Anyway, I don't see how the users output will help you to estimate the charge time... It's not easier to just calculate the time elapsed for a percent of charge, and estimate the remaining time based on that info ?

e.g. : starts charging at 15%, when the battery is 16% charged, 2 minutes have passed. So the remaining time is (100 - 16) * 2 = 168 min
and each percent gained, you redo the thing, so when battery is 80% charged, you know that (80 - 15) = 65 percent takes 50min, so 20% left will take (20 * 50 / 65) = 15 minutes