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When jump starting a car A using a car B, can one connect the chassis of both cars together instead of connecting the negative battery terminal of car B to the chassis of car A. In theory this seems to work as the negative battery terminals of both cars are connected to the chassis but wouldn't there be extra electrical resistance ?

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    There would be extra resistance. That doesn't mean it wouldn't work. It may work, but it may not. Are you not able to access the battery terminal on one of the cars or something? I believe the main reason to connect to the chassis on the dead car is to avoid the final spark being near the dead battery to reduce the risk of fire. – Justin Jun 10 at 19:47
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    You always connect the negative jumper cables to the chassis. Negative is always the last connection on each vehicle. That's for the (normal) negative ground vehicle wiring. If either (or both) aren't negative ground then it's different. – JRE Jun 10 at 19:53
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    Most jumper cables say to connect directly to the good battery negative while connecting to the bad cars chassis ground. – cde Jun 10 at 20:05
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    @sam on topic there dies not mean off topic here. – cde Jun 10 at 23:11
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    Back in the days of chromed steel bumpers, one of our cables turned out to be defective. We were able to jumper by touching the bumpers. – Keith McClary Jun 11 at 22:18
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The intent here is that the last connection to be made is not connected directly to the battery. One of the byproducts of the operation of a lead-acid battery is hydrogen gas, which may accumulate within and around the battery. When you make the last connection and complete the circuit, there will probably be a spark. Sparks and hydrogen seldom end well.

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    +1 for mentioning the crucial safety issue. – Elliot Alderson Jun 10 at 23:24
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    OP is asking about the donor connection not the dead battery. – cde Jun 11 at 0:38
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    That's not the reason. There isn't a significant amount of hydrogen, and it's remarkably hard to burn hydrogen in free air. The reason is to have the chassis add resistance, otherwise the current flowing from the charged battery to the discharged one is only limited by the internal resistances of the batteries, and those internal resistances mean both batteries get hot. Adding a resistance in the chassis reduces the current drawn and hence stops the batteries being damaged through overheating. – Graham Jun 11 at 9:28
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    @Graham if that were true we'd just build reliable resistance into the cables. – candied_orange Jun 11 at 11:36
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    @Graham Lead-acid batteries don't exchange a significant equalizing current, mainly because their internal resistance is quite non-linear. One always wants less resistance between (working) battery and the starter motor. That's why thick, heavy, expensive jumpstarting cables work better. – fraxinus Jun 11 at 12:10
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  1. Safety. Connecting the second cable makes a spark. You want it away from the battery. A depleted and over-discharged battery may outgas hydrogen. Hydrogen is explosive.

  2. Lower resistance (not much, but still) - when starting, the current flows in general from the running alternator of the working car to the starter of the dead car. An exposed metallic part of the engine (if any) is even better place for a connection than the chassis.

  3. Battery management system - a lot of modern cars have a current measuring shunt somewhere between the minus battery terminal and the chassis. The system expects no current flowing around the shunt and may react in unpleasant ways if that happens (up to and including shutting down the engine and requiring service).

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    Hydrogen is explosive No it isn't, not to any degree worth talking about here. It's a very popular misconception, but it isn't true. – Graham Jun 11 at 9:30
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    Technically, it is the hydrogen/oxygen or the hydrogen/air mixture. I have a scar on my chin exactly because the hydrogen in a car battery IS explosive and can be ignited by an electric spark. Well, it was 35 years ago and the car batteries didn't have the safety features they have now. Now it is way harder to ignite the hydrogen inside the battery, but if you do, the same unpleasant thing happens. – fraxinus Jun 11 at 9:49
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    @Graham - hydrogen is certainly flammable, and has one of the broadest range of flammable mixtures in air (from 4% lower explosive limit to 75% upper explosive limit). – Jon Custer Jun 11 at 12:52
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    @Graham That might explain why I still have eyebrows. – Harper - Reinstate Monica Jun 11 at 15:52
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    @Graham: Any time you have a volume of any flammable gas well mixed with air at a ratio that supports combustion (and for hydrogen, the combustible range is very wide), igniting it (which hydrogen also does very easily, having a low ignition energy) will almost surely produce an explosion of some sort because there's nothing stopping the flame front from freely propagating through the whole volume. Even if it doesn't detonate, a deflagration can still be plenty explosive enough. – Ilmari Karonen Jun 11 at 22:34
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In theory it will work. In practice, it will also work. Yes there is extra resistance as you have increased the path to ground. In a perfect system each ground would be equal, but it is not a perfect system. Due to the large surface area it is minimal but there will be some resistance. And due to the large current needed in charging and turning over a motor, the voltage droop/rise over that resistance will be non-zero. It will also likely be negligible in the use case provided.

So yes you can connect to the chassis, frame, or engine block of the donor car as long as it is clean and grounded. Obviously there are edge cases if you have a positive chassis vehicle (unlikely, antiquated setup), 6 or 24v vehicles (antique bikes and commercial trucks, helicopters, RVs and boats). But for your average car it's fine.

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Car batteries are extremely well-connected to chassis

Almost all cars use the chassis of the car as the (negative) ground.

Chassis is extremely well bonded to the battery negative terminal, since this is the normal current return path for the starter, which can pull up to 1000 amps.

So no, there will be as little resistance as you can find.

Your biggest problem will be paint or body rust; I solve that on my cars by bolting a lead terminal to somewhere appropriate on the frame. Such lead terminals are sold as "top post converter kits" intended to give a top-style post to a side-post battery.

Doesn't matter. You don't pull 1000A down a jumper cable.

That's not how jump-starting works. The real power of jump starting happens in the minute or two before you attempt to crank. You're either doing one of two things:

  • You have a battery that is merely flat (discharged) but otherwise perfectly capable of doing its job. You are refilling its energy "tank", and the start energy actually comes 90% from this battery.
  • You have a battery that is "at the cusp of end-of-life". It can still store enough energy needed for a start, just not for any length of time. So you are both boosting that battery's charge temporarily, and also, heating it up - batteries store more and perform better when they are warm.

A lot of people just dive right in, hook up and crank; but really, the heavy lifting is being done in the minute or so after they've connected but before they crank. It's better to rev up the donor engine (just a little bit; 1500 RPM is plenty; don't throw a piston rod!) and hold it like that for about one song on the radio (3-5 minutes). At that point you could even unhook and still get a start.

If you're trying to start a long-dead car whose battery hasn't started an engine in 2 years and is basically a rock, then yeah, you're depending on the jumper cables to carry all starting current. But most people do that kind of thing in nice weather :) When temperatures are warm, oil is thinner, and engines and motors turn easier. Cranking amps are in the low 100's typically, and it's conceivable you could get that out of jumper cables.

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  • I remember helping someone start their car late one night - they'd clamped the final jumper lead onto a metallic looking bumper part that wasn't connected electrically to the chassis. In the old days of metal bumpers that could have worked, but this was chrome-on-plastic. – Criggie Jun 12 at 0:37
  • +1 for this: "At that point you could even unhook and still get a start." Yes, but just turn off your engine. There is no sense in sending the surge from the target car starting into the donor cars electrical system. Source: Watching oscilloscope of Engine starts for my first job out of college and that fact that I have done this for 30 years and the target cars start fine without the donor car running. Just let the target car charge for a while. – Be Kind To New Users Jun 12 at 18:17
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The current path of interest is the battery to the starter motor on the car being started.

Each battery has a cable running from the negative terminal to the engine block or transmission near the starter motor. If you attach the cable to the engine block on the non-running car you can bypass one of the cables (which can only be better).

I usually attach the cable to the negative terminal of the battery or to a piece of metal on the engine block like a bracket (assuming you can find something made of metal).

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  • Ops talking about the donor car not the dead one – cde Jun 10 at 20:13
  • @Passerby Yes, I know, the answer is intended to be general. – Spehro Pefhany Jun 10 at 21:41
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To add to the other reasons given, there's one more important reason why you should connect to the chassis, and use the proscribed connection order: to avoid shorts.

Yes, there's a risk of sparks igniting hydrogen as noted above, but a car battery is also capable of delivering 200+ amps for the starter motor. You don't want to short it; the jump leads will quickly get extremely hot, potentially melting the wires and setting the insulation on fire. I suspect it might also damage the car's electrical systems, as the voltage is likely to do odd things for a moment.

It's usually pretty cramped getting big clips to the battery, and using the chassis for the negative terminal avoids getting the clips close together, reducing the risk of a short.

The proscribed connection order (connect +ve wire to both cars, then the -ve wire to both cars) means avoids a number of potential risks:

  • If you connect both wires to one car, there's a risk of them touching when you carry them over to the other car. Some idiot I'd given a jump start to nearly shorted out my car when he disconnected both wires from his car and carried them to me. They were swinging from his hand, and came within seconds of touching before I managed to remove the negative from my car.

  • If you connect the negative first, then drop the positive wire when trying to connect it to the second vehicle, or accidentally touch the chassis when trying to manoeuvre it into position, you'll get a short.

  • By connecting to the chassis, it reminds you that the chassis is part of the electrical system, something it's easy to ignore generally.

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Connect it to the terminal on the full battery, so the current doesn't have to go through battery -> chassis. But if the recipient car has the starter motor connected to chassis, then the path with least resistance is the chassis near the starter or the starter motor terminal.

Then again, usually you need the recipient battery to be charged a bit, so it helps in providing current. Depends on the setup, but some cars won't start if the empty battery is drawing current and you need to rotate the motor too.

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This works in theory and may work in practice if there's enough current getting to the starter.

On many (most?) cars, the starter is grounded through the engine block and subsequently the chassis. For example, here is a wiring diagram for a Miata starter - there is no wire going directly from battery ground to the starter:

Miata starter wiring diagram

In theory, if you are, say, jump starting one Miata with another you could connect both chassis instead of connecting battery grounds. In practice there are three issues with connecting chassis:

  • You need to get a good hold of the connection point with the cable clamp.
  • The connection point must be unpainted.
  • Whatever part the connection point is attached to must be well connected to the rest of the chassis (this is also why engine grounding is important, and why many no-start troubleshooting guides instruct looking at engine ground wires/terminals and ensuring they are in good condition).

If you try jump starting a Miata with a truck in the summer, you'd probably be fine connecting chassis together. If you try jump starting a Miata with another Miata (smaller battery capacity) in winter, chassis to chassis connection may not be sufficient.

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  • The chasis is a very large conductor, and the voltage drop over it should be negligible, unless the section you're clamped to is damaged by rust. The fact that this car uses the chasis as the ground wire for the starter emphasises this point. – Dan W Jun 11 at 15:45
  • This is why I said "part connected to chassis" and not "chassis". – D. SM Jun 11 at 16:43
  • Sorry - I don’t follow; your answer seems to imply that it ‘might’ work for ‘some’ cars. In practice jump starting via the chassis works fine - I’ve done it plenty of times. The battery negative is connected to the chassis, which is why it works, and the chassis is a large conductor - many times larger than the jump leads - so has negligible resistivity. The wiring of the starter motor isn’t relevant. – Dan W Jun 11 at 17:10
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    The problem is not in the chassis but finding a good point to attach the cable clamps to. – D. SM Jun 11 at 17:14
  • If you are working with a car that is designed to be jump started by connecting to chassis (i.e. has explicitly marked jump starting point on the chassis), this is solved for you by the manufacturer. – D. SM Jun 11 at 17:15
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If concerned about the effects of increased resistance while actually starting the engine, you can just connect to 2 vehicles for a little while to charge the dead battery enough to start the engine. i.e. Charge the dead battery from the other vehicle vs. starting the engine.

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