The output from the charger should be fine, what may be an issue is that the n900 expects the data pins on a dumb charger to be shorted (connected together), its normally fairly easy to fix this if you have a soldering iron and solder handy. I have a couple of car cigarette lighter socket to USB port chargers which both work after I performed the operation to fix this issue, they are rated at 500mA and 800mA.