This is a highly theoretical question.
There are these 100kF, 2.7V capacitors you can buy online. According to my calculations, 5 of them in series would give me the energy capacity close to a 44Ah, 12V car battery (about 0.5kWh), which is convenient, because the voltage rating of the arrangement would be 13.5V.
So, in theory, I could replace my car battery with a capacitor bank that costs 10x more. The problem is that the output voltage is linear to the charge, so the voltage would have to vary from, say 3V (not lower to keep the current manageable) to 13.5V, which is well outside the operating range for car's stuff.
- What kind of circuit/equipment I would need to stabilize [3V-13.5V] to 12V?
- What would be the efficiency of the process for the high currents needed for a car?