WARNING: polarity must be check 3 times to make sure before connecting battery think of this as one shot deal. These will effect warranty on product so if u got warranty don't try it PLZ. If you really want to do it go a head to next step.

Step 1: Materials

   The material u will need are:
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
2. connection 
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
apparently, the nxt is designed to run on up to 10 volts. I cannot remember where I read this. I believe the rechargeable batteries that actually are made for the nxt are 10 volts.
Not sure cause i mean it uses 6 battery they are 1.5 v each that would equal 9 volts there possibilities that can go up to 10 v. Not sure there little risk there you're choice I go by my experience on other things that Ive done.
Well, I did some research. It is charged by a 10v dc charger. I am guessing this indicates the maximum voltage the battery can be. Although the nxt works on nine volts, it probably can withstand a little more. Source: http://www.amazon.com/LEGO-NXT-Rechargeable-DC-Battery/dp/B003VT952G
makes sense kinda but sure you could be right.

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