMH cells, similar to NiCds require charging with a constant current with the rate of charge typically specified on the case of the cell, with this rate not to be exceeded. Within the boundary of constant current charging, there are a number of techniques that can be adopted to stop overcharging.
Timer charging - Using a timer to determine when the battery is charged assuming the battery or cell is being charged from a known state of charge. One issue with this method is that if the cell has lost its charge capacity, the charger is expected to deliver 100% charge according to the time estimated with potential overcharging degrading the cell.
Thermal detection - Detecting the end of a NiMH cell according to the cell temperature can be difficult. While the cell often feels warm when it has been overcharged, sometimes the temperature rise can be tricky to assess accurately because the centre of the cell will be much hotter than the exterior. Also, if the cell is being slow-charged, the temperature won't rise as much.
Slow charging NiMH batteries - NiMH battery slow charging is not recommended as they do not detect the end of charge. With voltage changes of only about 5mV at full charge rates, the smaller changes arising for a slow charge are impossible to detect. Additionally, temperature rises indicating the end of charge are far lower and not easily detected.
If you have any questions about NiMH batteries, please do not hesitate to contact Vapextech today. Our team of battery specialists will be more than happy to help.