This is an interesting thread. Am I right in assuming that the energy contained in a capacitor would bare a linear relationship to the voltage across it so in order for a capacitor to discharge some of its energy the battery voltage would have to sag? If so to half discharge your capacitor, battery voltage would have to sag to half which would require a huge current draw. What would be the leakage current across such capacitors? Could they be left sitting across the battery bank when the car is off so that they don't unduly stress the batteries on start up? Surely the initial connection of the capacitors to the battery would need to be via a resistor to limit the current.