# Supercapacitors as car batteries

I've seen YouTube videos of supercapacitors replacing car batteries. Is this practical? And if so, why haven't they been offered in the automotive market place?

the type supercapacitor he is using has the following data sheet:

http://www.nooelec.com/files/2600f.pdf

Note that it contains 8,125 Joules of Stored Energy.

Then if you go to http://www.rapidtables.com/calc/electric/Joule_to_Watt_Calculator.htm and enter 8125 in the box and let's say 5 seconds of starting (it shd start up in 1 second in actuality). You then get 1,625 Watts. Remember 1 HP = 750 Watts , so you have just over 2 HP of starting power. Remember he's using six of them. 6 x 8125= 48,750 J. @ 16.2V.
(for a 2 sec. start it's over 24,000 Watts(32 HP) of instant power) Easily enough to start your car. Without a battery too.

A good car battery would have 700 CCA. @ 14V = 9800 Watts(13 HP). Quite a difference. (The average starter is 1.9 to 2 HP)

Here, as time goes by, I will post more videos to bolster my claim:

http://www.extremetech.com/computing/183839-new-supercapacitor-technology-could-store-conduct-power-on-the-same-copper-wires

http://scitechdaily.com/graphene-based-supercapacitors-may-significantly-boost-power-electric-vehicles/

http://phys.org/news/2015-09-micro-supercapacitor-unmatched-energy-storage.html

https://www.tecategroup.com/ultracapacitors-supercapacitors/industry_news.php

http://www.skeletontech.com/news/skeleton-technologies-launches-markets-highest-energy-density-ultracapacitor/

Bottom line is, if you have enough Farads, you have energy density.

And this really settles the matter once and for all...

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What has your research taught you about energy density and capacity of batteries vs capacitors? – transistor Mar 8 at 19:27
Very similar question a month ago on Mechanics.SE: Replace car batteries with capacitors? – Nick Alexeev Mar 8 at 21:21
Part of the problem with your new calculations is that you don't get that full energy figure unless the capacitor is fully discharged. You would have to assume some voltage drop (say 12V to 8V) and calculate the energy difference between those voltages. And knock off the repeated edits. – W5VO Mar 11 at 23:54
@TimSpriggs Most starter motors don't work too well at 0V, so quoting the full energy stored is misleading. You can only get that energy if you fully discharge the capacitor. At some point the voltage on the capacitor isn't sufficient to drive the starter motor. – W5VO Mar 12 at 0:26
back in the 1970's, when most motorcycles had a kick starter (some also had an electric start), replacing the motorcycle battery with a very large capacitor was practical. – robert bristow-johnson Mar 23 at 19:56

It is not practical. I do not know why people do this, there is no benefit whatsoever. It amounts to misuse of something useful. Simply put, those videos are by people who don't know what they are doing and are misusing supercapacitors for a bizarre and senseless application they are neither well-suited to nor even practical. And they are offered on the automotive market, just not as battery replacements, for the same reason headlights are offered on the automotive market, just not as car stereo replacements. Because that wouldn't make any sense.

The sole reason supercapacitors exist is power density. They have terrible energy density, and that terrible energy density comes at many many times the cost. The entire point of a battery is bulk energy storage. Using supercapacitors to do the thing they are the worst at instead of something that is cheap, readily available, and proven for over 100 years is... the kindest but much too weak word I can use to describe that is "silly."

Those videos exist, but just because there is a video of it doesn't make it a good idea. It isn't.

What is a good idea is using supercapacitors for the reason they exist, which unsurprisingly is the exact way they are being used in automotive applications. Batteries have great energy density, but compared to supercapacitors (or any capacitor), batteries don't even come close in power density. Beyond that, forcing a battery to provide high amounts of power is hard on it and will reduce its long term life, and the quicker you drain a battery, the lower its apparent energy capacity will be. A battery will last much longer if drained at a 10 hour rate vs. a 1 hour rate. Meaning, at a rate that will discharge it in 10 hours vs. just 1 hour. Higher power means a higher discharge rate.

This power density weakness is bidirectional: batteries are bad at delivering huge spikes of energy, and bad at accepting them. They like things nice and steady. That is where super capacitors come in. They have terrible energy density, but great power density. 99% of the time, the big power spikes demanded in automotive applications are also brief - things like braking, a burst of acceleration, the inrush current of the starter motor, that sort of thing. The only reasonable (and intended) way to use a supercapacitor is in addition to a battery, never in replacement of a battery. They perfectly complement each other. A battery deals with storing tons of energy, while capacitors deliver it at high power when needed. They permit things like capturing nearly all of the energy back from regenerative breaking, because all that energy can simply be dumped right into them and they'll handle it like champs. It can then be siphoned back into the battery at a controlled rate that the battery can deal with.

Supercapacitors can let even an extremely weak battery in extreme cold start the car, because the battery is relieved of power demands. But that weak battery will keep working and still slowly but surely recharge the capacitors and stay charged long after those video makers' cars will be dead in the water.

Long story short, they are used in the automotive industry, and the people in those videos are simply spending money to make their cars inferior in many important ways by misusing supercapacitors in a way that is only detrimental. They are not replacements for batteries because batteries store tons of energy, capacitors do not. Used in tandem, however, they are a very good match and pick up the slack in the areas the other is weak.

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Congratulations, the only one so far among the answers who knows what a car battery is. :) – pipe Mar 8 at 20:06
"Breaking" your car isn't recommended, suggest replacing it with "braking". – hobbs Mar 8 at 21:29
There's a sentence buried in your answer that deserves more attention. You said: "...forcing a battery to provide high amounts of power is hard on it and will reduce its long term life..." In other words, using a SuperCap as an adjunct to a battery can make the battery last longer, especially in "bursty" applications. – james large Mar 9 at 1:55
And if we travel too far west, we shall surely fall off the earth. – Tim Spriggs Mar 9 at 21:00
@TimSpriggs I thought he was pretty clear. There's no benefit to replacing a battery with supercaps, but there is a benefit to using both at the same time. This is an excellent and well-written answer. You comments suggest that maybe you weren't really looking for someone to answer your question. If so, I'm afraid you've come to the wrong site. :-) – Adam Haun Mar 11 at 23:14

This comparison is kilo-for-kilo.

• Lower energy density than batteries.
• Higher self-discharge rates. Loses charge over time.

• Can receive charge more quickly than batteries.
• Can output more current than batteries.

Supercapacitors are better than batteries as a source of power, but they are worse than batteries at storing energy. In the cars, supercapacitors are sometimes found in KERS (Kinetic Energy Recovery System), where they absorb a large amount of power as the car slows down, only to throw this power back into the motors seconds later. But supercapacitors can not serve as a practical replacement for a gas tank at the present state of technology.

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Car batteries are not used to drive cars, the are used to start cars. – pipe Mar 8 at 20:05
@pipe Unless it's one of those newfangled Teslas. – Nick Alexeev Mar 8 at 20:16
A lot of hybrids use batteries to absorb energy from breaking, for the purpose of putting it to use a few seconds or minutes later. Large caps could potentially be better than batteries for such a purpose. – supercat Mar 9 at 0:22

First off, it would be nice if you would post a link to the video you're talking about, so that we can look at it ourselves, and not have to guess at exactly what you're talking about.

So no, it's not practical. It can be used, for instance, as a temporary storage buffer for energy recovered by regenerative braking, but that's not remotely enough.

EDIT - With links to the video and data sheet, I can respond more fully.

First, I will grant that my response was better suited to a discussion of electric cars, so I will try to do a better job of focusing on starter and other issues.

The video is a fairly classic example of a guy who has discovered what seems to be a neat idea, but who has not gotten over the first flush of enthusiasm, and I'll start by digressing from starter issues, just to get this out of the way. This is most obvious in his comments about solar charging. Let's run some numbers. Start by making a convenient assumption: A fully charged supercap array will be charged to 15 volts, and discharge stops at 7.5 volts. With 8 kJ energy for a charged supercap, the delivered energy at half voltage is 3/4, or 6 kJ. With 6 supercaps, total delivered energy is about 36 kJ. To recharge the supercaps with a 13 watt solar panel will obviously require $$t = \frac{36000}{13} =2769\text{ seconds} = 46\text{ minutes}$$ which is not exactly "a few minutes". In fact, at a nominal 6 hours of charge time per day a solar system can support about 16 charge cycles. Given what was accomplished in the video, this suggests a very small amount of useful work can be accomplished. But that was implied by the 13 watt solar panels, so it shouldn't come as any big surprise.

Now, on to starting. Consider a supercap array of 6 caps, each with 2600 F. Effective capacitance will be about $$C =\frac{2600}{6} = 433\text{ F}$$ Also note that the data sheet lists maximum current as 600 amps. So, at start, the supercaps can put out less than a 700 CCA battery can put out after 30 seconds. Worse, assuming constant current (which isn't entirely reasonable, but it simplifies the arithmetic) the voltage across the supercap will drop by $$\frac{dV}{dt} = \frac {i}{C} = \frac{600}{433} = 1.4 \text{ volt/sec}$$ and, for instance, the supercap voltage will have dropped by 7 volts, or nearly 50%, after only 5 seconds. Since CCA is defined as the current which can be supported for 30 seconds at a cell voltage of 1.2 volts (7.2 volts for a 12 volt battery), a supercap will run out of steam after only 5 or 6 seconds, compared to a regular battery's 30.

This is not entirely to the point, since starter motors don't actually draw 600 amps. Instead, 100 amps is a more reasonable number. Under these circumstances the supercaps will lose about 1/4 volt per second. Is this an issue? I note that the current edit of the OP suggests that a starter only needs to provide power for about 1 second. This suggests that the poster lives in a warm climate and never drives older vehicles that do not start easily. For such folk, a supercap will do nicely. For the others, not so much.

There are other factors as well. How long can a conventional battery provide less-than-starting power compared to a supercap? A usable number for the supercap has already been established: 36 kJ. What about lead-acid battery? Note that the vide weighed a battery and came up with 30 lb (66 kg). The supercap data sheet linked gives a supercap energy density (actually, specific energy) of 1-10 W-hr/kg, and battery energy density of 10 - 100 W-hr/kg. The supercaps shown have a specific energy of 4.3, so let's assume a specific energy of 40 for a car battery. Actually, Wikipedia says 33 to 43, so let's go with 35. Then a new lead-acid battery of the size shown in the video will contain about $$S.E. = 35\text{ W-hr/kg}\times{66} \text{ kg}= 2310\text{ W-hr} = 8.3\text{ MJ}$$ Unlike a supercap, a battery has a nonlinear voltage vs available energy curve, with a long plateau followed by a sharp falloff in voltage as the battery reaches discharge. As a convenient number, let's say 90% of available energy is found for a voltage above 50% of nominal. This is actually conservative. This gives a usable energy of about 7.5 MJ which is 208 times greater.

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Car batteries are not used to drive cars, the are used to start cars. – pipe Mar 8 at 20:04
@pipe: Hybrid cars' batteries are used to provide propulsion; sometimes for long periods, and sometimes just as an adjunct to help the vehicle get up to speed. – supercat Mar 9 at 20:08
@supercat Technically true, but if you go to a store and asks where they have car batteries I'm pretty sure they are not going to assume you want new batteries for your hybrid. – pipe Mar 9 at 22:25
No, but my point was that while supercaps may not replace all the batteries in a car, they might usefully replace some of the batteries in some kinds of hybrid. – supercat Mar 9 at 22:45
How many links do you want? – Tim Spriggs Mar 11 at 22:43

A car battery (in a traditional petrol/diesel car, i'm going to ignore hybrids and electric cars here) is used to start the car and also to provide power for various loads that are used when the engine is off.

Capacitors tend to have more effieicnt charging/discharging than batteries and no real issues of wearing out with lots of cycles but they have a MUCH lower energy density. So a small supercap bank can start a car successfully (as those videos demonstrate) but it will also discharge real quick if you have any significant loads turned on with your engine stopped. In the video linked in the question you see the voltage rapidly dropping when he turns the headlights on.

That is going to be a major PITA if you have to make roadside repairs at night.

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A battery can leave you stranded just as easily. Shorted battery, dead cells, frozen battery, etc. – Tim Spriggs Mar 11 at 20:36
@TimSpriggs Not 'just as easily'. I've had those faults about five times total in my life. A super capacitor will leave you stranded, every time. – EJP Mar 11 at 22:27
You don't know that because you never put the same supercap setup in your car. Have you now? : ) – Tim Spriggs Mar 11 at 22:46