I understand your approach, and the fact you need data to get the charging curve.

But there is a problem, how to be sure that the charging curve is constant in the time (and I'm pretty sure it won't, problem is N900 has been released since a month only), and how to be sure that there are not (or there won't be) different models of battery ?

In addition to the data you'll get, the best is not to collect data on the phone where the application is installed at each charging, so the estimation will remain the closest of the reality ?