The Traxxas parallel Y-harness lets you combine two 7.2V NiMH batteries for extra-long run time in 1/16 scale vehicles. 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.