You didn't mention what package the batteries were in. "AA"? "AAA"? Some form of button cell? If they're "AA" batteries, just slip them into a penlight and run them down. Recharge for the recommended time, then put them in the penlight and run them down again. Do this for three or four cycles, and they should be back up to par. Be careful and remove the batteries from the penlight when the lamp drops to a yellow glow so that you don't reverse-charge one of the cells.
If a smaller battery, connect them in series into a similar penlight circuit with a #222 lamp for a load to discharge them.
The worst thing you can do with NiCd batteries is to leave them on the charger all the time. Wait until they're about ready to cash it in, and then recharge. NiCds that are constantly on charge and then used for a little bit and put back on charge will only give you a little bit of usage as they acquire a "memory".
If the mouse came with NiCd cells, consider replacing them with NiMH cells if you can find them. Wal-Mart is now carrying several brands of NiMH in both "AA" and "AAA" now. NiMH cells are compatible with NiCd chargers.
Dean