I had a similar problem for a Thinkpad, except there is more than one cell at different capacities that are switched between. The existing battery manager would not tell me which battery was actually being used, and whether there was still a secondary battery waiting to be discharged.

I wrote a very quick hacky program for X11 that stays always visible that will display the information for any number of batteries: https://gitlab.com/danbarry16/bat_mon

It ends up being 50kB with minimal optimization and sports a lightweight X11 library (GUI) and JSON parser (configuration).