MAKE:Blog links to instructions on how to mod a wireless mouse so that it has some seriously long battery life.  All it takes is a micro contact switch and a bit of time.
“I tend to have to recharge my mouse at the most inconvenient times, & though with my luck it will always go like that, I did a mod to my wireless mouse that has so far took a mouse that needed to be recharged everyday, to a mouse that can go double to triple the amount of time before needing to be recharged!!! (Not to mention the fact that this is THE BEST way to save batteries with wireless mice, the only way to save more than that would be to either have the mouse off when you’re trying to use it!”

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