AA rechargeable batteries
Additional resources
Rechargeable AA-sized batteries was introduced on a concept to uses back the same battery by recharging them over and over again. It is based on Li-Ion chemistry in the beginning but the voltage produced, 3.6 volts was a bit higher than a normal alkaline battery which make it not suitable to use in most AA-based devices.
Besides the high voltage and mAh issues with the Li-Ion rechargeable battery, these cell also has self-discharge disadvantage where a fully charged battery will loss it charge automatically when not in use for a significant period of time.
Then a new generation of rechargeable battery, NiMH technology has a low self-discharge its feature and loss less power around 8% after 60 days. Normally NiMH AA rechargeable batteries are sold in the a pre-charged form which are ready to use after purchase.