Trying to help...

a) Mine is near-linear, I can calculate percentage charge to within a few points (checked by removing charger). Fiddling with it is bad tho.

b) It doesn't have to be by-the-second. Batteries in N900 are quite similar and charge current is 500 mA until such is the time to throttle it. Can't we just use a conversion table? Say, every 10 % or so, and guesstimate that a 30% battery will charge in 55-65 minutes? We'll call it "about 60 minutes". 40% would be "about 50 minutes", etc. Community could help with figures.

c) Can't a software self-profile? When plugged in, it times the charge time from current level to full _assuming_no_interaction_ (if the user plays with it it's no longer fully charging). It should be able to do decent estimates after -say- 3 charges at 5, 30 and 60 percent?