You might want to take a look in the NickelMenu thread from
message #999 and
message #1000.
You can telnet to your Kobo and take a look at what you get from looking at the contents of /sys/class/power_supply/battery/.
Edit: for the Libra Colour, you will need to look at the contents of /sys/class/power_supply/bd71827_bat instead.
Message to self: READ TO THE END OF THE THREAD BEFORE REPLYING!