Step 1: Materials
1. Any rechargeable battery pack you have or you can get
NI-MH adn NI-CD batery are acceptable
make sure they are 7.2 volts NOT over
i recommend NI-MH battery
give better run time
RC 7.2V Battery Pack Connector Repair Kit
you can get it at radioshack or any broken charger or Toy
3. Last thing is the NXT robot
Step 2: Solder connections or the way you want to do it.
before you solder them make a nod then solder them into place.
In total these should be $25 or nothing if you already have the battery and the other stuff.