A capacitor is not a "battery" at all. Think of a capacitor as the resivoir behind a dam. Power comes into the capacitor and it "fills up" with a charge. Then it's output is regulated at a consistant voltage.

Now, when there is a higher requirement for power on the output side that is not being delivered by the input side then some of the reserve power is sent out to accomidate that need.

On the same token whenever there is a surge of power from the input side the "reservoir" absorbs the surge and still only lets a regulated voltage out to the unit you're powering.


So if you put a cap on your fuel pump it wouldn't really help much unless you're getting horrible voltage spikes or extreme underpower to the pump. I'd say don't waste your time.