First of all, it’s best to keep using the battery for cordless drill. Use your tool at least once in 20 days. However, this is not always possible. Storing the battery the right way is also essential to its health and overall life.
When putting away cordless tools for a long time:
1.Put the battery in a cool and dry place and away from dust.
2.Put it in the original plastic case it came in. Put in a box or wrap it in a soft cloth.
3.Do not store Li-ion batteries fully charged or dis-charged. Store them at 30-40% charge level. Do the same for Nickel based batteries, though it does not make too much of a difference as they lose their charge fairly quickly anyway. Many manufacturers will recommend that you discharge your Nickel based batteries completely before storage. Follow the manufacturers instruction if your battery for power tool comes with them.
4.Fully charged NiCd batteries lose charge quickly. 20-30% in 24 hours. 10% the day after that and 1% for every day after that. Be ready to charge your NiCd and NiMh batteries after a long period of rest before you can use them again.
5.Li-Ion batteries retain their charge for longer. Specially if they are kept in a cool place.
6.NEVER put your batteries in the refrigerator or the freezer. This can damage them permanently. Technically, NiCd pwer tool batteries can be stored at a temperature ranging from -20°C to 45°C. But freezing is still not recommended due to the possibility of ice build up. For a Li-Ion battery 15°C is about the ideal storage temperature. But as long as things are not overtly hot, they should be fine.