This Geeklet will show a Low Battery warning text in big red letters when.
- External power has been disconnected
- Internal battery level drops below 10% remaining<
Default refresh is 10 seconds. Keep on Top is enabled so you can place it in the menu bar if you wish. It set mine at position 640x0 and size is 165x20. This puts it in the middle of my 1440px wide MacBook Pro screen.
EDIT 4/14: Fixed a typo where the warning text would show at the wrong time.
