# Improve a car battery's performance with capacitors

These days, it got very cold in my area, and I started to think I need a new car battery. But then, as long as temperatures are above -10°C, my battery works fine. It just can't support the high current needed to start the engine when the temperatures are below -10°C. So my thought was:

Can I add capacitors to my car battery, that will help supply the needed current?

I found 6 Capacitors with $$\2.7\text{ V}\$$ and $$\500\text{ F}\$$ each on ebay for a total of 25$. If I put them in series, their capacitance should be $$\\frac{1}{\frac{1}{500}\cdot6}\approx 83\text{ F}\$$ at $$\16.2 \text{ V}\$$. The energy in watt seconds would then be: $$\0.5\cdot83\cdot16.2^2\approx 10891 \text{ Ws}\$$, which should be enough to start my small little engine (small gasoline engine rarely takes more than a second to start, even in cold temps, so that would be about 130 Amps for one sec). Unfortunately, that's about as far as my knowledge of electronics goes. Can I really just attach the 6 Capacitors, which I connected in series, to the Battery? Is there something here I am not seeing, which would make it extremely impractical to do that? I can't seem to find anyone on the Internet doing this, which makes me sceptical. There are some people that use capacitors instead of batteries, but that seems like a bad idea to me since it won't last more than one start... • If you can't find anyone, you are not searching hard enough. There are lots of youtube videos about people starting their cars with ultracaps. Jan 17, 2017 at 10:18 • A crazy suggestion: assuming the battery is still in good enough condition and has enough energy stored: why not just turn on the lights or some other power consumer in the car so that the battery has to deliver some current. Leave that on for a couple of minutes. This will heat up the battery somewhat which might be enough so that is it able to start the engine. Jan 17, 2017 at 10:23 • @FakeMoustache: You mean like all the people in cold weather countries do it all the time? That assumes you don't do short distances constantly though. Jan 17, 2017 at 10:27 • 0.5*83*16.2² is the total energy stored - unfortunately this is erroneous as (a) the battery voltage (and hence the capacitor voltage) is more likely to be around 13V and (b) the capacitor voltage can only fall the same amount as the battery so the amount of energy available from the capacitors will only be a small fraction of the total. Jan 17, 2017 at 10:34 • If you can't start the car in -20°C, either the battery is a bad, or the rest of the car is. -10°C should be easy. Something is faulty or insufficient. Throwing capacitors at the problem will only mask it, it won't solve it. Jan 17, 2017 at 11:18 ## 7 Answers Putting capacitors in series to add up their voltage ratings isn't very wise. Their difference in leakage current will lead to an imbalance in the voltage they'll have across them (see https://electronics.stackexchange.com/a/80589/107479), and some of the capacitors could easily experience overvoltage (especially given the low voltage ratings supercapacitors have). The solution for that would be to add a resistor ladder in parallel to compensate this, but that would increase your standby battery drain, which is not a good idea either. Moreover, your computation is wrong, since the capacitor will be charged to the battery voltage (~13V), not the full capacitor voltage rating. And I doubt it requires less than one second to start. And, when you say in your comment: The top rated answer in the post that PlasmaHH linked, suggests doing exactly what I want to do, using capacitors in addition to the batterie, so the battery can store lots of energy, and the capacitor can deliver it at high rates for short times. you're mistaken: this answer you're referring to, when it mentions "short times", it means "a few ms", for example to smooth the big current spikes due to the inrush of the starter. Not for the starter the whole starting time. This is where it begins not making sense. Finally, if your car doesn't start when it is -10°C outside, you simply have a problem somewhere with your car. Bring it to your mechanic. He'll fix it, and he surely won't fix it by adding capacitors. He'll find the actual faulty component (which, by the way, you don't seem to have really checked - you're assuming it's the battery, but it could be anything: glow plug/starter motor/some sensors/whatever/...) and replace it (because when such a component starts to fail, trying to fix it by adding capacitors or duct tape is futile - you replace it). • I am sure it's the battery, and I don't have a mechanic. I usually fix my car by myself. I will replace the battery. I knew that this was the most intelligent thing to do from the beginning on, I'm just naturally curious and wanted to know whether it was possible to build something like this (capacitor supported battery) for relatively little money. After all, they are using it nowadays in cars that have a start-stop-automation (but it's ridiculously expensive, 700€ for a 1200F capacitor in addition to the starter battery). Jan 18, 2017 at 16:47 • Actually, the price is not that irrelevant compared to what you planned to buy. You said you could have the equivalent of 83F for 25$. That would be about 360$for 1200F. Depending on quality (lifetime, ESR, ...) the price for a cap of a given capacitance and voltage can easily range from one to twice the price... Anyway, I'm glad you're going for the sane choice of replacing the battery. That would be the best usage of your money IMO. – dim Jan 18, 2017 at 20:28 There is a company that sells capacitor banks that completely eliminate the need for a lead acid storage battery for automotive as well as other applications. Since they are non chemical based they are not affected as much by extreme low temperatures. Here is a link to check out and tease yourself silly with ideas. http://www.maxwell.com/products/ultracapacitors/cells This link contains specs as well as product line and company contact info. I read somewhere a while ago that these units will charge in about 3 secs. • Can you show where the company recommends ultracaps to replace lead-acid batteries in gas-powered cars? Dec 9, 2019 at 16:58 Using supercapacitors to boost overall starting current is good, but there are several practical limitations: 1. You would need short (and thick) wires to connect the supercap pack to the battery (so you do not add too much wire resistance in series with caps), which means the supercaps will have to most likely sit inside the hood. Engine can get very hot and so do the caps. You will have to make sure the caps do not get damaged at the highest temperature they can see inside the hood. You may have to find a place where it is not too hot and measure the temperature at that spot with a thermometer when running the car for certain time to verify that it is indeed a good spot to anchor the capacitor pack. 2. Nominally the alternator may provide 13.8V to charge the battery. But I have seen it going to 14.4V when I crank up the engine. Since we are dealing with the magnetic circuit of the alternator, load dumps, etc. it is possible that the battery will sometimes see voltage spikes that are much higher than 14.4V, or 15V or even 16V. If a supercap is rated at 2.7V and you have six or seven in series across the battery, you will have to make sure that each cap will not see more than its rated voltage any time, i.e. you may need protection circuitry (mainly transient absorbers) across each supercap in the pack. You will have to then watch out for the leakage current in the transient absorbers when they are not active. 3. If you connect a bunch of capacitors in series, the voltage across them when charged may not be equal. The voltage depends on the actual value of the capacitance of each supercap. Usually there is a tolerance to the value of all capacitors. I have seen supercaps having as much as +/-20% tolerance in their value. I.e. a supercap with nominal capacitance of 500F may have actual value anywhere between 400F and 600F. Unless you can screen many supercaps and pick those with values within a small range, you are stuck with what you get. A +/-20% variance in capacitance translates to a similar variation in voltage across the caps when connected in series and connected across the battery. i.e. If you have six supercaps in series, connected across a 12V battery, they will not have 12/6=2V across each (Q=CV, so V=Q/C, all caps in series will have the same amount of charge because the current going through them when charging will be the same since they are in series). Caps with smaller capacitance (say 400F) will have higher voltage across them than the caps with bigger (600F) capacitance. You will have to have enough caps in series so that because of tolerance of capacitance, voltage across a cap never exceeds its rated voltage or it will develop defects and either become useless (i.e. develop intrenal short circuit) or burn up. People use charge (i.e. voltage) balancing circuits across each cap to equalize voltage across all caps. But these circuits can have leakage current, so when the car engine is off, they are draining the battery. 4. Even without charge balancing circuits across them, supercaps have their own leakage current. Leakage current can be tens to hundreds of micro Amps. That is not much, but if you have not started a car in a long time for any reason, it all adds up. There are super-low-leakage supercaps but they come with price premium. You can make your supercap string across car battery work, but please make sure you have addressed all the above issues. If you have overcome them, please post your solution so the rest of us can learn from it. • Anoop Hegde • Interestingly about load dump, it's probably much less likely that both battery and supercap become disconnected simultaneously, leading to a load dump condition; and they'll definitely cover for the load dump of a disconnected battery (if indeed a load dump situation even occurs, since the voltage won't be allowed to rise and continue the reaction). Regarding leakage, they will balance by capacitance value initially, then stabilize by individual leakage, which is higher near rated voltage; I wouldn't worry about balancing resistors, I think. Feb 27, 2023 at 11:30 Conceptually sound but not likely to work in practice. The starter needs lots of current, and you will need a lot of capacitance to make a difference. • A lot of Capacitance is cheap nowadays, as you can see on youtube, people are building capacitors large enough to power a starter motor for around 40$ Jan 17, 2017 at 12:32

Simple solution - get a car starter kit and keep it somewhere warm. For example

The market offers plenty of small jump starters for every purpose, but which packs the most punch? Here's our verdict

I added six 2.7V 500F supercapacitors in series with a protection board and that drained my car battery in six hours, so I decided to remove all the protection boards and not use them. Now I have seven 2.7V 500F supercapacitors in series and paralleled with my car battery. Everything works perfectly. The capacitor does not over charge since my alternator maximum charge is only 13.8V.

My supercapacitors in series have 18.9V and 71.4 farads total.

Make sure you install an 80A fuse as protection.

• ... and a second thing: fire protection, e.g. a fire proof enclosure. Jan 17, 2020 at 6:20
• So does it work? Can you start the car from the capacitor bank if the battery is dead? Jan 17, 2020 at 14:07

What a lot of these people seem to misunderstand is what is nede dto stat s motor is power. As the bat can not cope with demand its votage drops, and as the volts drop the cap releases.its stored power. Lots of vids on u tube. Seems like drop in voltage starting is less than a third. EVEN BETTER when it's cold and battery IS LESS EFFICIENT. Yes people have tried caps only but I would be inclined to have both. Need to disconnect caps when goes to garage for service, and probably precharge before connecting to bat. I EXPECT REDUCING STRESS on battery will give it a longer life so I would do it to cope with a year then get new bat which will last longer.
Best of luck and please feed in reports on how it worked. Ps I understand in deep cold caps are more efficient must be even better..

• What is needed to start a motor is current, voltage at the start can be low, so, you do not (necessarily) need a lot of power. Dec 9, 2019 at 20:53