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I've been reading about the science behind jump starting a car and everyone keeps talking about the correct order and why the negative shouldn't connect to the dead terminal, etc... but no one explains why the negative cable has to be connected in the first place. This might be the most stupid question you've heard but I can't wrap my head around it.

Electricity flows from + to -, so why not simply connect the good battery + to the dead battery + and ignore the - ? Since the dead battery is still connected on both end, the circuit is still complete and we only take positive charges from the good battery. Also, if we do need to connect the negative, why not connect it from the good battery - to the good car bare metal, instead of the dead car ?

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  • It gives more power if the old battery is not connected; if connected it will draw power from the good battery. The amount drawn by the old battery may or may not be critical. A few times I have not connected the negative but pushed the metal bumpers into good contact . You can then connect cables end to end for twice the length. Feb 16 at 0:39
  • I do not want to write a third answer, but both current here are correct. It is easier not to short anything if you connect - last. To any grounded place in the vehecle, while the + really should go to one spot. (Frog), And electricity does not flow from + to +, or rather a tiny tiny electrical change might. Possibly not enough to light a bicycle light. And then it would stop. You really need a connected circuit. If it helps, think about the battery as a water pump, pumping a stream around.
    – Hennes
    Feb 16 at 7:20
  • I forgot. OLD style bateries which needed refillign with distilled water now and then also lost that water. Including loosing it when being (over)charged and then the water splits into an explosive gas. Highly explosive gas, concentrated acid and sparks are not a recommended situation.
    – Hennes
    Feb 16 at 7:21
  • Sorry, I'm even more confused now. You're saying that the dead battery is not even necessary here and can be removed, the point is to start the car with the good battery and then let the car recharge the dead one right ? Next thing you're saying that we need to complete the circuit, how is connecting the good battery to the bare metal completes it ? Does electricity flow through the car's metal and if I touch it I'll get shocked ? I would have understood if by completing the circuit you meant connecting the + to + and - to - but here we are connecting the - to the metal.
    – Dnkey
    Feb 16 at 13:34
  • The only explanation I could come up with is that connecting the negative from the good battery is not necessary and we only do it in case the positive cable touches the car's metal and we want electricity to flow back to the battery instead of through a person touching the car. But I've never seen anyone give this reason, so I'm pretty sure I'm misunderstanding something.
    – Dnkey
    Feb 16 at 13:35

4 Answers 4

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if you don't connect the negative from the good battery to the bad battery you don't benefit from the good battery because it does not complete a circuit between the two batteries. the reason the last connection is from the good battery negative to the dead car ground instead of at the battery terminal is prevent sparks that can cause an explosion with some batteries.

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    I jumped many batteries without incident , but my brother, the college professor, managed to explode the side off a battery. It is possible. Feb 16 at 0:41
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In reality electricity flows in a circle, the good battery tries to push it round and if there’s a break in the circuit then it won’t flow. The main thing to look out for when fitting jump leads is to fit the positive one first. If you fit the negative one first then when you come to fit the positive clip it can short against almost any part of the vehicle, with bad consequences (melted leads, exploding batteries). Similarly, remove the negative lead first when you’re done.

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Electricity flows from + to - on the same device (i.e. the same battery). Current flows in a loop through a conductor (metal) and can be stopped by insulators (rubber tires, the ground, etc) The negative terminal on the dead battery isn't connected to the negative terminal on the good battery any more than it is magically connected to my battery likely hundreds of miles away. So the circuit is not completed.

This is why the negative terminal of the good battery must be connected to the negative terminal of the dead battery (or some other exposed metal on the chassis of the dead battery car).

Why can you connect it to the metal chassis on the dead car? Because the negative battery terminal is directly connected to the cars chassis. Why do you not get shocked when touching exposed metal on your car? Because you are not completing a circuit. You are not also touching the positive battery terminal. Lets say you are touching metal on your car and then touched the positive terminal, even then you would not get shocked (as long as your hands are dry) because dry skin is not a good conductor and the voltage of the battery is low (12 volts). Its a different story (and not safe) if you touch line voltage at 120v.

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    Alright I think I'm starting to get a clearer picture now that I understand that the negative terminal is connected to the car's chassis and I understand why connecting the good battery - to the dead battery - or dead car metal is the same, but we prefer the metal to avoid sparks near the battery.
    – Dnkey
    Feb 17 at 0:00
  • Does that also mean that at some point the positive cable ( can I call it the hot wire ? ) connects to a load - which would be what in the car ? The other side of the load then connects to the car's chassis which goes back to the battery negative terminal ? Is that the full circuit ?
    – Dnkey
    Feb 17 at 0:07
  • @Dnkey "Does that also mean that at some point the positive cable ... connects to a load" Yes, absolutely, I think you are getting it now! One thing the positive cable connects to is the starter motor, in fact this is the most important thing it connects to, with the biggest wire as its the highest current draw. For the pedantics out there it connects indirectly through the starter solenoid which is controlled by the ignition system, but I didn't want to complicate things.
    – Glen Yates
    Feb 17 at 14:06
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Electricity actually flows from minus to plus: in metals, electricity is carried by electrons and electrons are negatively charged. The idea that current flows from positive to negative is a historical mistake made before it was realized the current carrier actually has a negative charge.

So, by your reasoning, slightly corrected, you should connect only the minus. That's untrue.

The problem here is that a battery is a charge pump. If the electric potential between its terminals is less than about 13 volts, it takes an electron from its positive terminal and moves it to its negative terminal (actually they are different electrons to be precise, but the effect is the same, electron disappears from one terminal and appears at another terminal). This forces the potential difference to be always about 13 volts (ignoring internal resistance here).

Let's assume the failed car battery has exactly zero volts, for simplicity.

Let's also assume the failed car battery has a potential difference with respect to Earth of plus 3000 volts (very typical, cars are notorious for creating static electricity and static electricity has a surprisingly high voltage). Both negative and positive terminal is at 3000 volts.

Let's now assume the good car battery negative terminal has a potential difference with respect to Earth of plus 1500 volts. This means the good car battery positive terminal has a potential difference with respect to Earth of plus 1513 volts. The battery as a charge pump maintains the difference of 1513-1500 = 13 volts.

Now if you connect the negative cable only, it will rapidly equalize the negative terminals. If we assume the grounded parts of both cars to have same charge storage capacity, they will cause the connected-together negative terminals to equalize to the average of their original voltages, which is 2250 volts. This happens by electrons flowing from the good car battery negative terminal to the bad car battery negative terminal. You may see a very small spark of static electricity during the connection, but you'd have to be in a very dark environment and have an excellent eyesight to see the minor spark of static electricity.

The good car battery is a device that ensures the potential difference between its terminals is always 13 volts. So the charge equalization between negative terminals ramps up the voltage difference of negative terminal with respect to Earth. This threatens to cause the potential difference between its terminals to fall below 13 volts. The battery as a charge pump thus takes electrons from the positive terminal and moves them to the negative terminal. This causes the positive terminal to move to a higher electric potential.

So now you have this:

  • Good car battery negative terminal: 2250 volts
  • Good car battery positive terminal: 2263 volts
  • Bad car battery negative terminal: 2250 volts
  • Bad car battery positive terminal: 2250 volts

There will be no charges moving after that. You haven't completed a circuit by only connecting the negative lead.

Now what happens if you connect the positive too? Because the good car battery positive terminal is at 2263 volts, higher than bad car battery positive terminal at 2250 volts, electrons want to start moving to a higher potential, because they are attracted by the positive charge of the higher potential. So electrons in the bad car battery positive terminal start moving towards the good car battery positive terminal. This gradually raises the potential of the bad car battery positive terminal, to 2263 volts in fact, because less electrons means more positive. The electrons moving towards the good car battery positive terminal lower its potential, so the potential difference between the good car battery positive and negative terminals threatens to fall below 13 volts. The good battery is a charge pump, so it moves electrons to the negative terminal, maintaining the 13 volts of voltage difference.

What you then have is the following equilibrium:

  • Good car battery negative terminal: 2250 volts
  • Good car battery positive terminal: 2263 volts
  • Bad car battery negative terminal: 2250 volts
  • Bad car battery positive terminal: 2263 volts

Now there are two possibilities. Either the bad car battery is completely dead. This means the 13 volts of voltage difference between its terminals no longer causes any charging current. If this happens, the situation stays in the equilibrium. However, it's also possible the continuous 13 volts of voltage difference between the bad car battery terminals causes a continuous charging current to flow (in which case to be precise the potential difference at different points will be different due to Ohm's law, resistance of metals, and resistance of batteries, but if we assume most internal resistance is at the bad battery, then the same voltage difference of 13 volts is a good approximation). If this means, you have the following circuit with continuous current:

  • Electron at good car battery positive terminal moves to good car battery negative terminal (actually this is not the same electron as it causes chemical reactions to happen, the chemical reactions slowly deplete the battery state of charge)
  • Electron moves from good car battery negative terminal to bad car battery negative terminal
  • Electron moves from bad car battery negative terminal to bad car battery positive terminal (actually this is not the same electron as it causes reverse chemical reactions to happen, the reverse chemical reactions slowly increase the battery state of charge)
  • Electron moves from bad car battery positive terminal to good car battery positive terminal

This continues as long as the state of charge in the batteries is different. Once they have the same state of charge, the process stops.

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  • Awesome example, so if I understand it correctly, when the car is using the battery, the current is moving from negative, onto the chassis, through the loads and back to positive. And when it's being charged it's probably the opposite, though I would ask how do the loads keep working with electricity flowing in the other direction.
    – Dnkey
    Feb 18 at 15:14

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