Squidge has released a new application for the GP2x. Here is what Squidge said:

I wanted to put a battery monitor into my SNES emu to warn you to save when the batteries start getting low rather than just crashing, so I wrote a simple program that checks the voltage of the batteries. It can be used to check that your adaptor is functioning correctly too – ie, outputing a voltage within specification, so could be useful to some people.You can get the executable here: http://www.squidgey.plus.com/voltage.gpeThe voltage is constantly checked until you press the START button to exit.For rechargable batteries, 2.4V and below is considered”empty”, 2.5v – 2.6v is considered”medium”, and 2.7v+ is considered”full”(same as the battery status in the settings menu).

http://www.gp32spain.com/foros/showthread.php?t=27108