Interesting article about the relative potentials of batteries versus capacitors.
In essence, they are good for regenerative braking where cars go from 60 to zero, and usually waste that as heat with some recharging the battery. Gradual slow down braking wastes less heat and more of the braking power is used for battery recharging.
In affect a capacitor is like a spring. Capacitors also decline in voltage as the charge is consumed since the charge equation is Q = 1/2
CV*V where charge Q in coulombs, Charge C in Farads and Voltage V in volts.
That said, there is a niche for greater braking efficiency with supercaps.
https://interestingengineering.com/could-ultracapacitors-rep...