Super Capacitor Jump Starter Circuit

Jeffery Vahrenkamp

Joined Jun 16, 2016
53
EDIT:
Alright, i'm gonna head this off, yes I am aware that I can buy a lithium based battery jump starter for cheaper. That is not what this is about. This is about me seeing something and going "How does that work and can I make that?" So if your reply is going to be "This is a stupid idea/waste of time/Just use a battery" please don't. Not trying to be rude, I just want help with the problem I am actually interested in, not peoples opinions about the product i'm trying to emulate.
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I am trying to build a super capacitor jump starter like the one shown in this video. (go to 12:00 to get past all his talking and see it in action)

I have a capacitor bank that I am able to jump start my car with. The jump starter in the video uses what I am assuming is a boost circuit to charge the capacitors with the semi-depleted battery car battery (or an internal one if the battery is lower than 5V). It is connected via alligator clips to charge from the battery. Then when it has charged to 15V the same alligator clips are used to start the car.

I have done all of these steps but I disconnected the + terminal of the battery for the start. But this assumes you have the tools to disconnect the terminals, or two people when you need to start, both of which are assumptions i'd rather not make. I have been trying to figure out how they can charge using the same cables that start the car. Obviously there can't be a closed circuit to the battery because this would discharge the capacitors back into the battery they were just charged from. It would also mess with the boost circuit. So I am assuming there must some sort of very high current MOSFETs (like this?) that are switched when the internal circuit senses the key has been turned.

I am hoping someone here either has torn one of these down or can help me figure this out.

Attached is my preliminary circuit. Hopefully the circuit diagram purists can appreciate my alligator clips . The capacitor bank is 16V 60F from my tests. I am looking for two things.

1) Any major obvious pitfalls in the current design. I think the boost circuit will just shut off when the MOSFETs connect the input and output (i'll check that out when i get it). Other than that I don't see issues, but this is a hobby so they may abound.

2) I am looking for a way to sense the key has been engaged and trigger the MOSFETs.

My first thought was to use a microcontroller and look for a voltage drop when the battery tries to turn the starter, which should drop it close to zero. I think this would work, unless the battery is totally toast (<3-5V).

My though for that scenario to use a voltage divider, where R1 is much lower than R2, so when the solenoid was engaged the resistance for R2 would drop (since the starter and solenoid have much lower resistance than the batteries ESR from what I can tell) and this would increase the voltage to the MOSFET gates, but I realized the resistances we are talking about (20mOhms for the battery ESR typically) would essentially be a dead short for the voltage source :-/ .

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oz93666

Joined Sep 7, 2010
737
The guy in the video might be a good car mechanic , but he hasn't got a clue about electronics ...many errors ...

There's really no need for any circuitry ....

First you need to specify exactly what it is you are trying to do .... Presumably partially recharge a flat battery so it can start an engine , this will probably not be a regular occurrence .... capacitors are not appropriate...

You just need rechargeable 3s lithium pack capable of delivering high current , one like this , only $20 . When fully charged it has 12.6Volts , just right . When connected to a flat battery it will bring it's voltage up , give it 30 secs ... then the two together will start the engine can deliver 160A !!! , just keep it fully charged at all times , then its ready to connect across a low battery .... The device in the video is just a flashy gimmick with numbers indicating state of charge ...waste of time. Last edited: Thread Starter Jeffery Vahrenkamp Joined Jun 16, 2016 53 The guy in the video might be a good car mechanic , but he hasn't got a clue about electronics ...many errors ... There's really no need for any circuitry .... First you need to specify exactly what it is you are trying to do .... Presumably partially recharge a flat battery so it can start an engine , this will probably not be a regular occurrence .... capacitors are not appropriate... You just need rechargeable 3s lithium pack capable of delivering high current , one like this , only$20 . When fully charged it has 12.6Volts , just right . When connected to a flat battery it will bring it's voltage up , give it 30 secs ... then the two together will start the engine

can deliver 160A !!! , just keep it fully charged at all times , then its ready to connect across a low battery .... The device in the video is just a flashy gimmick with numbers indicating state of charge ...waste of time.
I am aware I could build a jump starter with a lipo battery or just buy one, and I'm sure it's a waste of time, but it's my time to waste and it's a project I'm interested in.

oz93666

Joined Sep 7, 2010
737
Well , If you're determined to use capacitors the way I would do it would be to get 8 AA alkaline batteries in series and leave them permanently across the capacitor ... this way the cap will always be charged and ready ... the cap will self discharge very slightly , I would imagine the batteries would last at least a year . The batteries can deliver around an Amp , so it would take a minute or two to recharge after firing

shortbus

Joined Sep 30, 2009
9,643
It is connected via alligator clips to charge from the battery. Then when it has charged to 15V the same alligator clips are used to start the car.
I have to ask, have you ever jump started a car? How big of a cable was on the jumper cables? Do you seriously think that the small diameter wires and small size alligator clips on the device shown are going to supply the needed current to start most cars? Just because it's on Youtube doesn't mean it's true.

Papabravo

Joined Feb 24, 2006
19,300
This one is a real knee slapper. You do know that a stater/solenoid combination can draw several hundred amps for 30 seconds or more on a cold day in January. How is a capacitor going to do that?

shortbus

Joined Sep 30, 2009
9,643
How is a capacitor going to do that?
He claims to have one already. Just want's a way to charge it from his dead battery. They show it on Youtube so it must be true.

Quote from the first post - "I have a capacitor bank that I am able to jump start my car with."

Papabravo

Joined Feb 24, 2006
19,300
He claims to have one already. Just want's a way to charge it from his dead battery. They show it on Youtube so it must be true.

Quote from the first post - "I have a capacitor bank that I am able to jump start my car with."
Great - I guess there has been a suspension somewhere. Maybe Rhode Island.

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oz93666

Joined Sep 7, 2010
737
He claims to have one already. Just want's a way to charge it from his dead battery. They show it on Youtube so it must be true.

Quote from the first post - "I have a capacitor bank that I am able to jump start my car with."
I was about to question that .... 60F @ 16V ... if charged to 14V ...1/2 CV2 = 30 x 14 x14 = 5,800 J

Realistically half of this is available 3,000J ...

cranking a car @ 150Amps , 10V .... two seconds cranking , more than enough to start a good engine ....

shortbus .... As for croc clips and thin wire ... you'd be surprised , I've started a car with 1 sq mm copper wire and small croc clips , normally only takes half a second , too short a time to melt the wire or even the plastic insulation .

shortbus

Joined Sep 30, 2009
9,643
shortbus .... As for croc clips and thin wire ... you'd be surprised , I've started a car with 1 sq mm copper wire and small croc clips , normally only takes half a second , too short a time to melt the wire or even the plastic insulation .
And I've seen discount store supposedly good jumper cables melt. Most times, at least around here, people needing a jump start are people that don't keep the car running correctly and tuned up. A half second is not what I've ever seen to start a car needing a jump. Then again most times you need a jump in cold weather that adds to the problems.

Jeffery Vahrenkamp

Joined Jun 16, 2016
53
Wow, okay a few replies since I stepped away from this.

Lets start off with, you can all suspend your disbelief about this product. People have been starting cars with super caps for at least 5+ years from what i can see on youtube (I know, people fake crap on youtube, but there are plenty of examples and from sources that are reliable). Secondly, I have done it myself. I put screws through the terminals of my cap bank and pressed them directly to the disconnected cables of my car and it started right up. If you don't believe me, spend \$40 and get one here and try it yourself. I know this works, it's not even a question. @Papabravo I sure don't want to live where I need to crank my car for 30 seconds to get it to start. Where are you, Siberia... or North Dakota? I am not starting a 1 ton diesel truck in -30F weather. This 50-60F 15V cap bank is more than sufficient for the weather and vehicle I deal with.

@oz93666 I was going to use one or two lithium cells as a back up to charge the bank.

I know how to charge the cap bank, this is also very easy to do and not an issue.

@shortbus I am not talking about little dinky gator clips with small 20agw wire for jump starting the car. I either going to use the gators from a busted pair of jumper cables or something like this. From experiments I know that even standard jumper cables with 6ft 8 agw wire have too much of a voltage drop to crank the engine. Fortunately for me I don't need long cables, and i can use higher gauge wire.

The half second time step was about how long the maximum current draw was. You can see in this video what the current draw profile looks like, and also be amazed at a guy starting a car with a super capacitor bank. I've confirmed this myself with my own oscilloscope.

I already have all these details worked out. The question is almost purely about how you would detect when a car is trying to crank its motor, and what kind of components you would use to switch that much current.

Bicmichum

Joined Mar 6, 2022
2
I have to ask, have you ever jump started a car? How big of a cable was on the jumper cables? Do you seriously think that the small diameter wires and small size alligator clips on the device shown are going to supply the needed current to start most cars? Just because it's on Youtube doesn't mean it's true.
I've jumped a e350 full size van with an 18v Milwaukee battery using 12 guage wire... Current doesn't flow through a capacitor, it can only store energy in the for of voltage(basically its a volume tank or reserve). That being said, there isn't a need for very large wires being the amperage that flows through the circuit when starting the vehicle does not pass through the capacitor...

Jeffery Vahrenkamp

Joined Jun 16, 2016
53
I've jumped a e350 full size van with an 18v Milwaukee battery using 12 guage wire... Current doesn't flow through a capacitor, it can only store energy in the for of voltage(basically its a volume tank or reserve). That being said, there isn't a need for very large wires being the amperage that flows through the circuit when starting the vehicle does not pass through the capacitor.
Current may not pass "through" the capacitor but when charging and discharging a capacitor it does flow through the wire, and thus there is energy dissipation and voltage drop. Technically voltage doesn't pass through a battery either, but look inside your engine compartment and you'll find that your car battery has very high gauge wire connected to it. You are likely able to start your car with your Milwaukee battery and 12 gauge wire because your car battery probably wasn't that depleted, and your Milwaukee battery charged it up enough to turn over the starter. I bet if you tried totally disconnecting your car battery and starting it from your power tool battery and 12 gauge wire you'll find that it's probably not sufficient. And you'll also find that that 12 gauge wire will get pretty warm has your passing hundreds of amps through it.

Bicmichum

Joined Mar 6, 2022
2
Current may not pass "through" the capacitor but when charging and discharging a capacitor it does flow through the wire, and thus there is energy dissipation and voltage drop. Technically voltage doesn't pass through a battery either, but look inside your engine compartment and you'll find that your car battery has very high gauge wire connected to it. You are likely able to start your car with your Milwaukee battery and 12 gauge wire because your car battery probably wasn't that depleted, and your Milwaukee battery charged it up enough to turn over the starter. I bet if you tried totally disconnecting your car battery and starting it from your power tool battery and 12 gauge wire you'll find that it's probably not sufficient. And you'll also find that that 12 gauge wire will get pretty warm has your passing hundreds of amps through it.
I agree with that, but in terms of the wire size on the super capacitor jump starter, and this is me assuming, but I believe the capacitors discharge would be directly to the battery and not the entire circuit of the starter, because like water, current will take the path of least resistance.

DickCappels

Joined Aug 21, 2008
9,318
Getting back to the circuit itself, when your switching transitors turn off, there will be a magnetic field in the starter, and that field will begin to collapse. When that happens large voltage can develop across your switching transistor. Often a snubber circuit of sorts is used to absorb the current during the collapse of the magnetic field. Below is on such example.

The diode needs to have a peak current rating of at least the current in the circuit it supplies.

If you have the car battery in the circuit it should protect the switch, how would you find out?