Jump Start a Car with Super Capacitors




Can super capacitors (supercap) alone jump start a car:




But of course they can, if they are rated properly of course. If you want more detail on what supercaps are, you can read this Wiki Page. But in general these especial capacitors have huge amounts of capacity compared to other types of capacitors. They are the bridge between electrolytic capacitors and batteries. At much smaller size, they can store much more electrical charge. The supercaps I used are Eaton XV3560-2R7407-R, which are 400F, 2.7V, 3.2mOhm which can deliver 220A peak at 15 degrees temperature rise. Well I drew more from them and I’m not sorry about it!

Their downside is that majority of them are rated for 2.7V, which is pretty small. One has to put more of them in series in order to get higher voltage rating. But that ends up reducing the total capacitance of the battery. As we know the total capacitance of series capacitors is:

Series Capacitors

When all capacitors are equal, if we have 6 capacitors for example, the total capacitance would be C/6.

Now to crank a car we need a high voltage capacitor. With 6 supercaps we get a maximum of 16.2V. You may say we could remove one capacitor to get 13.5V total to use in place of a 12V car battery. But as also shown in the video when car is charging the capacitor, the voltage is usually regulated to 14V or sometimes higher. So the more margin the batter.



Now cranking a car takes a lot of current as seen in the plot below. Initially as the crank motor is not running, the only thing holding the motor current back is the motor coil resistance which is generally very small especially for these motors (their inductance is very small). So the current jumps to maximum which in this case is around 740A.

A typical car crank current

A typical car crank current

As the crank motor starts spinning, the back EMF (voltage generated on the motor coils when turning) picks up and reduces the current into the motor. So the motor current drops to 200A and goes lower. Those ripples in current are due to the change in motor speed as it tries to turn the engine, which is also audible. As soon as the engine turns on, it turns the alternator and charges the battery. This is when the current reverses back into the battery which is around 40A.

The picture below is the voltage of battery when cranking the motor. The battery has some internal resistance which results in voltage drop when current is drown from it. As seen below the voltage almost drops 4V and we know the current was 740A. This means the battery series resistance is around 5.4mOhm. Of course this is a charged battery resistance and will be smaller if the battery is not charged well.

Battery voltage when cranking

Battery voltage when cranking

Also from the plot above we can calculate the coil resistance of the crank motor, which is around 8.4V on the battery divided by 740A current, or around 11.4mOhm.

Now the supercaps I used have an ESR (Equivalent Series Resistance) of 3.2mOhm each. With 6 of them in series we will have around 6 x 3.2 = 19.2mOhm resistance. This means that when we crank with capacitors, the voltage across the capacitor will drop much more and they provide a smaller in-rush current. This would result in the motor turning slower. This is what we observe in the video as well, and the plot below shows how the crank current is smaller.

Cranking voltage and current with supercaps

Cranking voltage and current with supercaps

The crank current is around 460A and the voltage drops by around 7V from 13.6V. This means the capacitors have 7/460 = 15.2mOhm resistance which is better than expected.

As you see from the plots, while the battery could start the car in less than 600mS, the supercaps started the car in closer to 800mS, which means as expected the crank motor is turning slower. But that is good enough to start the car. Right after the car starts, the capacitors charge and are ready for another crank.

There are two issues with supercaps:

  • They have large leakage current, around 1mA for my supercaps, which will drain them fully in about 13 days.
  • My supercaps are only good for one crank per charge, meaning if you open the doors and in-cabin lights or other devices turn on, their voltage drops and there won’t be enough juice to crank.

Of course you can get much higher capacity supercaps good for multiple cranking tries before car starts and recharges the capacitors.

Also in many cases when they want to use super capacitors in place of car batteries, they also include a low power battery parallel to the supercaps. The battery can provide the 1mA leakage current for very long period of time. Also if one crank didn’t turn the car on, you can wait for the battery to charge the supercaps for another try.



28 thoughts on “Jump Start a Car with Super Capacitors

  1. Thanks for the video/article. I’d be very interested to see you build another one, incorporating the parallel battery to maintain the supercaps. I wonder how long something like a laptop battery could keep them charged, and what manner of circuit would be required to recharge the “trickle” battery [simple resistors to manage the current passed from the car battery, through the capacitors, and into the small battery?].

    I’m still very new to the world of electronics, but I’m doing my best to keep learning every day.
    Your videos inspired me to first start googling Ohm’s law and I love double-checking your resistor values, ensuring that my math arrives at the same value as yours, and staring at your circuit diagrams until I understand them [or at least START to understand them].

    Thanks for your awesome productions. Keep doing what you do. =)

  2. You can balance these caps very easy. And reduce the leakage current to a degree of uA.

    Just research this IC. ALD810026

    I would be interesting to see a video about this topic.

    Balancing caps ( super-caps )

    Thank you

  3. Good job. The numbers are exactly what I needed, although the amp value is pretty much different from every car. Mind to tell me your car is, and the fuse used by motor (for cranking) is it using? My car is 1.6T engine with 300A fuse for starter/generator(or alternator?) but i have no equipment to test the amp like you

    • It is provided in their datasheet. But you can also measure them. It involves having a series resistor with the cap, send a square wave to it at >10k frequency and look for jump of the voltage when pulse transitions. The jump is due to series ESR of the cap. Having that jump you can calculate the ESR.

  4. Can you let us know whats ur dc power suply and the circuit diagram of the capacitor bank. And also please let us know a way to store 12Kv 60mA from a capacitor bank with the diagram of ur suler capacitor bank. Thanks!

  5. Hi! Can you make an article/video about surge protectors? Specifically, I’d like to know if there’s any impact, positive or negative, on other appliances in the same house not plugged into them. Thanks in advance!

  6. Greetings Mehdi,

    I’m thinking of buying a current clamp-on meter and I noticed that you’re using Mastech HH2002 multimeter in this video. How’s this multimeter? Would you recommend it?

    Thanks

    • It is a low cost basic multimeter that works. I haven’t checked its accuracy but I’m not expecting high accuracy out of it anyways. Other that that, it works well.

  7. Pingback: Jump Starting a Vehicle with a Bank of Super Capacitors « Adafruit Industries – Makers, hackers, artists, designers and engineers!

  8. Ur experiments rock man i have been watching your videos of experimenting since 2015 and i really need an oscilloscope i am a student of hamdard university having Electronics Engineer as a major,i hope u understand

  9. My son is graduating from UVIC as an Electrical Engineer and I want to buy him a present. Where did you buy that sweet Electrical Scope thing ?

  10. In the video you mentioned you were giving away your old scope. My old high school physics professor has an oscilliscope from what feels like the 80s. I fact I noticed when I was using it that it was completely miscalibrated until I adjusted it back to normal.

    I was the first to notice that it was giving weird readings and brought it up to him. He blamed it on error on my part, as he believed the oscilliscope which he had bought for the class had to have been perfect. After a long three month argument I finally proved that it was infact miscalibrated and I fixed it. Honestly the whole ordeal kind of severed his relationship with me as a friend. He didn’t like being wrong about something, and as a high school student I probably didn’t handle the situation as appropriately as I could have.

    Anyway If you could email me regarding the oscilliscope and maybe potentially sending it to me so that I could give it to him it would be greatly appreciated. He teaches ap physics ap calculus and engineering.

    This is especially a difficult year for him because they just changed the physics curriculum and he is struggling with it because he has always been very very successful with the old one.
    The tests are on a 1-5 scale with 5 being the best. He was very proud of his test scores in all ap classes being around 4.5 or greater on average, which is ridiculously impressive for a school that makes all ap students required to take the ap test at the end of the year.

    I appreciate that you took the time to read this post and I would greatly appreciate if you found the time to reply.

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