FWIW, `POWER_SUPPLY_TIME_TO_EMPTY_NOW` is *extremely* variable on my end (Forma): it very much appears to be computed on-the-fly *right now*, with zero smoothing.
(As in, it can fluctuate by a factor of ~4, depending on the immediate power draw, and the thing is *very* finicky).
For people playing at home, it also requires a recent device anyway (Mk. 7 for sure, possibly Mk. 6).
Quick'n dirty script:
bat_ttl.sh