I think that @Patcher has the right idea in that the only way to get an accurate measure of the battery charge is to integrate current in/out (+/-) of the battery over time (as confirmed by @KF5YFD's wikipedia link). Fortunately the AXP209 includes a coulomb counter which should do the trick. I'm just getting started with Armbian on an Olimex Lime A20 myself, but Igor's script is a great starting point and example of communicating with the AXP209 via I2C. At some point when I have some time I might try to sort out a technique that uses the coulomb counter to track battery charge. If I do I'll be sure to post here. In any case I think the coulomb counter is the way to go.
