For using an alternative charging power source I found out which voltage matches that the N810 (or a Nokia phone) charges and shows 'Charging' when plugging in and displaying the animating battery icon.

Using a 8.3 volts solar panel connected via a car charger does charge.
Using the same panel without car charger (i.e. the 8.3 volts directly to the N810) displays 'Not charging', which I think 8.3 volts is too high.
But when I put a single diode in series (0.7V volrage drop) does charge the N810 (and my 5800 XM phone).

Measuring the charge currents was in all cases 200-230 mA (battery was nearly full, all but one blocks in the battery icon was filled).

Does someone have experiences with charging voltages / alternative charge sources?