Self-discharge is an event in which, battery charging level is reduced when the battery is stored open-circuit for a while. Self-discharge (losing charge) is caused by electrochemical processes which act like a small external load. If lead acid batteries being stored at normal temperature, estimated self-discharge rate is about 5% per month. This estimated rate could become more critical as a result of temperature increase or battery aging. In order to reduce self-discharge, it is recommended to store cells and batteries at lower temperatures.
Remember to Keep your battery charged when it is not in use!