memoryeffect

The memory effect, also known as the charge-recall effect or hysteresis, is a phenomenon observed in certain types of rechargeable batteries, particularly nickel-cadmium (NiCd) and older nickel-metal hydride (NiMH) batteries. It refers to a temporary decrease in a battery's usable capacity. If a battery is repeatedly recharged after being only partially discharged, it can develop an "internal memory" of this shallower discharge depth. When subsequently discharged, the battery may behave as if its capacity has been reduced, seemingly cutting off at the previously recalled discharge voltage.

This effect is primarily a consequence of crystal growth within the battery's electrodes. In NiCd batteries, repeated partial discharges can lead to the formation of larger, less reactive cadmium crystals. These larger crystals reduce the surface area available for the electrochemical reactions that provide power, thus lowering the overall capacity. While the battery has not lost actual material, its ability to release stored energy is diminished under these conditions.

The memory effect is largely a concern for older battery chemistries. Modern lithium-ion batteries, which are prevalent in most consumer electronics today, do not exhibit the memory effect. For NiCd and NiMH batteries where the memory effect was a problem, a common remedy was to fully discharge and then fully recharge the battery, a process sometimes called "conditioning" or "deep cycling." This can help break down the larger crystals and restore some of the lost capacity. However, for NiCd batteries, the presence of cadmium also raises environmental concerns.