I have a "force" branded micro-USB car charger.
When plugged in the N900 said it was charging, but it must have been getting less current than it consumes, at least with backlight and GPS on. The battery level continued to drop.
It was only a 2-wire cable, and I replaced the cable and micro-USB connector. Now it works fine. I don't know how the old one was connected to make it appear to charge, but not really. Could it just have been too much resistance in the cable, so lower voltage?