The reviews are all praising this widget, but I may be missing the point of it…
MAC OSX already offers a convenient way to check the battery % remaining for any paired Apple mouse~keyboard~trackpad by 1st clicking "Show Bluetooth in menu bar" in the Bluetooth System Preference pane; thereafter, simply click on the Bluetooth menu bar item & drag down to the device you wish to check. When one of those paired Apple devices needs battery replacement, the menubar icon will begin to blink.