It looks as though you are talking about powering the car and only have access to a 2s lipo rx pack. If you look through the Tekin RS thread you will find a diagram where is shows how to splice your lipo battery into the switch leads. This is a path internal to the speed control past the power section and only applies your extra battery to the BEC circuit. By using a 2s lipo you will end up seeing 3s equivalent at the BEC since it and the main battery are inline but it works fine. I had some 240mah single cell lipos that I wired inline and have had great results. A ten minute run uses around 38mah from the rx battery.