If you need an external charger find a cheap e-cigarette or phone battery charger and solder on two jumper wires for the + and - terminals and figure out how to make them stick without bending the contacts, this will directly charge the battery externally. You can find plenty of youtube bits on doing this to charge batteries and other stuff. I used to have an OpenMoko phone which has a problem similar to the n900 dead battery - cant start bootloader to begin charging, I just hacked off an old USB mouse and gave the battery 5 min at 5v, it worked and the phone was able to begin charging when I put the battery back in.