Most batteries will lose 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. However, if the battery gets too hot after using it, you'll need to allow it to cool down before connecting to the charger.