How does jump starting a car work if current only flows when there’s a difference in voltage?

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Jump starting a car refers to recharging a dead (uncharged) car battery just enough to get the starter motor rotating. To jump start a car, you need a second car with a fully charged battery; you then connect the positive terminals of each battery to each other, and the negative terminals to each other (WARNING: read below). Since both batteries should generate the same voltage, the voltage between the two positive terminals or between the two negative terminals should be 0, meaning no electricity will flow. So how does this result in the dead battery being charged?

The answer lies in the side effects of draining a battery. Batteries die not because they lose voltage or the ability to store charge, but because resistance between the terminals increases as the internal electrodes corrode. However, most battery types – especially the lead-acid batteries used in cars – also lose a bit of voltage as they lose charge (this is how battery indicators know when a battery is dying). In the case of car batteries, a fully charged battery will measure about 12.6 volts between the terminals, while a completely dead battery will measure 11.9 volts. Thus, there actually is a voltage between the two batteries which will conveniently cause charge to flow from the full battery to the dead one.

WARNING: Above I mentioned that to charge a battery, you simply connect like-terminals on each battery. Though you can connect the terminals in any order, you never should. Here is the usual order for connecting battery terminals:

  1. Connect one cable to the positive (red) terminal of the dead battery
  2. Connect the other end to the positive (red) terminal of the full battery
  3. Connect the second cable to the negative (black) terminal of the full battery
  4. Connect the other end to a shiny, non-painted metal surface on the car of the dead battery (eg. the chassis or the engine-block).

There are two reasons for this ordering: first of all, creating a short-circuit could cause the battery to overheat and explode, spewing dangerous acid all over the place – this ordering minimizes the chances of a dangling cord accidentally causing a short-circuit.

Secondly, since before jumping you can never be sure the battery is at fault, there’s a chance that the "dead" battery may actually be full, and you’ll be overcharging one of them. Overcharging a car battery produces hydrogen gas, the explosive gas responsible for the Hindenburg Disaster. Needless to say, because of this possibility you’ll want to try to generate as few sparks as possible, and keep them away from the batteries. This not only explains the ordering, but why you connect to the chassis instead of the negative terminal – the circuit stays complete either way because the negative terminal and the chassis are electrically-connected in all cars (unless a wire broke!).


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One Comment

  1. Andreas says:

    Two minor points: The minor voltage difference is between a full and an empty car battery is not enough to charge the empty one in reasonable amount of time. Using your example and assuming an internal resistance of 25mOhm, you would have a charging current of ~7A (at that only while the voltage difference was that high), which would be a pretty meager charge current for a car battery which typically has 40Ah-50Ah.

    Also, if you waited long enough then you would end up with two batteries that are half full (since the net capacity has to stay the same). The reason it works lies in the behaviour of the battery under load.

    The voltage of an empty battery will break down if you try to extract a high current, partly because of the higher internal resistance, chemistry etc. Which is the reason why you might be able to still turn on your car (lights, etc.) even when you can't start it. The needed current for those small loads are small enough that the battery voltage stays high enough to still work.

    When you now connect a second battery in parallel you can start the car with the empty battery it will use the energy provided by the full battery to start, running the motor and therefore the alternator will then load the empty battery.

    As a sidenote: You don't want to start a car with a car that is not running, since you don't want to discharge the battery if the car that is working.

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