You can not necessarily just take any battery and hook it up to LEDs and expect them to work properly. You will first need to get your LEDs and find out what voltage they are rated for... then you will need to find out what voltage the RC car's battery pack is. You will then need to (most likely) put in the correct resistor to lower the voltage from the battery so as not to fry your LEDs. I would also suggest installing a two lead on/off rocker switch inline with your positive from the battery pack to the resistors/LEDs. If you need help figuring all this out.. give us a bit more information please (voltage of battery pack, how many LEDs and what they are rated at.