Memory effect

Memory effect, also known as battery effect, lazy battery effect, or battery memory, is an effect observed in Nickel Cadmium (NiCd)  and Nickel Metal Hydride (NiMH)  rechargeable batteries that causes them to hold less charge. It describes the situation in which nickel-cadmium batteries gradually lose their maximum energy capacity if they are repeatedly recharged after being only partially discharged. The battery appears to "remember" the smaller capacity.

The effect can be overcome by subjecting each cell of the battery to one or more deep charge/discharge cycles. This must be done to the individual cells, not a multi-cell battery; in a battery, some cells may discharge before others, resulting in those cells being subjected to a reverse charging current by the remaining cells, potentially leading to irreversible damage.