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This may seem like a very simple question, but I've searched all over the place and haven't found an answer.

When jumping a car, we connect the + end of the charged battery to the + end of the dead battery, and the - end of the charged battery to the chassis or other metal part of the car.

I always thought that you need a closed circuit for current to flow. But this circuit appears to be open: we are connecting the - end of the charged battery to the ground! Thus, how can any circuit connected to ground have a current?

I believe another way to ask this question is: will jump starting a car still work if I connect the - end of the charged battery to a third (powered-off) car, instead of to the chassis of the car with the dead battery? If so, why? (I've heard people say that jump starting a car only works because the chassis is connected to the electrical components of the vehicle, thus providing a closed circuit since the battery is also connected to the electrical components of the vehicle).

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    \$\begingroup\$ You are confusing circuit ground, chassis ground, and earth ground. An automobile is isolated from "earth ground" by the rubber tires, but the 12V battery negative terminal is connected to the car chassis forming the "chassis ground". The circuitry in the car is also grounded to the chassis, albeit with some other protection in between, but this has nothing to do with the current flowing from one car to another during a "jump." If anything, this circuitry can be destroyed from spikes in the battery voltage. \$\endgroup\$
    – Kurt E. Clothier
    Commented Jul 5, 2013 at 7:42
  • \$\begingroup\$ When you jump a car, you are using the second car's battery to provide the necessary current to turn over the engine's starter. Once the car is running, the alternator will (usually) recharge the battery enough to allow you to start the car on its own a time or two before it is dead again. The alternator is also what is really powering all of the electronics in the car while it is running by keeping the battery charged. If that fails, you will notice your lights get dim or your radio go out as the battery dies while your are driving, but the engine will continue to run. \$\endgroup\$
    – Kurt E. Clothier
    Commented Jul 5, 2013 at 7:46
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    \$\begingroup\$ One reason for making the final connection to the dead car's chassis that a discharged battery can outgas Hydrogen. Since there is likely to be a brief arc as you complete the circuit to the live car's battery, you'd really rather this happened away from the dead battery! It would be safer yet to make the final connection at the live car, that much further away from the dead battery. \$\endgroup\$
    – JRobert
    Commented Jul 5, 2013 at 17:12
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    \$\begingroup\$ @Spoon: The bare essentials includes the fuel pump which is probably the most power-hungry device in the car. In the days of carburetors and mechanical fuel pumps, you could drive quite a lot longer (hours) than you can today with a total-loss electrical system. \$\endgroup\$
    – JRobert
    Commented Jul 5, 2013 at 17:15
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    \$\begingroup\$ Almost 2 years on :-) : Note that the vehicle body is ALMOST at -ve battery potential. Of most importance is the B+ - starter relay - starter motor - B- circuit. The starter motor almost invariably bolts onto the engine block and makes ohmic contact via this path and a large low resistance strap extends from battery minus to the engine block and through the block to the starter. Sometimes the strap attached near the starter motor sometimes not. The block is not usually made of especially high conductivity material BUT the conductor (= the block) is VERY thick so resistance is low. \$\endgroup\$
    – Russell McMahon ♦
    Commented Apr 1, 2015 at 5:07

3 Answers 3

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"Ground" is just a code word which, in this case, refers to the "current return common" circuit node. There is a complete circuit because everything electrical in the car, such as the starter motor, also connects to ground in order to return current to the minus terminal of the battery through the ground. The car's chassis is used for this return network, and so the entire chassis is an extension of the minus terminal of the battery.

During jump-starting, we connect the boosting battery to ground rather than to the dead battery's - terminal for the simple reason that this provides a more direct return path to the good battery which is powering the dead car: the return current does not have to travel through the dead battery's minus terminal hookup cable and then to the jumper cable, but can go directly from the chassis ground to the jumper cable.

A more direct return path allows for better current flow and less voltage drop, like plugging a big appliance directly into an outlet, rather than via an extension cord.

In case you're also wondering why the plus jumper connections are made first, then the minuses. This is because there is no harm done if you leave the minus jumper dangling in the chassis of the car. Anything it accidentally touches is likely to be ground. If you connect both alligator clips on one end before connecting the other end, the other end is now live and you can accidentally touch the clips together to create a short circuit. If you connect the minuses/grounds first and then go to connect one of the pluses, you can create a short circuit, because the opposite side plus is probably dangling and touching something that is grounded.

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    \$\begingroup\$ Thank you! You got right down to my core misunderstanding. My epiphany occurred on the idea that "the entire chassis is an extension of the minus terminal of the battery". I'm still wondering why cars are designed this way, but that wasn't in my original question. Job well done. \$\endgroup\$
    – Ken Levinski
    Commented Jul 5, 2013 at 18:31
  • \$\begingroup\$ @Kaz, if the entire chassis is an extension of the negative terminal, what is the reason that we don't get shocked when we touch a car that is turned on? Is the voltage/current not enough for us to feel anything? \$\endgroup\$
    – Josh Beam
    Commented Sep 23, 2014 at 17:31
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    \$\begingroup\$ @JoshBeam It's essentially for the same reason that you don't feel anything if you touch your tongue to just one terminal of a 9V battery: you have not completed an electric circuit. To be shocked, you have to touch two points that are connected to opposite terminals of the battery. (And the voltage is low, yes). \$\endgroup\$
    – Kaz
    Commented Sep 25, 2014 at 17:56
  • \$\begingroup\$ See, I knew that the chassis is an extension of the '-' terminal, but it never occurred to me that the chassis might be a lower-resistance connection to the starter etc., and I was always like"what's the difference?" On a related note: I had a car once where the chassis was painted--I had to scratch the paint if I wanted to make electrical contact. If it's contact resistance we're worried about, maybe it would have been better to clip to the terminal? \$\endgroup\$
    – Frank
    Commented Feb 24, 2016 at 20:37
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    \$\begingroup\$ If the entire chassis is just an extension of the minus terminal of the battery (given the simple model that everything is connected in parallel to the battery), wouldn't the lower resistance be negligible? I am guessing that the difference would probably be less than 10 milliohm. \$\endgroup\$
    – Alex
    Commented Mar 1, 2017 at 14:51
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The - end of the charged battery is already connected to the chassis, the engine, and, particularly, the starter motor. The whole car is designed to work that way. Everything use earth return. All the lights have one wire, and one connection to the chassis. The spark plugs have one wire, and one connection to the engine block. And so on.

OK, on a modern car you use a second wire for the starter motor, because it draws a lot of current and you would get a significant voltage drop and power dissipation in the earth return path. And diesel engines don't have spark plugs. And high power driving lights are driven through a relay like the starter motor is. And lights mounted in plastic fittings need 2 wires.

Given that the -ve end of the battery is connected to the chassis and the shell and the engine, where is the best place to connect the return wire?

Traditionally, you connected it to the chassis or the shell or the engine, so that you didn't have to lean, with a live wire, into the engine compartment, across the engine (hot moving machinery), on to the battery (hydrogen explosion risk, with sulphuric acid as well). But this assumed that the battery was difficult to reach, and that the engine and chassis had a very good connection to the battery (required for the starter motor earth return).

Nowadays, some people make the return connection direct to the battery, if they can reach it easily.

Even if you connect one end of the return wire to the chassis, you normally connect the first end direct to the first battery. This because until the wire is connected to the first battery, it is not a live wire, will not spark when you touch it to something, is not particularly dangerous. After the return wire is connected to the first battery, it is live, and is dangerous. So you connect it to something safe that is easy to reach.

No, you can not connect the return wire to a third car. Cars sit on rubber tires, and the tires insulate each car from other cars. You need a complete circuit from your battery to the other battery.

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  • \$\begingroup\$ Hey David! Thanks for your answer. I was a little overwhelmed with the details because I don't think I know enough about electronics in order to appreciate your answer. Nevertheless, I'm sure that it'll be useful in the future once I've studied a little more into the area. \$\endgroup\$
    – Ken Levinski
    Commented Jul 5, 2013 at 18:40
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The main reason for connecting to the positive terminal on the battery first is to do with volatile gases possibly being emitted from the battery. If you connect to both terminals at the battery terminal this will usually cause some kind of spark as you first touch the cable to the terminal, whether you do positive or negative first is irrelevant. Connect the positive at the battery terminal first(no danger of a spark as a complete circuit is not formed), then connect the negative cable to a point on the chassis away from the battery, so the resulting spark is not in the area likely to be affected by any gases. that way you have ruled out the possibility of igniting the gas and have avoided any chance, however small. of an explosion.

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