Most batteries will loose their charge over a period of time. Approximately, 15-20% the first day out of the charger. The second day is between 7-10%, roughly 3% on the third day, and about 1% thereafter. It would be best to recharge the battery before it is used again to be sure of 100% runtime.
The batteries are not affected by memory. They can be placed on the charger at any time and do not need to be drained before recharging.