Replace your car battery with capacitors!

[Bicycle Bob]

Stealth RV is finally getting some wrench time. I'd like to install about 60 AH of deep-cycle capacity for life in camp. Would it make much difference to get a battery without enough cranking capacity for a 1 litre, and use caps to start it? I've got a handy little DC-DC voltage converter that could probably charge caps even from a low battery.

[Enki]

It should work, but make sure you have enough cranking capacity on the cap bank to not drop more than about 3 volts or so; this way you don't drain the caps further than the battery voltage, and wind up killing the battery anyways due to the still relatively large hit it will take.

Ideally, you'd want the caps isolated from the battery with some kind of charge controller to prevent overworking the battery (since the caps act like a dead short).

[Bicycle Bob]

Yes, I'm sure I could make it work. However, since I want a heavy battery anyway, is there much point in choosing one that can't handle moderate starting loads? Very cold weather will sometimes prevail.

[Enki]

Generally speaking, the higher rating amp hour deep cycles will have enough oomph to turn over moderate draw engines on start; that said, if it gets really cold where you are, you could always build a jump box like what I have for jump starting other vehicles since my car basically has a motorcycle battery in it and can't really handle that task too well on it's own.

The only time I think you'll need a hybrid setup with lead acid and caps is if your lead acid system isn't capable of running your highest short term load (starting the car) most of the time, but is properly sized for your long term low drain loads. In that case, you should endeavor to figure out how much your engine requires to start, and try to build a bank that has a farad rating of Amps / 2 (assuming no current limiting from battery to cap bank).

If I were in your situation, and since I know my car requires 500 amps to start, I would design a bank that has a farad rating of no less than 500 / 2 or 250 farads; this way I know my system voltage won't drop below 13.8 - 2 = 11.8 volts during start, thus minimizing the drain to the battery as much as possible without having a massive cap bank taking up space as well.

I have a couple ideas on a higher voltage cap bank setup (14) with a lower voltage battery setup (12.8) where the cap bank is smaller and isolated from the battery until start, but I really have no way to know if it will work or is even possible.

[redpoint5]

Without knowing exactly how stubborn the RV is to start on a cold day, I would still venture to say the capacitors are not beneficial enough to justify their expense or build time. If the RV can be reliably started without causing excessive damage to the batteries, then the capacitors aren't much benefit.

That said, I have had a lot of fun experimenting and learning about super caps. I even had a neighbor that needed a jump due to a dead battery, and my supercap bank easily got them going.

I don't see much point to using supercaps unless vehicle parasitic load is low enough to run only supercaps, or weight can be saved by using a severely undersized battery paired with supercaps.

Update on my projects:

My plan for the Prius when the original battery fails is to replace it with a small LiFePO4 pack since it lives in the passenger compartment where temperatures are less extreme and peak demands are low.

My plan for the Acura when it's battery dies is to build a larger capacity LiFePO4 bank and run an alternator kill switch, recharging at home.

Both plans will not require the use of supercaps.

Finally, despite my instruction to my dad to let me build a supercap solution for the diesel truck, he dropped several hundred dollars on 2 new batteries. I'm still angry about that since I had already spent $200+ on building the supercaps/balance circuits/solar maintainer.

I'll never buy another lead-acid battery again in my lifetime.

[oldtamiyaphile]

Originally Posted by redpoint5

I'll never buy another lead-acid battery again in my lifetime.

If only. Chances are that if you buy a new car, it's charging system will be optimised for Pb cells, and anything else will cause issues. Most new cars have 'smart' alternators that look for charge acceptance and LiFe's which continue to accept charge when full can cause the battery monitor to think the battery still needs charging

I do want a LiFe pack for my Prius though.

[Isaac Zackary]

Wow! Just read the whole thread and watched most of the videos. I had thought about doing this before. Nice to see that I can pick these puppies up for cheaper than I had thought.

Here's a couple of thoughts I don't think have been discussed yet on this thread.

For starters, (is that a pun?), where I life it gets super cold. Like -40° F (-40° C) cold. There are several problems with running batteries. The lead acid ones of course have a hard time putting out amperage at those temperatures. I'd bet LiFePo4s probably do to. Not only that, but if the lead acid batteries drain completely out then they turn to water on the inside, freeze, and can even bust open the battery! Lithium batteries normally aren't rated for that cold of weather either. So the idea of ultra capacitors that can put out full amperage even at those kinds of temperatures is really appealing to me.

But what about the self discharge rate. Solar panels with a foot of snow on top probably won't work. One idea I had was to install some sort of permanent magnet alternator that I could hand crank to charge up the caps. Another idea would be to have a battery connected by a diode to the charging system so that it only charges. I could have an emergency switch (probably with a resistor in series) to connect it to the caps if my caps drain too much.

[oldtamiyaphile]

A 350uf supercap bank capable of starting a 4 cyl car has a capacity of around 200mah, you can charge it in seconds, but not with a standard 12v car charger, a large jump starter pack would be the way to go (just because caps will suck a lot of current). That's roughly the same total capacity as two 9V smoke alarm batteries, so you don't need a big back up battery if you keep currents in check.

On the Suzuki Swift I got four starts back to back from mine before the caps were too weak to crank it.

[Enki]

The self discharge is pretty bad the bigger you go farad wise, but will level off and even completely stop after a while. I'm pretty sure I posted that specific info in this very thread quite a while ago.

[Isaac Zackary]

The longevity really interests me too. A lead acid battery is usually rated for about 200 complete charge and discharge cycles, and in real life might only last a few years. A LiFePo4 battery is about 2,000 cycles, or in real life at least 10 years. A cap is rated at a million cycles. Of course you do use more of it's cycle, but still these will probably outlast the vehicle.

Also if you completely discharge a lead acid battery just once, that takes off half of its life right there. Lithium batteries are even worse. But if you completely discharge a capacitor it doen't harm a thing.