Only Traxxas Y-harnesses let you run dual-battery power for increased speed or runtime with the proven convenience and performance of genuine Traxxas High-Current Connectors. Use the series harness (#3063) to combine battery voltage and unlock the 50+mph speed potential of your model. Choose the parallel harness (#3064) to combine battery capacity for extra-long run time. Dual-battery operation requires attention to precautions outlined in the E-Revo VXL manual. Follow all precautions to assure peak performance and reliability of your model and batteries. Traxxas parallel and series Y-harnesses are for use with 2/3A battery packs only. Required Parts Installation between batteries and ESC