At one point I mused that it would be nice to be able to power the mice with the BL-5C Lithium Ion batteries that power my cell phone. They recharge quickly and hold a lot of current. Only problem is they are 3.7 volt batteries and the mice are designed to run on a couple of AA's in series... 3 volts. I wondered if the extra .7 volt would be a problem. One way to find out. A couple of jumpers later, I had my answer... the mouse seemed quite happy with 3.7 volts.
The hard part was finding a way to stuff the battery inside the mouse. You have to chop a lot of the battery compartment plastic out, remove the springs, and install a connector assembly that will match up with the BL-5C terminals (if you want it to be removable, more on that later).
Step 1: Not a step, just a pic
I also installed a small slide switch with which I can turn the mouse off to conserve battery power.