batlvl reads the battery voltage (requires the installation of the cs1168 package.
this is a dependency) and then turns on the LEDs appropriately for 5 seconds.
When the battery voltage is less than 20% it blinks the 20% light rapidly for 5 seconds.
This lets the user know the battery is on its last legs.

to use the batlvl app you need to bind it to the wps button (the name given to big button in the center of the LED ring).
here is a /etc/config/system entry that will do that.

config button
        option button 'wps'
        option action 'released'
        option handler '/usr/bin/batlvl'
        option min '0'
        option max '5'

this entry depends on hotplug and the 00-button script as defined here
http://wiki.openwrt.org/doc/howto/hardware.button