The memory effect is a phenomenon of rechargeable batteries. It occurs mainly with NiCd batteries and to a lesser extent with NiMH batteries if they are not fully discharged and are recharged in this state.
If a battery is only partially discharged before charging, the effect occurs that the battery has "remembered" this partial discharge and no longer provides its full rated capacity during subsequent discharges. This "remembering" is known as the memory effect. If such a battery is loaded with nominal power, the nominal voltage collapses as if the battery no longer has any charge. The memory effect is caused by the formation of crystals on the cathode, which is made of cadmium. The formation of crystals reduces the capacity of the battery and thus its performance.
The memory effect can be reversed by deep discharging, which is done in some chargers by a refresh function. Complete discharge prevents the memory effect and extends the life of the battery. After a deep discharge, the battery has its full nominal capacity again. The memory effect can no longer be detected in modern rechargeable batteries.