thx venemo.... i wanted to post the same thing..... i never got a full charge when it was charging with usb....
just an example....
its simple...

500mAh usb
dont know atm what the wall has ... lets say 900mAh

lets say the n900 consumes 170mAh in idle...
50mAh are lost through heat while charging...

on usb 500 - 170 =330 - 50 = 270 mAh charge...
on wall 900 - 170 =630 - 50 = 570 mAh charge....

that example tell me that usb will need more than the twice time to charge....

IF all the values are correct.... and they are not correct .... beacause i dont have no wall chafrger here at car.... and i dont know what the n900 is consuming while idle....