I've gone through five pairs of 2500 mAh NiMH rechargeables and two pairs of one-use batteries in my Magic Mouse since November. The one-use batteries lasted 25 and 32 days, the first tow pairs of rechargeables 17 to 19 days. I read that NiMH batteries needed to be deep cycled after several years use and mine are over five years old. So the last three pairs were totally discharged before re-chargeing. That increased the life some, but I also discovered that they loose about 1% charge per day with no load.
I found an article at:
http://www.consumersearch.com/batteries/review that describes this and the hybrid batteries by Sanyo (Eneloop) that are only rated 2000 mAh but have a much slower no load discharge rate so as to actually last longer in lower discharge uses like the Magic Mouse. Their findings agree with the no-load discharge rates that I have experienced.
My current pair of 2500 mAh batteries have been in for 24 days now and Mouse Preferences still indicate 55% charged, so having cycled this pair twice seems to have improved them and also putting them into the Mouse the day they came off the charger meant it was 80% charged from the start instead of 64 to 70% before. (NiHD batteries only charge to 1.2 volts instead of 1.5, so you never see a 100% charge by the way Mouse Preferences measures it.)
It's a pain to have to figure out how to have a freshly charged battery ready for the mouse when you don't know when it will need it. I think I am learning though to not put the batteries on the charger until I actually get the low battery warning. That has given me 24 hours warning so far, which is just enough time to charge up a new pair. I do plan to try the Sanyo hybrid batteries, however.
