The Memory Effect of Rechargeable NiCd Batteries

If a NiCd battery has not been completely discharged before it is charged again, the charge will not be complete and so the energy capacity will be slightly lower. When this happens over and over again it will appear that the battery is no longer holding a charge. This is called the "memory effect" since the battery seems to remember the point where it was charged from and when it is discharged to that point the output voltage drops as if the battery has been fully discharged.

The memory effect can usually be reversed by putting the battery through several complete discharge and recharge cycles - special chargers are available to do this.