What is "memory effect"

A partially discharged battery at times does not fully discharges before recharging. It sort of "forgets" to do so. This is what is the memory effect. For e.g. if you fully charge your battery and then use only 50% of its capacity before the next recharge, eventually the battery will become unaware of its extra 50% capacity, which has remained unused. The battery will remain functional, but only at 50% of its original capacity. To deal with this, always fully discharge your battery, before recharging it. This can be done by, unplugging the device's AC adapter and letting the device run on the battery until it ceases to function. This will help your battery remain healthy.