Replace your car battery with capacitors!

[Isaac Zackary]

This is the self discharge rate I came up with for a 6 pack of caps to fall from 14V to 12V. Obviously a fall from 14V to 10V would take twice as long.

FCell mA 2V change
310 .45 63.8h or 2.7 days
350 .3 108h or 4.5 days
650 1.5 40.1h or 1.7 days
1200 2.7 41.2h or 1.7 days
1500 3 46.3h or 1.9 days
2000 4.2 44h or 1.8 days
3000 5.2 53h or 2.2 days
3400 18 17.5h or 0.7 days

I calculated the number of hours or days based off of Maxwell's data for their capacitors. Of course in the footnote it says "After 72 hours at 25°C and rated voltage. Initial leakage current can be higher". So at first it'll discharge quicker. But after that, at lower than rated voltages the leakage current should be less.

[Enki]

The thing is, you can't use the raw Maxwell data to determine how bad leakage will be; at some point, leakage will nearly stop.

See this to know what I'm talking about:
http://ecomodder.com/forum/showthrea...tml#post432060

Also, my bank has been sitting untouched for months and probably still has enough in it to start just about anything.

[Enki]

Just checked, my bank was sitting at 12.6 volts, whereas my spare Braille AGM was sitting at 11.7. Oops. Charged the bank up to 14.25 volts and I'll check when reminded to on here.

Really, though, it's the first charge that's terrible with leakage. After that, it drops significantly, so in a car application, it shouldn't be too bad at all (especially considering it will be constantly recharged while driving, thus further reducing leakage).

[Isaac Zackary]

Originally Posted by Enki

The thing is, you can't use the raw Maxwell data to determine how bad leakage will be; at some point, leakage will nearly stop.

See this to know what I'm talking about:
http://ecomodder.com/forum/showthrea...tml#post432060

Also, my bank has been sitting untouched for months and probably still has enough in it to start just about anything.

That's what I was getting at. Even going by the raw data worse case scenario they wouldn't leak all that bad. The worse ones (3400F) would theoretically take over 34 hours to leak down enough not to start the car. And in real life it will take longer than that (unless the car's drain is significant, but not with my old clunkers!)

I'm thinking of going with the 3400F caps. That'll run me about $400 per car from Mouser.com. Not bad given the longevity. Buying a $150 battery every 3-5 years isn't as economical in the long run.

[redpoint5]

Originally Posted by Isaac Zackary

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.

I would certainly build a bank of 6-series connected 350f or 600f supercaps to help with cold starts in those extreme temperatures. I've started my TSX with just a bank of 350f supercaps that were charged to 10v. The rate of self-discharge below 13v is very low. The parasitic drain of the electronics of the vehicle will exceed the self-discharge rate of the caps.

LiFePO4 cannot be charged below freezing temperatures, so it makes them a bad candidate for your cold climate.

You might be able to use a solar panel to maintain the charge of supercaps as long as they have a balance circuit or if connected to a lead acid battery that will drain the excess charge. Without a means of bleeding off excess charge, supercaps can very quickly be over-volted.

Originally Posted by Isaac Zackary

This is the self discharge rate I came up with for a 6 pack of caps to fall from 14V to 12V. Obviously a fall from 14V to 10V would take twice as long.

FCell mA 2V change
310 .45 63.8h or 2.7 days
350 .3 108h or 4.5 days
650 1.5 40.1h or 1.7 days
1200 2.7 41.2h or 1.7 days
1500 3 46.3h or 1.9 days
2000 4.2 44h or 1.8 days
3000 5.2 53h or 2.2 days
3400 18 17.5h or 0.7 days

It would take many times longer for self-discharge to go from 14v to 10v compared to 14v to 12v. I would say months. The self-discharge is fairly high near the voltage limits and drop exponentially as the voltage drops. I've got a bank that I charged up about 4 months ago and the LED balance circuit is still giving off a lot of light.

I don't think self-discharge is much of a consideration.

I'm a fan of the kit on Ebay for the 350f caps that comes with the printed circuit board and balance hardware. It was fun to assemble and reasonably priced at the time I bought.

[redpoint5]

Originally Posted by Isaac Zackary

I'm thinking of going with the 3400F caps. That'll run me about $400 per car from Mouser.com.

Are you going for battery elimination, or just supplementing the cranking amps of the battery in the vehicle? Generally I would say running capacitors only is a bad idea. If even the glove box light is left on you will likely be unable to start the car. You also can't listen to the radio for extended periods of time with the engine off.

If going with the BCAP3400, you might consider running just 5 series caps instead of 6. They are rated at a slightly higher voltage than all of the other caps. Using as few caps as possible in series will increase the overall farad capacity. 5-series connected 3400f caps have 680 farads of charge, while 6-series connected caps have 566 farads. That's close to a 20% difference in capacitance, and it costs more to purchase the 6th capacitor.

I bought 5 of the BCAP3400 to eliminate 2 large batteries from my truck. The truck has a very high cranking demand of over 400 amps, but has an extremely low parasitic draw of just 5.7 mA just to maintain the presets on the radio. It would be almost zero if the radio were removed. I also have a solar charger to maintain charge after long periods of sitting unused.

If you're just going to supplement the battery in your car, go with 350 or 600 farad caps. The kits make them easy to assemble.

[Enki]

FYI my bank has discharged itself to 14.1 volts over approx. 51 hours; this comes out to 75 amps or 0.4084 milliamp-seconds of self discharge. If I charged it to 14.25 again, it would probably drop further and be closer to zero.

[Isaac Zackary]

Originally Posted by redpoint5

Are you going for battery elimination, or just supplementing the cranking amps of the battery in the vehicle?...

If going with the BCAP3400, you might consider running just 5 series caps instead of 6. They are rated at a slightly higher voltage than all of the other caps. Using as few caps as possible in series will increase the overall farad capacity.

I guess I haven't quite decided yet quick way I want to go. I know what it's like to have to be careful of not letting even a small light stay on. (That's because I had been running a bad battery for almost a year. It would barely start the car only if it was fully fully fully charged.)

I understand how the capacitance changes with the addition of more capacitors on a series. If I do get the BC3400's I'll consider only getting five.

[cts_casemod]

The alternator will most likely be subject to a bit of s strain while charging a capacitor from 9V to 14V, for example. It's a 'dead on' short circuit.

I currently have 12.8V LiFePO4 battery on my car and even that ended up blowing the bridge rectifier due to the CV charge employed.

I say a hybrid would be the best approach. Say 10Ah lithium battery than can be had for $60 coupled with the capacitors to aid on starts. Probably a 200F would suffice.

I like the idea of a small solar panel to trickle charge... never used use, but I'm seriously considering it.

[oldtamiyaphile]

Originally Posted by cts_casemod

I currently have 12.8V LiFePO4 battery on my car and even that ended up blowing the bridge rectifier due to the CV charge employed.

How long did you get out of if? I've got 14,000km and roughly 7300 starts on my supercap/ lead acid combo so far (from new).