Nickel Metal Hydride
Nickel Metal Hydride (often abbreviated NiMH) batteries are a proven rechargeable technology. These batteries are often lower cost than other chemistries but suffer from lower densities than LiPo. NiMH batteries require less stringent charging curves, which lower the cost of the chargers. NiMH are often found in lower cost electronic devices such as toothbrushes and cordless shavers where output voltage is less of a concern (you’ll notice your toothbrush running more slowly but continues to work).
Each cell outputs nominally 1.2V. This is very similar to alkaline batteries of the same size that output 1.5V. Combining four AA NiMH will get you a 4.8V pack which should run most 5V systems but will drop in voltage as the pack discharges.
Charging and Discharging
NiMH batteries on their own have no discharge protection circuits. A discharge protection circuit prevents the battery from discharging below a certain voltage level to prevent damage to the battery. More information on NiMH batteries and over-discharge can be found here.
Because of their similarity to regular consumer batteries, charging NiMH batteries is often done with chargers that plug into the wall. We recommend NiMH for applications where a device has already been designed to use AA type batteries.