Myth
The best way to keep your rechargable batteries is to keep them partially charged or better to keep them fully charged if you can. There is battery memory where if a battery is left at a low level for a long period of time it my not charge up to maximum levels later. ( google it ) For the average guy, just try to charge it up when in storage. For the person that uses it every day for work, charge it up at the end of the day. These days it seems just cheaper to get a new drill/charger/battery when you old one wears out.
Actually, "battery memory", is a myth, the problem with ni cad rechargeables, is the charger, most are not to smart, and over charge the batteries.
Lithium batteries are a whole different animal, they should be stored for long periods at 50% to 75% charged, storing at 100% is not good for them.
Lithium batteries are far superior, and will hold a charge on the shelf almost indefinably, and recharge very quickly.
I prefer double DD,s myself.
FAST