The unit of capacitance is a farad. Capacitance is given as Q(coulombs) = C(farads) x V(volts). A 1-farad capacitor can store one coulomb of charge at 1 volt, and will deliver 1 amp-second at 1 volt. Charged to 12 volts, the 3F capacitor built to sustain this voltage could theoretically store 36 coulombs, and would discharge to provide 36 amp-sec at 12 volts, or 432 amp-sec, or 0.12 amp-hr to a 12 volt load. Unfortunately, a capacitor's discharge voltage also drops exponentially with time to zero, depending on the load's circuit resistance.
Might be good for some dc motor-starting applications or other excessive (very short-term sub-sec) peak transient load requirements that exceed a storage battery's normal discharge capabilities, but methinks this is hardly a valid candidate for outright battery replacement.
For most applications in traction packs, it seems to me a much better expenditure would be for additional batteries instead of any "supercaps."
Just my $.01 worth, your mileage may vary...