Replace your car battery with capacitors!

[AlexanderB]

I looked what hobbyking has on offer nowadays, and found this:
https://hobbyking.com/en_us/turnigy-...case-pack.html
5Ah, and a 70C (-->350A) 10 second burst rating, and only 32 bucks. For a bigger engine, just use two. I guess I'll have to buy quickly before you guys buy 'em all.

[Enki]

I went a different route; 4s4p bank of LiFePo4 cells at 8 AH each.
This is for my daily driver/race car (daily racecar?) and weighs around 12 lbs. I went this route after looking over lead acid/cap combos as well as just straight caps.

If I were constrained for space/weight, I'd probably do a boost converter setup with a single larger iron cell and a charger dedicated to that and a cap bank just big enough to start whatever it was I'm working on; however the size of this setup is rather small at less than 6.5" x 6.5" x 6" high, with posts.

With my recently added requirements of being able to run the water pump and fans with the engine not running, caps no longer made sense.

[redpoint5]

Originally Posted by AlexanderB

I looked what hobbyking has on offer nowadays, and found this:
https://hobbyking.com/en_us/turnigy-...case-pack.html
5Ah, and a 70C (-->350A) 10 second burst rating, and only 32 bucks. For a bigger engine, just use two. I guess I'll have to buy quickly before you guys buy 'em all.

Not sure that would be a good fit for 12v vehicle battery replacement. The reason people choose LiFePO4 despite the lower energy density is that a 4s configuration very closely matches the voltage of the lead-acid battery and alternator output of a vehicle. Not only that, but it's more tolerant of the non-managed charge/discharge cycles that the battery would be exposed to.

You would probably need to build a charge circuit to run the battery you listed, and it might be damaged by the low voltage since it wants to be at 15v.

[redpoint5]

:0)

[Enki]

Originally Posted by redpoint5

The reason people choose LiFePO4 despite the lower energy density is that a 4s configuration very closely matches the voltage of the lead-acid battery and alternator output of a vehicle. Not only that, but it's more tolerant of the non-managed charge/discharge cycles that the battery would be exposed to.

This is the other reason I went lithium-iron, and got a 270 amp underdriven alternator internally regulated at 13.8v hot. Ancillary testing by another person with an 18650 iron pack on his car showed that the overcharge current was negligible at the factory alt/ecus 13.8 float voltage, so it all makes sense for me to go that route.

[AlexanderB]

The battery, between 17.6V full and 12V empty indeed averages at 14.8V and thats a little high for the average car.
I wouldn't mind modifying my alternator to charge to 16 volts instead of 14, running a slightly higher voltage would also increase the performance of the fuel pump and other components, my only worry would be the lightbulbs, though those are cheap, easy to replace, and I could always hook up a dc-dc converter for stuff that really doesn't like 16V.

3 lihv cells (13v full) is just a little bit too low, unless I modify the alternator to put out less than the full 14+ volt, which would then further reduce the power going to the fuel pump, which in my application would be bad, not better.

LiFePO4 is a good straight swap for a car battery, but they're either pricy or not very good so you need more of them in parallel and then they're still pricy, or you need a cap bank thats pretty pricy too.

Oh, and balancing would either be done by just balance-charging it every now-and-then, or by building a little balancing board to permanently connect.

[freddyzdead]

I guess we've drifted a bit off the original topic; me, I've only just arrived, and am all set to go as soon as the bits I've ordered arrive. I've bought 6 of the Chinese 500F capacitors and a 7.6 Ah LiFePo4 battery, which is internally arranged already to charge from a system designed for lead-acid. It cost me $AU99, which is more than I wanted to pay, but I don't have to modify it, it should just work. 380CCA, I think that is more than enough for my little 659cc 4-cylinder engine. I could have got a smaller one for $20 less, but I'm trying to do a proper job with this, so, what the hey.

I am quite concerned about balancing for the capacitors. Although the idea of using LEDs for this purpose shows some inventive thinking, the result is much less than ideal. The conduction knee for LEDs is not very steep, and they start conducting way before you need them to. If you pad them with diodes, the top end goes out of range. When they are really needed to perform, it seems that they cannot drain away the excess voltage quickly enough. I'd probably try zeners, they have a sharper knee. Whether balancing is even necessary in this application is another point. In my experience, obtaining components that are all from the same batch usually means they are pretty well matched. But these capacitors are specified at +/- 20%. If you happened to have one on the high side and one on the low side, you could be in trouble. I don't really know what happens if you over-voltage one of these, but I assume you get a dielectric breakdown, which could, I suppose, develop into a full-blown short. That wouldn't be good. Somewhere back in this thread, I recall someone mentioning something about voltage drops increasing the nearer you get to the negative terminal. What? As I recall, Kirchoff's law says the current is the same in all parts of a series circuit. So that can't happen. Ok, so anyway, I went ahead and ordered a balancing board so I can stop worrying about it.

Originally, I was thinking of trying a 4s arrangement of Li-ion cells (I've got lots of 18650s lying around), realising that they won't fully charge, but maybe that's not such a bad thing; you won't get their full capacity, but you should get more cycles. I would put them in the passenger cabin to save them having to put up with the nasty engine compartment environment. The car would be relying completely on the capacitors to start, and I have no idea what kind of cycle life I'd get from the Li-ion cells. But the battery I bought makes all that obsolete. I'm hoping I can install these things and forget about it for the life of the car. I'll let you know what happens when everything arrives.

[cts_casemod]

The good thing about super caps is their ability to work with any voltage, unlike a lithium battery that has a defined soc for a given capacity.

6*2.5V = 15V. Since most vehicles charge at 14.4V, I would personally find 4% headroom slightly too risky.

If instead you choose a 8 stack super-cap bank, each can be kept under 2V and a very simple, yet effective, balancing mechanism could be a standard 2.4V Zenner.

The point here is to know the knee. A 2.4V zenner starts conducting at 2.2V or below, requiring a system voltage of 13.2V or below under a 6cap arrangement (So it be on and likely burn during charging) Bump that into 8 and it will remain in the off-state well into the 17.6V. If a unbalance occurs, the cap would be kept at 2.2-2.4V - a perfectly safe voltage.

There will be a reduction in the available energy of the capacitors under the lower voltage, but reliability will be very high.

[freddyzdead]

There will be a 25% reduction in capacitance, which is already suffering from series string disease. I don't know that it really hurts them if they occasionally nudge a little over 2.5 volts. The weak link is the dielectric layer, which in this case is very thin. Excessive voltage will cause a breakdown, which may or may not be self-healing. One day if/when I have a spare one that I don't need, I will run some tests. It certainly would be useful to know what is the failure mode of these things.

Connecting a series string of zeners directly across the capacitor/battery is a bad idea. A transient spike, perhaps from the starter motor, could zap one or more of them, resulting in disaster. The proper way to do it is to have a transistor across each capacitor, turned on when the zener starts conducting. Probably cheaper and safer to just buy a balancing board. The LED idea is appealing because of its simplicity, but it doesn't really do the job, and can't dissipate enough power to drain away the excess charge quickly enough.

[freddyzdead]

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