I know a little bit about the battery parameters in the M5StickC now. The power controller is the AXP192. The manufacturer makes a comprehensive AXP192 data sheet available. Unfortunately for me, most of it is in Chinese. With the help of Google translate and code published by M5, I was able to figure out quite a bit. (There is an English data sheet available, but it's only 4 pages, compared to more than 50 pages for the Chinese data sheet.)

• The AXP192 targets charging to a battery voltage of 4.20v, plus or minus 5%.
• M5 has configured the AXP192 to automatically shut things down at 3.00v.
• M5 provides an API called GetBatteryStatus(), but it returns an 8-bit value that is a collection of flags from an AXP192 register. The datasheet told me what each bit was.

(M5's AXP192 class includes helpful methods for reading and writing AXP192 register values. Alas, they are all private methods, so I can't use them directly to get things that M5 didn't expose. It's not insurmountable, but it's inconvenient.)

I found out experimentally that if I plug in the device when it's a little below 4.20v but within 5%, the AXP192 doesn't start charging it. And, since the AXP192 switches things to run completely on the USB current, the battery won't discharge and things will stay that way. OTOH, when the battery is being charged, it only got up to about 4.18v. That's within that 5% range (in fact, only about a half percent difference), but I expected it to be more on target. I don't know if that's due to AXP192 logic or if my specific battery won't take more charge.

I recorded the voltage as the battery discharged, as described in my previous note. The lowest recorded voltage was 3.53v. That's quite a bit higher than the 3.00v shutdown voltage I expected. I'm going to run a few more charge/discharge cycles to see if that low number changes. When the numbers settle down, I will probably still use a linear calculation for the percentage. That's probably close enough for this kind of experience. What you really want to know is when the device is going to quit due to a low battery.

The application logic I am using to record the voltage range at this point:

• For any new lower voltage, record it regardless of charging or plugged-in status.
• If the device is plugged, not charging, and the difference from the measured voltage is more than 5% different from 4.2v, record it as a new high water mark. (I'm not really sure what happens if the AXP192 can't get the battery up to the target voltage range.)