briefly charging a low battery causes shutoff on unplug

[Selene ToyKeeper]

On Ubuntu Touch devices, when the device thinks the battery is very low (regardless of whether it is actually low), plugging it in to USB for a minute (or less) and then unplugging it causes the device to immediately power itself off.

I'm not sure how long it must be plugged in to avoid this issue, but I have seen it happen after being connected for 3 seconds or 60.

If the battery is actually low, this means charging it for a short time to get a little more usable battery life... can actually make it run for a shorter time. Normally I'd expect a brief charge to give me at least a few more minutes before it needs to shut down, but instead it shuts off immediately after unplugging.

If the battery is not actually low, due to a variety of bugs in the charge estimation code, this means the plugging in the phone briefly can make it power itself down even when it's full. For example, after testing bug 1476468, I plugged the device in to get some logs and check some things in /sys, then unplugged it to see what the kernel said about the battery while it wasn't plugged in. The device shut itself off immediately, then when I booted it again the charge indicator said 67%. It went from 1% to 67% in the time it took to reboot, resetting itself to a state which was based on actual voltage measurement instead of ... whatever it is it does to guess at the percent.

In any case, if it wasn't low enough to shut itself off before USB was connected, it probably shouldn't shut itself off when it gets unplugged (after recovering a bit of charge).

I've seen this happen on both krillin and arale, both in the past and on today's latest image.