Hello everyone,
I had a similar case a while back, dead battery and phone not charging it. I think I solved the issue quite good, so I will share with you how I fixed it.
My N900 was in reboot loop, and the battery was almost dead (it does not keep the charge even today, it's quite old).
Since the OS is not able to start, the phone can't charge the battery.
Without the battery however, reflashing is impossible.
What I did:
- remove the phone battery
- get a 4.5 battery (http://www.consumablestore.co.uk/pro...pg&w=360&h=272)
- let the 4.5 battery discharge for about 3/4 hours, for example putting it in a flashlight (http://www.lighting-gallery.net/gall...artapetrix.jpg)
- measure the voltage after the discharge with a tester, discharge further until reaching 3.7 volts
- using small electrical leads (http://www.harborfreight.com/media/c...mage_16393.jpg) connect very carefully the battery to the connectors on the back of the phone - avoid short circuits placing insulating material e.g. a tiny paper piece or adhesive between metal parts
- connect the phone to a computer equipped with the flasher, and reflash the phone
- once the phone is up & running again, turn it off and put its battery back in place
- turn the phone back on and it should happily resume charging its own battery

Hope it will help someone!
M