When jumping a car battery the standard advice is to connect the red (positive) cable first. What's the physics explanation for this?
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5$\begingroup$ PSA: All answers to circuit related questions should include a circuit diagram :-) $\endgroup$– DanielSankCommented Jan 6, 2017 at 23:14
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4$\begingroup$ @DanielSank hmmm if you need a circuit diagram to understand how a car battery layout works , you probably should ring your local garage. :) $\endgroup$– user140606Commented Jan 6, 2017 at 23:49
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8$\begingroup$ @TaMeCeart the point of websites like this is for people to learn how things work. I hope such learning removes the need to ring the garage. $\endgroup$– DanielSankCommented Jan 6, 2017 at 23:52
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2$\begingroup$ Related on Motor Vehicle Maintenance & Repair: Jump starting a dead battery: Connect black wire to negative pole of battery or grounded metal? $\endgroup$– FerrybigCommented Jan 7, 2017 at 12:21
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2$\begingroup$ Note that this assumes (what is somewhat misleadingly named) the "negative earth" convention. It's widespread, but not all vehicles (as questions like mechanics.stackexchange.com/questions/1934 show) have it, including vehicles manufactured in the U.K. from the late 1930s to the mid-to-late 1960s. $\endgroup$– JdeBPCommented Jan 7, 2017 at 15:19
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5 Answers
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This is more of an automotive question but...
The reason you connect the reds first is to minimize the likelihood of a short. Remember that you're typically in control of one clip at a time, so one of them is not fully in your control.
The particular trouble case is the last clip that you put in place. If you attach the negative sides first, then one positive clip, the other positive clip is now at 12V with respect to the "ground" of the other car's body. If the other red clip touches the other car almost anywhere in the engine, you'll create a short circuit. This is really easy because there's a lot of metal in an engine compartment!
If you connect the positive clips first, and then one negative clip, the other clip is now at roughly 0V with respect to the "ground" of the other car's body. If it touches anything in the engine, it's no big deal. The only way to create a short circuit here is if you explicitly touched the black clip to the red one, and that's a lot harder to do by accident.
answered Jan 6, 2017 at 23:10
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This question is a bit vague and not obviously entirely confined to physics. In order to make it a physics question, I'll focus on which order is better if the main concern is what happens to the circuit if we accidentally drop one of the cable's connectors.
Suppose we connect the black cable first, i.e. the one that goes on the "minus" terminal of the battery. Suppose also that the minus terminal is connected to the car chassis. Now suppose we connect the red cable to one of the batteries, and then accidentally drop the other end of the cable on somewhere on the car chassis. See the diagram.
Now we've shorted the battery, so lots of current will flow. There will probably be sparks etc. and if you leave the connection for too long (possible a very short amount of time) the battery will probably get hot and do I don't know what (i.e. catch fire/explode).
If you drop one of the connectors, it's easy for it to touch the chassis of the car because the chassis is big. On the other hand, dropping a connector onto the positive terminal of the battery is much more unlikely.
Therefore, if our main concern is what happens if we accidentally drop the cable, it's better to connect the red one first.
answered Jan 6, 2017 at 23:09
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8$\begingroup$ The sketch is not very intuitive for your description. The short shown would even happen if the black cable where disconnected. The short that is prevented by not connecting the black cable would happen if open end of the red cable would be dropped to the other car's chassis. $\endgroup$– dronusCommented Jan 7, 2017 at 14:52
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$\begingroup$ @dronus: If one begins by connecting the red wire on the car with the depleted battery, the severity of a short even if the red wire were dropped on that car would likely be far less than in a car with a fully-charged battery. $\endgroup$– supercatCommented Jan 7, 2017 at 18:09
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$\begingroup$ If the the battery which is short-circuited is the fully-charged one, the time required to overheat and melt the wire could be well under a second, so the phrase "too long" might not really be applicable. $\endgroup$– supercatCommented Jan 7, 2017 at 18:10
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$\begingroup$ @dronus agreed. Please feel free to edit. I might not get to it for a while. $\endgroup$ Commented Jan 10, 2017 at 22:49
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According to (IIRC) an old Haynes manual for an MG Midget, the true reason for connecting the reds first is that lead acid batteries produce hydrogen, and when you connect your jumper cables to the battery terminals there may be a spark in the event of a potential difference between the batteries (one of them is probably somewhat dead, so this is likely).
I have heard a first hand account of this causing an explosion, but did not see it with my own eyes. Damage was said to be limited to singed body hair, but surely this is better avoided.
Therefore, the correct procedure for jumpstarting, as I was taught, is as follows:
Connect the red cable to both positive terminals, in whichever order you like. No spark, as there is no difference in potential.
Connect one end of the black cable to the negative terminal of the good battery. The car may be running and this is the easiest/safest spot to reach. Still no sparks.
Now connect the other end of the black cable to some part of the engine block away from the battery of the dead car. This way any sparks produced when you connect the final cable will be away from the battery & any potential outgassing. The engine is connected to the negative terminal of the battery by a large cable, so you will still make a good connection.
Removal is the reverse of assembly...
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$\begingroup$ Connecting the red terminal to the dead battery first would minimize damage if the other red clamp were accidentally dropped so as to contact the frame of that vehicle. $\endgroup$– supercatCommented Jan 7, 2017 at 18:11
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$\begingroup$ @supercat The potential between the positive terminal of one vehicle and ground of the other is still zero(ish) until the ground cable is hooked up, so it shouldn't matter either way? I remember some even older advice about parking the vehicles so that their (chrome) bumpers are touching for better ground connection, but I assume we are out of the 50's here. ;-) $\endgroup$– jkfCommented Jan 7, 2017 at 19:00
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2$\begingroup$ @jkf: You really want to avoid your vehicles being in contact while jump starting. $\endgroup$ Commented Jan 7, 2017 at 22:57
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$\begingroup$ @LightnessRacesinOrbit Yeah that is not something I was recommending, but I know I've seen it suggested in vintage manuals. I don't think it would make anything worse though, in the case that the vehicles in question were vintage? If the bumpers are both chrome and in contact the theory seems sound enough -- for instance if your jumper cables were too skinny to turn over a cold engine you could touch bumpers and double the cables up on the positive terminals? Again not recommended unless you are living in the 50's... $\endgroup$– jkfCommented Jan 8, 2017 at 4:10
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1$\begingroup$ @JerryCoffin What you describe amounts to using the running vehicle as an (inefficient) generator/charger, and will work if you have lots of time and/or a battery which is not too badly discharged. But the reason decent jumper cables have very heavy gauge wire is to minimize their resistance (trying to add some physics to this!) so that the large amperage needed to turn the starter does not result in too much voltage drop to start the car. You can start a car with some dead battery cells or even no battery at all (do not do this, very hard on electronics!) this way in an emergency. $\endgroup$– jkfCommented Jan 8, 2017 at 23:06
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Depends on how careful you are with your wrench, I zapped myself a few times by using a wrench which accidently made a connection between the positive pole and the car body. Disconnecting the negative first is better. This is for removing the cables
For setting up, connect the positive cable first. Older, non sealed batteries do sometimes outgas and a spark around that is not what you need.
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OK. Here is the real reason. Batteries sometimes produce hydrogen gas which is extremely flammable. When jump starting it is assumed you will not connect the negative cable to the negative battery cable at all but will instead connect it to a substantial piece of metal of the car, say a bolt on the engine. When you connect the cables, start with the positive/red one onto the positive battery cable and it will not spark. Now connect the negative cable (black one) to a piece of metal that is away from the battery, which means it is away from any hydrogen gas emitted by the battery. You almost always get a spark when connecting this negative cable. So the reason is to keep the spark away from hydrogen gas.
