Doesn't work with two internal batteries?

[tsbertalan]

I have a T470, which includes a 24 Wh internal battery in addition to the 68 Wh removable battery. I installed acpi-call, tpacpi-bat, and tpbat-utils-acpi after adding the respository ppa:morgwai/tpbat. After reboot, (which involved a password dance to disable UEFI secure boot), I ran battery-set-normal-charge-levels, after which battery-get-charge-levels reports

start charging at 50 (relative percent)
stop charging at 85 (relative percent)

However, when I unplugged, let the machine run down the battery a bit (the history chart in the Ubuntu's Power Statistics - Device History" view reports a "Cell charge" of about 42% 2.5 hours ago), and then plugged back in the charger, it charged back up to 91%, as of time of writing.

Is this as intended, or is there some bug with my setup? How could I troubleshoot this further?