The Traxxas Y-harness doubles your voltage by combining two 7.2V NiMH batteries in series to unlock the 50+mph speed potential of your 1/16 scale brushless model. Dual-battery operation requires attention to precautions outlined in the model's owners manual. Follow all precautions to assure peak performance and reliability of your model and batteries. Traxxas Y-harnesses are for use with (#2925X) 7.2V NiMH battery packs only.