I don't think you would gain anything by putting a big capacitor in parallel with the battery...you'd still have to charge it (the capacitor).

IIRC, the point behind the big caps is to give a little power reserve for some of these guys with the big amps and big subwoofers...you know, the guys you can hear 1/4 mile away. It is, I think, supposed to keep the amp from going into distortion on those bass peaks. Of course, half the cars that have those amps rattle so much on the peaks, it probably doesn't matter!

There was about 40% waste in my calculations, but even if it's a lot more I can't beleive it's 1400%.

During those bass peaks, the amp pulls a LOT of power. If the battery and alternator can't keep up with those spikes fast enough, a capacitor is supposed to help.

Caps in a hybrid are allowed to have their voltage go up and down a LOT as they absorb regen energy and then give it up during acceleration. Those voltage swings would not work on your 12 volt electrical system. The big power amps draw 2000 watts at times on the 12 - 14 volt power line that translates to 166 amps surges so the BIG farad(s) size caps provide that surge to make up for the loses that many amps put on the wires going from the battery to the amp and also for the batteries inability to provide rapid load response to the amp draw. The Ultra caps work great for quick stops and starts but don't really handle long hill energy levels which can really charge up a battery in even a Prius fully so Cap are not the ultimate solution.

i read headlights=2hp somewhere maybe it was wrong
but what your basically saying is theres a possibility that it could be .3 hp

set your scangauge to GPH or HP (if you have it set up for that) and let your car idle. cut the lights on and then off to see what the difference is. that will tell the tale.

if you don't want to waste gas to do this, wait until you are at a stop light. this is also good because you will be at operating temp and in closed loop. i did this and there was a very little difference. I think the HP gauge showed a few tenths difference. I did do this a while back and several have wondered about the accuracy of that X-gauge on the scangauge but it at least told me that there is some difference but not the huge difference that most have talked about. also, at idle, my car only produces 5-6 HP. please remember also that it is only running 600ish RPMs as well.

I think mine was .05gph difference with the lights on and off.

Caps are physically a simple device. It's just conductors with an insulator between them. I was looking into DIY Tesla coils a few years back and found some people used nested yogurt cups with aluminum foil in between as caps. Granted these were not being use with 12V, more like 800-10kV AC.

As to the benefits of a big cap on a car, I really don't see it improving anything, unless it was hooked up to some sort of specialized charging device and or controller.

A cap takes power, and then can give it back quickly (with loss, like any device). So your not saving energy, unless the power to charge it up would otherwise be waited.

All and all, i think, a cap would only make improvements on a electrical system which had large power spikes (ether in usage, or charging).
The only applications i can think of would be electric motors, and HHO devices that were on complex controllers.

I think the question is, do capacitors store and release energy more efficiently than batteries...

it depends how long you need to store it, and how the current will be drawn.

Oh, thought of an application! for people who turn off there cars in N. A cap for restarting would likely be quite useful.

What I'd like to see is a capacitor big enough to REPLACE the battery in either an electric or a hybrid car. Would have to be a pretty dang big capacitor...but it might get rid of the charging time problem...