If you are going to go that route, you can just get a 3s LiPo battery and wire it inline with a charger that automatically turns on with ignition. You would only need something like a 2,200 mAh battery which weights less than 1lb and should provide between 11.1V and 12.6V. Here is an example:
http://www.hobbyking.com/hobbyking/s...dProduct=10279 Just 7 ounces. Supposedly a car draws 20-100 on cold cranking and that sucker should be good for up to 110A peak (peak C rating times capacity in Amps... so 50C x 2.2A capacity = 110A of current).
Edit: Just so you know my familiarity with those... we use them to spin up electric motors for flywheels at 36,000+ RPM. They are
little 130 sized fuckers can draw up to 16 amps are are pretty beastly.
Edit again.....
http://www.hobbyking.com/hobbyking/s...dProduct=45130 That battery is better because it can provide 143A continuous and 286A peak (10 second load). Look how cheap it is too
