Self Discharge is a phenomenon in Batteries in which the charge in the battery drains without any external load. This happens when the battery is kept in storage.
The reason for self-discharge are chemical reactions within the battery. These reactions are still not properly understood.
Different types of batteries have different rates of self discharge. The discharge rate is typically high for the first month of storage and then drops to a lower rate.
The rate of self-discharge is found to be linked to the temperature. The higher the temperature, the higher the rate of discharge. Hence, batteries should be stored at lower temperatures.
A battery which has lost its charge due to self-discharge will regain its power once it is charged again.