How a Hacker Jump Starts a Car

Here’s the Scenario: you need to get somewhere in a hurry. The problem is that your car has a dead battery and won’t turn over. The Obvious solution would be to call a friend for a jump. But is the friendless hacker out of luck in such a situation? Not if you can whip up a quick parts bin jump starter.

Clearly, [Kedar Nimbalkar]’s solution would be practical only under somewhat bizarre circumstances, so we’ll concentrate on what we can learn from it. A spare PC power supply provides the electrons – [Kedar]’s 250W supply pushes 15A at 12 volts, which is a pretty respectable amount of current. The voltage is a little anemic, though, so he pops it up to 14.2 volts with a 150W boost converter cooled with a PC fan. A dual panel meter reads out the voltage and current, but a VOM could substitute in a pinch. About the only thing you might not have on hand is a pair of  honking 10A diodes to keep current from creeping back into the boost converter. [Kedar] claims he got enough of a charge back in the battery in five minutes to start his car.

As jump-starting goes, this hack is a bit of a stretch. It’s not the first time we’ve seen a MacGyver’d jump starter, though, and you never know when the principles and hardware behind these hacks will come in handy.

71 thoughts on “How a Hacker Jump Starts a Car

    1. I believe that’s what most of the lithium ion “jump starter” devices have in them. I’ve seen some advertised that suggest they use 3S packs and others 4S. I seriously doubt they’d have 18650 cells (or similar) in them considering their spec’d discharge rates are so low.

      1. Pretty sure they actually have a fairly large capacitor (100,000uF at 16V) as well, turns out you can pull multiple C at 10% duty cycle IF you monitor the negative interconnect for any sign of heating. As these are usually Al on budget lipos they oxidize very easily and ruin the pack. Don’t ask how I discovered this “feature” … !
        Also this charging methodology does not seem to harm the electrics in the car as long as it doesn’t go much above 17V too often.

        1. No Caps! you can calculate yourself, how many Microseconds a 100.000µF cap would need to discharge at 100 Amps or more. :-) And yes, this LiPo jump start packs have normally “11.1V” -> 3s.

    2. Yep, also did it once, just soldered a T-dean plug to a pair of hobbyking AWG14 cables and it was really fast. Used a 25C 4S 5Ah battery (so 5*25 = max 125A, i.e. 1.9kW, while the starter motor is rated 1.8kW) and it didn’t get warm. The charger described in the video probably wouldn’t survive 125A?

    3. I have done this also, was out camping in the middle of nowhere years ago and came back to a flat battery. Luckily I had large 4S laptop battery I used to use to powering my gear that still had enough juice left to get the car started.

          1. I’ve not paid a lot of attention to his videos, just seen ’em around. People seem to be of the impression that he’s for real. Or at least that he’s doing stupid things for real, whether he does them knowingly or not. The premise of a comedy doesn’t seem to be there.

    1. In practise: No they don’t have a trim pot in there anymore. Those things are built to be cheap, and do the job, not to be useful outside a PC (where 12V+/- 5% is plenty accurate).

    2. It is unnecessary for a PC PSU when you calculate the values in the first place. The ATX voltage specs can easily be met by component tolerances. The proper values can be calculate in the first place. When you add a trimpot, someone on a factory floor has to calibrate it manually. That costs money and add failures as someone could have set it wrong.

      – There is one feedback and it is on the 5V rail.
      – There are voltage monitors on a decent power supply. 15V is overvoltage.

    3. Does it need to be more than 12V anyway? Minus whatever the drop on the diodes was. Even 11.something should be enough to start a car up, after a while it’ll float at that voltage and the current will stop flowing. But even before then, you should be able to start the car.

      If he didn’t have the boost convertor, I bet it’d still be OK. Surprised the convertor actually worked, surely the battery, which can pull essentially an infinite number of amps, would pull the voltage down? He didn’t use a resistor. Except perhaps the wires to the battery.

      1. If you had a ~11V PSU which could source the required current yeah sure that would start the car no problem, some others mentioned 3S lipo as an example. But that’s not what is going on here.

        It’s really a fast charge, not a jump start. Engine cranking current is high, over a hundred amps initially and probably at least dozen (or more) once turning. The PSU is charging the battery, not cranking the engine.

        1. Yeah I know, my point is, does it need a 14.4V booster? Couldn’t you just charge the battery from the 12V supply?

          It will mean that the battery might only charge up to 11 or so volts, but that should still do the trick.

          1. A lead-acid battery at 11V cannot supply the current necessary to start an engine. An ultracapacitor with enough capacitance at 10V can do the job, but it is supplying much more current than a car battery at even 12V.

        2. I was lucky enough to recycle a 1kw 13.8V transformer based PSU which was left over from a car audio demo stand.
          I made it into a stater cart. Yeah, no issues with flat batteries here. Or in fact needing batteries for engine testing :)

      2. Of course 11 or 12V are enough. The starter turns even with 6V. When it kicks in it tears down the battery voltage anyway. A dead/discharged battery also has less than 12V.
        It is also useless to have diodes in series with the output. The PSU has them already built in, we call them rectifier. :-)

    4. They don’t always have trim pots but you could look up the PWM IC and see what resistor value to change.
      The problem though is every PSU is different inside and parts may not be marked with standard numbers.

    1. 12 is the nominal voltage, part way thru its discharge. It takes 14+ volts to charge them, and they are flat at about 10v. Like an alkaline AA says it’s 1.5, but starts closer to 1.7 and is dead at about 1

  1. I think I would leave out the diodes: the way a boost converter works, it already has a diode at its output. When the boost converter is “off” or “not powered”, the only component that is connected to the output is the diode (blocking current further into the circuit) and a capacitor.

        1. it’s a boost converter, so even if the output rose above the 14 volts it’s outputting, the input voltage would not increase at all, since there’s a diode in between them. the current would lower to almost nothing and the psu would see almost no load.

  2. Hmmm… Nice idea, but actually, people who are equipped with such stuff and are in a hurry would actually do it like i did a few times and just take one of their Lab-Power Supplys outside and crank them to 15V at 15A to just give the battery a gentle kick in the ass :P

  3. A similar situation; car battery flat due to the light in the back. I have a lead easily accessible straight to the battery for charging RC plane batteries etc. 30 years ago I built a 40Amp 14V linear power supply. A few minutes connected and while still connected the car cranked over like it was on a sugar rush. Then removed the lamp from the light in the back.

  4. You charge a car battery at a little higher than battery voltage, but you only have battery voltage available to start a car. In the end, however, a simple DC starter motor will simply run a little faster or slower and your engine doesn’the care. Start it with 16 volts, start it with 10 volts, it will start. It is the CURRENT (internal resistance of the battery) that matters, and without enough of that, you won’t turn your engine (and voltage will drop).

  5. Drove a 36V Neighbourhood electric vehicle to and from work years ago. One night my colleague had a flat battery, so I hooked up my 100AH x3 series battery pack to his 12V battery (after disconnecting his positive wire, not to fry his car), after 4 seconds at 36V and around 400A his battery started to fizzle, so I stopped and his car started right up. He told me the day after that his battery felt as new again when starting after that treatment.

    1. No surprising it felt like new. Like Irsh wrote below, this knocked the built up sulfonation off the plates. This is what so-called battery rejuvenators did and folks had it done regularly back in the day. Practice has largely stopped now because alternator based charging systems generally don’t let the problem get too bad in the first place (and the places that used to do it would rather sell you a new battery when it does)

  6. In a real life scenario, if you stand there with your hood (or bonnet) up and a set of jumper cables in your hand, someone will stop to give you a jump usually pretty quickly. @ [Tore Lund], I think by jump starting your colleague’s car the way you did, his battery WAS for all practical purposes “new” again, as the fast inrush of current and the higher voltage knocked off deposits (i.e. corrosion) from the lead cell plates inside the battery which would allow electrons to flow freely through the battery once again without being slowed by the built-up plate deposits.

      1. Starter hill. Used longer before tractors where equipped with electric starters. Make sure the mag kill switch is open. Left gravity start the tractor, not the strong arm starter And yea if the tractor or any vehicle is parked on starter hill chances are the starting system is defective for some reason.

  7. Also relevant, this only surface charges the battery. For a proper rejuvenation it needs to run for about 24 hours minimum, ideally with average 12.8V pulsing up to whatever the battery will take at about a 10% duty cycle. This actually does work, sadly only on flooded not SLA or AGM.

  8. Took a 12V 2A power brick I had lying around from who knows what and plugged that in to a dead battery in my car since it was only sitting in my driveway and I wasn’t rushing to get it back driving. It brought the battery back to life enough to start the car up, so I drove it around a bit after that and then finally got a real float charger which kept it ready to go at all times thereafter.

  9. i guess everyone is too weedy to bump start their own cars? or is it because all cars are now unnecessarily huge and heavy 4×4’s with automatic gearbox?
    two people (one pushing) is easy enough if they understand the procedure, sometimes though its easier to just get it rolling jump in and start it yourself.

    1. Automatics are very common now. I had an old manual and push starting was fun, but difficult in winter when it’s likely to be needed. One morning, I think it was -15 F, the old honda wouldn’t start, ran the battery dead trying. Pushed it down the drive way, nothing. Had to have my brother in law pull start it on the road with his truck. Engine was so stiff it lurched and slid on the pavement a few time before it finally roared to life.

    2. There is another factor to consider, alternators need a field current to function. If your battery is dead flat you can turn the engine all you like but you’ll get nothing from the alternator. No electricity, no sparks.

  10. if you left the lights on and ran the battery to zero a straight 12 volts should work you dont need to get the battery fully charged.

    you need to get it up to maybe 8 or 10 volts and depending on how old the car is it may start and you can then leave the car run to charge.

  11. This will not work for a car battery that is actually dead. a PC power supply is barely capable of running the car.

    100Amps at 12V DC is easy. you need a big transformer 10:1 windings, and 4 really big diodes. Just let those pulses right through to that battery.

    Had a friend actually wind his own 10:1 transformer in a few minutes in the garage using a piece of pipe and 10 gauge copper wire.

    he then quadrupled the 10 gauge for the low voltage side to handle the 100 Amps, the high voltage size is only going to draw around 10 amps.

    Worked great, but that is when we found out you can’t take 20 5A rectifier diodes and parallel them to get 100A capacity. when one weak one goes, the rest go like firecrackers.

    1. you need to leave the power supply connected for a few minutes just like you have to do if you use a jump box through the cigarette lighter port because the wiring cant handle the current.

  12. I went a hard way. Took apart my PSU, found out voltage regulator circuit, added a trim potentiometer and got regulated PSU. Now i can pull 12a up to 16v until the safety circuit cuts off the PSU.

  13. I used 4s lipo 35C 6000mha battery more than 10-15 times to crank my 2000 Passat it only dropped 0.8v from fully charged. Since it is 35C it should provide about 210A constant current, and it if we are about to believe to manufacturing specs it should provide 70C for 1-2 sec bursts. 420A bursts !!! I think it could probably jump start smaller truck or a van without hiccup. Wires do worm up a bit, but nothing alarming.

    1. I used 6 maxwell 2600F capacitors to jump start volvo/mack trash truck diesels at my old job. charge the bank up to about 14 volts, disconnected the battery altogether and connected the terminals straight to the caps. would crank over those huge engines without the slightest hesitation.

  14. Has any one considered server psu’s for straight jump starting?

    Coming from the RC world lots of people mod 2nd hand server psu’s to use for charging batteries. I have a modded psu that can deliver 12 volts at around 150-200A and can be trimmed up to 15 if you so choose. Its a rated 2Kw psu from an IBM blade server thing and it only cost me £30.

  15. > A spare PC power supply provides the electrons

    Doesn’t the power supply just provide a potential so that the metal’s free electrons (or ions, or whichever kind of charged particles said conductor contains) can move, creating electric current? This is like saying we buy electrons from our power company… I normally don’t like being pedantic, but this one was way too obvious for me not to point out..

  16. > A spare PC power supply provides the electrons

    Don’t PSUs provide a potential so that the free electrons contained in the metal can move in a coordinated way, creating electric current? I avoid being pedantic, but this one was way too obvious for me not to point out…

  17. I have used amateur radio coaxial cables as an alternative to jumper cables more than once. Have two cables with UHF/PL259 connectors use the outer screen on each as one braided cable. Handy at radio events where one of the cars does not start and cables are a plenty. Use the shortest cables you have and connect the negative of the good battery to the chassis (not battery) of the bad one. Keep the good engine running and revving while you quickly jump the battery. At the same time start the bad car. Often it will start in 2 or 3 seconds allow both of you to move on. You are not jumping or charging the battery, just giving the starter motor enough voltage for a few seconds to crank.

    I have also parked my car on the top of a hill due to a faulty starter. When I arrived back car would not start to roll so I opened the door and pushed it a few feet flint stones style until it started down the hill.

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