Apparently, rechargable batteries loose quite a bit of their charge, on their own, in a month. Something like 20-30%. That's a lot of juice. What is means is this: recharge your batteries before a shoot.
It also appears that lower capacity batteries actually hold their charge longer. So check out 2000mAh instead of 2500mAh batteries.
[Coding Horror: Adventures in Rechargeable Batteries](http://www.codinghorror.com/blog/archives/001078.html)