I carrie a portable jumper pack with me in case my battery goes dead I can jump off of that instead of someone else's battery. I keep it in my trunk. When I shaved my handles, I just ran a wire to the trunk and mounted a button on the exterior of the car out of site. If my battery ever goes dead (hasn't happened yet) I can clamp the positive of the jumper on that wire and the negative on metal in my trunk, press the button, and PRESTO my door opens. You could just carry two of those 6 volt batteries used for lanterns in your trunk and wire them up together if you didn't want to buy a jumper pack. I just already had the jumper pack.
