So I've worked out why it wasn't charging... Monitoring of the battery temperature sensor is enabled by default in the AX803. Unfortunately, from looking at the Pinephone schematics, it seem that it expects a 10 Kohm thermistor in the battery whereas the battery actually has a 3 Kohm thermistor.
I can only wonder why. Unsurprisingly, the values that the AX803 uses by default for over and under temperature detection are wildly different to what the battery actually reports. As a result we get an UNDER temperature warning triggering, and charging is stopped. We also almost certainly would not get adequate protection against OVER temperature either, with the default settings. Obviously it's possible to simply disable the battery temperature sensor, but I'm not satisfied with simply disabling a safety device. I did test this configuration, though, and the phone does charge. Amasingly this seems to be the way that the issue has mostly been dealt with by the various Linux distributions, which makes me cringe. I decided to investigate the real solution of getting the temperature sensor adequately configured. Through trial and error, I've set conservative values that seem to work for my specific handset. The battery charges, and if I reduce the upper thresholds any more, the over-temperature cut-out triggers. The main problems are: * How to determine correct thresholds for over and under battery temperature that are suitable and safe for a range of devices in a range of environments * How to reset the PMIC after a temperature alert has been registered This second point is a bit unusual. Once the over-temperature threshold has been triggered, charging stops. After removing both AC power and the battery, then replacing the battery, it still does not charge, until the over-temperature threshold is set really high. It can then be reduced again to a more sensible level, and the battery will continue charging. Other observations: * It never charges to 100%, only to 99%. * The charging current stays at about 0.23 amps even when at 99%. * Connecting to a plain USB-A socket on my workstation allows it to charge. * Connecting to a USB-C power adaptor does not. This is probably expected.
