i use a sonyo eneloop externel battery http://www.eneloop.info/index.php?eI...729fcc5b6fc3a1

that has 2usb out ports. Using an Y-usb cable from a external harddrive with an adapter to micro usb i get up to 1000 mA out of the battery which seems to charge the N900 the same way the Nokia charger does, at least according to batterygraph. The battery has 2000mAh and was about 35€ on amazon, the adapter and the Y-cable should be about 10€. And my impression is that the battry holds the charge without much loss, i left it for about two weeks in my pocket and the recharged th N900 one time without a problem.