adjust battery-warning-level of magic mouse
Solution 1:
This thread has some scripts that can help you.
It seems that you wrap the results of this shell snippet in a nice AppleScript:
ioreg -c BNBMouseDevice | grep BatteryPercent
Solution 2:
In my macOS Sierra 10.12.6, there is no BNBMouseDevice. Script which works for me is:
ioreg -c AppleDeviceManagementHIDEventService -r -l | grep BatteryPercent | sed 's/[^[:digit:]]//g'
Source: Check the battery level of connected bluetooth headphones from the command line