I bought a new NOKIA BL-5J battery and it seems to be completely drained. How can i bootstrap starting the system? I still have the old one, and the cables work, etc.

It is hard to get N900 to charge totally dead battery. You might do some searches on the forum and wiki about flat battery recovery. One of the options is also use external charger or some Nokia Dumbphone to charge the battery. Some people have managed to revive flat batteries by leaving them in phone connected to charger for full 24h.

Edit: adventurous person could charge the new drained battery with the old worn battery by connecting the + to + and - to -, but this ofcourse runs the risk of blowing up both of the batteries on your face if you screw it up. Basically you just need to do it for short period of time to get the new battery just out of flat level so you can do rest of the charging on the phone.

If the "flat" battery is less than 2V (at the cell level - notice, that protective circuity could have cut-off terminals, showing 00.00V, while cell might be actually higher) is may be not worth to even try bringing that battery to life. LiIon's are really vulnerable to total discharge, you may end up having 100 mAh capacity one

/Estel

Dont worry just find an external battery charger and try or else find a supply with 5 volts and connect it to the battery for 5 sec repeatedly a couple of times and it should be fine to charge in your n900

5 Volts! Sure, feel free to do that if you like fireworks.