Well the average car battery is 100 Amp hours. Now a coulomb is one amp x 1 second. So a 100AH battery has 100 x 3 600 (seconds in 1 hour) – that’s 360 000 coulombs. Actually you should multiply that by 12 as it’s a 12 Volt battery, but let’s disregard this as we hope our capacitor will be rated at 12V as well. A one Farad (1F) capacitor holds one coulomb of charge (again multiplied by 12 if it’s a 12V capacitor), so you would need 360 000 farads of capacitance to equal the battery. As the usual 1F supercapacitors you get are rated at 5.5V, you’d need 3 of them to give you one coulomb at 12V or more. 3 x 360 000 is a fair bit over a million. The 1F 5.5V capacitors I have lying around are about half a cubic inch (I’m using inches for an American audience). So you’d need half a million cubic inches to store them – that’s a bit over ten cubic yards, or a bit under 10 cubic metres. So you’d need a big trailer to store them in! You can get bigger ones but even if they take a tenth the space they’d still need 1 cubic meter. You might just get them in a station wagon.
The other disadvantage is that your battery will hold its voltage until it’s down to 10 or 20 percent of full charge. Capacitors are more linear - by the time you’ve used 50 percent of the charge on your mega capacitor it will be down to 6 V and won’t be much good for turning the engine over.
So my advice, John, is stick to batteries for the moment! Supercapacitor design is progressing all the time, however, so watch this space….
