From the "About your Kindle 1st Generation" booklet online......
Tips for Conserving Your Battery
The Kindle battery will have a much longer life if you charge it
frequently, rather than waiting until it is fully drained to charge it.
The battery may need recharging if it has not been used for a long period
of time, whether or not it was fully charged before it was stored.
http://www.amazon.com/gp/help/custom...deId=200317150