Harness Overview: Traxxas Series Battery Wire Harness. Use the series harness to combine battery voltage and unlock the 50+mph speed potential of your model. Dual-battery operation requires attention to precautions outlined in the model's owner's manual. Follow all precautions to ensure peak performance and reliability of your model and batteries. Traxxas parallel and series Y-harnesses are for use with 2/3A battery packs only.