mouse keeps disconnecting

Mice are not connected via Wi-Fi, they use Bluetooth which is a totally separate wireless technology. Please use the following Troubleshooting wireless mouse and keyboard issues

This helped me in most cases if you use the magic mouse: com.apple.driver.AppleBluetoothMultitouch.mouse.plist - delete (or move that file to the desktop if you are more comfortable with that, can always move it back) and reboot the machine.

*Sorry forgot the location: ~/Library/Preferences/ can search for the file from there and it should come up...