Battery vs Capacitor Bank

I'm thinking of building a capacitor bank in lieu of using a battery in the Cadavalier. I'm tired of buying a battery for it, lugging it to the car, and then bringing it home with me just to let it rot away for months, weeks, or years at a time before buying another battery to use next time I want to go play with the car. There are no electronics to run in the car that are not necessary, so there should be very little drain.

I figured running some supercapacitors in a series that equates to 12-15v should do just fine and won't deteriorate sitting in my office. It would be a nice emergency tool as well.

Any thoughts on the matter? Anything I should be worried about?

They do that for magneto fired motorcycles / ATV's, so you might look into that.

I tried one on my old Honda motorcycle (the only use of the battery is to keep the tail light on when the engine is off) and it kept blowing out the tail light bulb. So... not sure why it didn't work.

No lights on this car, so that is one less thing to worry about. It will be a project to try out this month. I guess I'll get to ordering the parts.

FWIW: pretty sure a bank of capacitors is still a battery. I know the language has evolved so battery now commonly refers to individual electrochemical cells. Evidently the first electrical use of battery in fact referred to a bank of capacitors, oh well.

What it will come down to is which will better fit my needs if a bank of supercapacitors are safe. Fortunately I won't have to use leyden jars. IIRC the first batteries were capacitors, but batteries have evolved tremendously. I don't need the battery to hold charge for more than a few hours after use. I also don't need it to dump the power when I hit the starter button, although I could jump the car and let the bank sustain the charge...

I think it would be an interesting experiment. Got specs on the super-caps?

I'm not sure capacitors have the energy density required to start a car without dedicating a space larger than a car battery. Why not a lawnmower battery and a float charger? That will start a cavalier and only set you back $50 or so.

That said, it could be a neat experiment if you can get the parts cheap (surplus on ebay?).

https://www.youtube.com/watch?v=z3x_kYq3mHM

https://www.youtube.com/watch?v=8miq6sDy0wA

I guess a battery tender is probably the easiest and cheapest solution, but supercaps are probably more fun and are definitely more super.

What defines a supercapacitor? Most capacitors I have worked with can hold plenty of voltage (like 250, 500, 1000v, etc) for an automotive application, but not that many electrons. This suggests you would need a big capacitor or a big pile of small capacitors in parallel.

While a big bank of capacitors might be able to provide the charge to start the car, I'd be worried about the charging system and the rest of the electricals continuing to work the same way after the car is running.

Also, make sure you fuse that sucker, batteries have internal resistance that greatly limits 'runaway' current in the case of a short. Capacitors can self discharge MUCH faster than batteries, nearly instantly loading a whole ton of energy into 'something'.

Farads = Coulombs/Volt (Coulomb = 1 amp*sec). So, if you need 300 amps for your starter, for 10 seconds, you need approximately a 250 Farad capacitor. (This is a huge capacitor by normal capacitor standards, I think the biggest I've seen was 1 or 2 farads and it was the size of a football).

the energy in that cap is 1/2 * Farads * Volts^2, or 18,000 Joules. using kinetic energy comparison, that is about the kinetic energy of a 3000 lbs car moving at 8 miles per hour. If you dropped a car off a cliff, it would be moving at 8 mph after about 1-2 feet. So, the energy in your cap will be about the same energy as a car has after free-falling for 1-2 feet. You probably don't want that car landing on your foot. Just be careful.

bigdaddylee82 wrote: https://www.youtube.com/watch?v=z3x_kYq3mHM https://www.youtube.com/watch?v=8miq6sDy0wA

Cool. Those are awesome! Apparently my last post is a bit dated....

Just unhook the battery from the terminals???

Batteries and capacitors are two fundamentally different things.

Battery: Voltage depends on the chemistry (lead-acid, Lithium, etc.) and if charged or not, nominally 12V.

Capacitor (super or otherwise) Voltage depends on how much current you have dumped into them. Ranging from 0V to whatever maximum they will withstand.

There was a discussion about this a while back. A few links were posted of a guy on youtube who made this work for his DD. If I remember correctly he used a series of D cell sized capacitors. The whole assembly was considerably smaller than a battery.

Found it: https://www.youtube.com/watch?v=z3x_kYq3mHM

I found that video earlier and it was enabling me. The comments worried me, so that is what brought me here. The specs would be similar to his. So it comes down to that, or as Kenny had mentioned, a small battery.

Yamaha, the last few batteries I had sitting around eventually stopped working. One was my fault because I'm a dummy.

I use these and normal batteries in my neglected projects. No problems.

If you're really worried, add a solar panel. Search ebay for "VW solar panel" for surplus galore.

In reply to blizazer:

The battery comes home with me and goes in the shed. The car sits in a field and deals with whatever weather conditions the Midwest can throw at it. The battery may sit for a month or a year with nothing connected to it.

Although after consideration, I am thinking leaving leaving it out there may just be a simple option.

Capaciters leak down too. They don't sit there holding a charge infinitely. They also aren't light.

I'll agree with some others above, a $25 battery tender Jr would take care of your storage problem.

I know they leak down, but the overall life is tremendously longer than a battery. I thin the cheap battery will be my best choice as well.