The hardware appears to have the capability to generate a powerfail interrupt when the car power supply drops below about 10VDC. When this happens, the kernel does a number of things, including muting the audio, writing the 128byte flash savearea, and blanking the display.

Software also disables the powerfail detection, so that the interrupt doesn't just keep interrupting things -- polling is used to detect when the powerfail trigger goes away -- voltage above 10VDC again.

The player software "sees" this by looking at /proc/empeg_power, where there is a bit labelled "1 (Powerfail enabled)". When this shows up as "0", the player displays the battery icon. When it shows as "1", the player does not display the battery icon.

EDIT: there does not appear to be a way to read the actual voltage, but rather just the trigger for above/below 10VDC.

Cheers