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I recently watched this video on YouTube which basically shows the guy replacing a larger capacitors with some smaller capacitors which he had bought. The larger capacitors were actually in place of a worn out battery. Both capacitor packs were giving him the needed voltage to start his vehicle and appear to be working quite fine.

My questions are:

  • How can a capacitor work in place of a car battery?
  • How does a capacitor work in the first place?
  • Are there any modifications you have to do in order to use a capacitor instead of a battery?
  • Would there be any benefits to using a capacitor over a battery?
  • It appears in the video the capacitors are charging right back up to 14.1vdc. How long will a capacitor hold it's charge (in general terms - I know this is dependent upon the parasitic drain put on it from the vehicle in the first place)? I guess the better question is, will the capacitor hold enough charge over time to support starting the vehicle?
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Yes its possible to start a car with Ultra capacitors. These caps don't appear to have a cycle life that the traditional lead acid batteries have. The ability of these caps to provide starting current is very good. There are a couple of snags:

  • The caps at present are more pricey than the best lead acid batteries.
  • The caps are strung in series to get the nominal 12V which is fine when you use 6 of them rated at 2.7V each.
  • Don't use them on old cars with a generator and electromechanical regulator because their voltages simply are not accurate by modern standards.
  • It would be wise to include some balancing electronics to keep the voltage on any cap below 2.7V.

This is all not too bad so far but this is why I have not done this yet. The ultra cap has brilliant power density but bad energy density compared to the lead acid. It's like comparing strength to endurance. Modern cars that have decent charging systems are likely to have higher sneak currents that occur when the key is off. In other words you would be more likely to find that your car won't start after not being used for a couple of weeks. However charging them fast is easy.

  • Could you please qualify this statement? "These caps dont appear to have a cycle life like the traditional lead acid battery does." Great answer otherwise – Zaid Feb 7 '16 at 13:18
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    Because these caps can be and are designed to be discharged and charged very fast people built test fixtures that ;Cycled the caps " There does not appear to be a degradion of the caps thats related to the applied number of cycles which was very high ,In other words in a car they should outlast the lead acid battery and probably the car , – Autistic Feb 7 '16 at 20:54
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    FYI, standard English punctuation rules call for marks like . , ; : ! ? to have a space after them, and no space before. It's difficult to read your post the way it is. – Nate Eldredge Feb 8 '16 at 22:34
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    I will try to learn to post more clearly .My screen name is not entirely innaccurate – Autistic Feb 8 '16 at 22:57
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    @ Evren Yurtesen .To start a car POWER is what you want .You dont need much energy .The caps have the power due to thier low esr .If you size the caps to start the car the caps will be lighter .Many people have done this .BUT if you left lights on the caps would be terrible .The low energy means that you should not run stuff without the engine running . – Autistic Mar 14 '17 at 20:37
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How can a capacitor work in place of a car battery?

Barely : )

How does a capacitor work in the first place?

Capacitor works by holding electric field between electrodes, unlike lead-acid cell which stores energy in chemical reactions between electrolyte and plates.

Are there any modifications you have to do in order to use a capacitor instead of a battery?

Battery is great at stabilizing voltage, capacitor just holds any voltage you connect it to. It's basically a very small battery (in terms of capacity) but very powerful (in terms of peak current). If your car can live with widely changing voltage (or if you put enough capacity to never discharge below 80%) there are no modifications needed.

Would there be any benefits to using a capacitor over a battery?

No. That's why we're not doing that.

It appears in the video the capacitors are charging right back up to 14.1vdc. How long will a capacitor hold it's charge (in general terms - I know this is dependent upon the parasitic drain put on it from the vehicle in the first place)? I guess the better question is, will the capacitor hold enough charge over time to support starting the vehicle?

It's merely a matter of putting enough capacitance. If the central lock stand-by or self-discharge drain it too low, you just need more of them, so they'll last longer.

Now, let me explain why lead acid cell is so great. You just connect it to a circuit - and that's it. Whenever voltage in the circuit is higher than given voltage dictated by cell chemistry, a lead acid cell will drain it and store the energy (when it's fully charged, it'll just dissipate it as heat). Whenever the voltage drops below, it'll give out the energy. It keeps the voltage more or less where it likes it and keeps pretty steady voltage across almost all of it's capacity. So we pack them in batteries (in the meaning of row) that like about 12V total. You can say that lead-acid is self-regulating.

Capacitor on the other hand, is much more straightforward. It just holds any voltage you apply to it, and when it reaches your voltage it stops. Then, when you start discharging, it drops. Because the voltage of capacitor is pretty much proportional to the charge inside. Roughly speaking half discharged = half the voltage. That's absolute nightmare if you want to use capacitor as your primary power supply because you either need to work on any voltage from max to nearly 0, or put some power converter that will boost the voltage to keep it at steady level.

If you want to save weight in a racing car, then just put the smallest battery that will keep the engine running. You can charge it before every race, so it can be several times smaller than in a road-going car. //edit: If your race car has classic ignition system, it can be replaced by a magneto which ultimately removes necessity of any battery once the engine is started. Have a look at aircraft piston engines, they use magnetos because of reliability. And they're pretty light, too : )

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A typical "boost pack" of capacitors as shown in that video is 350 F (farads)

Car batteries are rated in Ampere Hours (Ah), converting units:

1 Farad = 1 Amp-second per volt

There are 3600 seconds in an hour, so at 12 Volts:

1 Ah = 3600 /12 = 300F

Even a cheap car battery is around 60 Ah ... your capacitor boost pack is about 50x lower capacity than the battery !

Good luck starting the car if it needs more than a second or two turning the engine over...

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    And that is the crux - if you only need to start once and the engine starts instantly then no issue, but if you need to turn the engine over for longer periods - to draw fuel though for example then you can run out of energy. – Solar Mike Mar 14 '17 at 15:36
  • it's even worse than this -- 6 350-farad capacitors in series only have 350/6 ~ 58 farads of capacitance at 12V! (I still think having a capacitor pack in parallel with a battery is a potentially good idea) – costrom Sep 28 '18 at 15:09
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I'm an engineer, so...

Short answer: It depends.

Longer answer?

Well, there are a couple of really important things missing in the above answers.

-> How can a capacitor work in place of a car battery?

The first thing that I notice here is that everyone is assuming that the capacitors are only being used to start the car. A car battery serves 3 main purposes. Starting the car (high current, short duration), running equipment within the car while it's running (generally low current, but also very low efficiency required because the alternator takes over for most of it, and uses the battery mostly as an additional regulator), and, finally, keeping the "passive" (as in, not required to run the car) systems like the clock and the lights working when the engine is off (low current, long duration).

A capacitor is fine for the first two, but really sucks for the third (and often most important) one. Ever turned off the engine and needed the lights inside or out? How about turned on the car and had all of the stored channels on your radio honked up and the clock wrong, or the other settings that you've set suddenly not work because you changed batteries? Major PIA.

-> How does a capacitor work in the first place?

A capacitor works by having a dielectric (a substance that insulates/prevents electron flow) positioned between two electrodes (wires of some sort). As the electric potential (the number of electrons "attracting" to the wanting protons on the other side) builds up, the capacitance grows to the limit of the device (its "capacity"), then it either stops working or, more likely, explodes like a lipo battery. Think of it like a jug of water at the top of a water fountain... you can put only so much in before it overflows, but it's very fast and easy to refill when it gets low or empty and it can be emptied very quickly.

And, everyone forgot to mention leakage. All energy storage devices (battery, capacitor, angry wife, whatever) leak the energy that is initially stored. Some faster leakage (angry wife), some slower leakage (battery). A lead acid battery can be expected to hold a charge WITH NO DRAW AT ALL for several weeks or months in the best cases. A capacitor won't last anywhere near that long, a few days or weeks at best. The wife just leaves, so that's almost zero, but then she's someone else's problem so no worries.

Leakage in a capacitor is the result of electrons "migrating" through the dielectric. No matter how good the dielectric is it's still not a PERFECT insulator, so it WILL leak.

The past 40-50ish years of work on super capacitors is around making this dielectric almost perfect and figuring that we can work on surface area later. But, just like how a straight piece of wire still presents SOME resistance (and thus gets hot), the same is true in reverse for a capacitor. You want the dielectric to be almost zero in thickness, have maximal surface area and have an infinite resistance. But, once the distance gets down to even nanometers (not to mention, femptometers -- 1/1000th of 1/1000th of a nanometer, which is probably what's required for a true super capacitor), physics starts to go a little crazy. Quantum tunneling gets involved and things get more complicated. To give you an idea, a single hydrogen proton is about 1.7fm... and an electron is A LOT smaller than that.

So, this question and its in depth answer is a lot more complicated than it initially sounds.

  • Hi Kevin. Great answer. The only thing I'd like to add to your statement of "When the electrical potential builds up...". If the capacitor were rated for say, 25 volts, the potential in the automotive charging system would never reach this level, so it wouldn't be an issue. – Dacid Salin Sep 30 '18 at 6:32

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