Traxxas y-harnesses let you run dual-battery power in your 1/16 scale vehicles for increased speed or runtime with the proven convenience and performance of genuine Traxxas high-current connectors. Choose the parallel harness to combine battery capacity for extra-long run time. 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 3063x and batteries. Traxxas parallel and series y-harnesses are for use with 2/3a battery packs only.