When you change batteries on your mac’s mouse, you lose your mouse connection. Follow these steps to re-pair it with your Mac from the keyboard.
1. Use command-tab to switch to the finder.
2. Press ctrl-f8. Remember, f8 is “mute” on some apple keyboards. Hit the ctrl-fn-f8 key if your f8 is “mute”.
3. The Time machine area will highlight in the upper right of the screen. Use the right arrow key to highlight the bluetooth icon.
4. Press down arrow until you see your mouse’s name, then press the right arrow again.
5. Select “connect”, wait 15 seconds.