What experience does your girlfriend's son have with RC's and in particular rechargeable batteries? Lipo batteries are a good upgrade to a RC vehicle. They provide longer run time and more power. The things to remember are they require a special charger and can be [B]VERY[B] dangerous if not handled properly. Since you are asking this question my guess would be that your new to the RC hobby. My advice would be stick with the stock NiMH battery that comes with it. Purchase a second battery and a better charger. If your unsure of what to purchase go to your local hobby shop or contact Traxxas technical support.