Flow of electricity when you jump-start a car

[Humberto]

I'm always forgetting how to jump-start a car, so that I have to look it up on the internet each time someone asks me to jump-start their car for them. I think the best way to remember how to jump-start a car is to understand how it works, what the rationale is for doing what we're doing.

So this is how I think it works:

In an electrical circuit, electrons flow from the negative terminal of a battery to the positive terminal.

This is how you connect batteries when jump-starting a car:

1. Connect jumper cable to positive (red) terminal on dead battery.
2. Connect jumper cable to positive (red) terminal on good battery.
3. Connect jumper cable to negative (black) terminal on good battery.
4. Connect jumper cable to engine block (not battery) of car with dead battery.

I've tried to figure out why this works and this is what I came up with:

The trick is to see the positive terminal of the dead battery not as part of the battery, but as the end of the electrical circuitry of the dead car before the circuitry re-enters the dead battery. Call the end of this circuitry (X); this happens to correspond to the positive terminal of the dead battery. Then:

• Electrons move from the negative terminal of the good battery to the engine block of the car with the dead battery.
• This causes electrons to move into the good battery though its positive terminal.
• This causes electrons to move out of (X).
• This causes electrons to move through the electrical circuitry of the dead car. These electrons don't enter the dead battery through its positive terminal, but enter the good battery though its positive terminal.

Hey presto, the dead car has current moving through its circuitry as though its battery were working.

In effect, we are by-passing the dead battery when jump-starting a car. The fact that we connect a jumper cable to its positive terminal has nothing to do with the dead battery itself; it is merely a way of moving current through the electrical circuitry of the dead car.

Please critique my explanation.

 

[Richardxtc]

connecting red on red and black on black is more simpler. you will never skip the dead battery as it is part of the "circuit" unless you completley disconnect it

 

[bruce_the_loon]

What you are doing when you jump-start a car, you are adding a second battery in parallel to the first. Since the engine block is grounded to the negative battery, it doesn't matter where you connect the leads to. Whatever is easiest to reach is the best choice.

The natural flow of electrons within the circuit will draw power from the good battery into the flat battery. If you left them joined, they would eventually end up with even charge. What you might be thinking is that if the flat battery is completely between the good battery and the circuit, that the dead battery will suck the charge in, but won't do it if it is off to one side. Electricity doesn't work that way, it doesn't follow the path of least resistance and not down the other path, it spreads across the entire circuit.

Effectively you have two circuits overlapping. The first is between the two batteries, the second is between the good battery and the starter motor. Only if you disconnect one of the flat battery terminals will you eliminate that circuit.

 

[etwylite]

Firstly the entire chassis of a car is negatively grounded. So connecting anywhere on the body or chassis will complete the circuit. The recommendation of engine block is that an unpainted surface produces a better uninsulated contact point. By NOT connecting at the neg terminal of the recipient vehicle, you avoid a spark and possible battery explosion from hydrogen gas.

 

[Sinbad]

Firstly the entire chassis of a car is negatively grounded. So connecting anywhere on the body or chassis will complete the circuit. The recommendation of engine block is that an unpainted surface produces a better uninsulated contact point. By NOT connecting at the neg terminal of the recipient vehicle, you avoid a spark and possible battery explosion from hydrogen gas.

This. To clarify though, you don't avoid a spark, but you keep sparks away from the battery.

 

[Ou grote]

connecting red on red and black on black is more simpler. you will never skip the dead battery as it is part of the "circuit" unless you completley disconnect it

That's how I remember it.

But these days, I have a warning buzzer if I leave my light on.

 

[pcdoc]

That's how I remember it.

But these days, I have a warning buzzer if I leave my light on.

Before disconnecting jumper cables from dead battery remember to switch on headlights, aircon, radio etc so that you you do not damage onboard electronic components

 

[Ou grote]

Before disconnecting jumper cables from dead battery remember to switch on headlights, aircon, radio etc so that you you do not damage onboard electronic components

How about posting a step by step guide for us.
I'll print it and leave it in my car just in case.

 

[SauRoNZA]

connecting red on red and black on black is more simpler. you will never skip the dead battery as it is part of the "circuit" unless you completley disconnect it

Just because its simpler doesn't mean it's correct.

That's the fastest way to not only melt the jumper cables, but also the components in the car.

And believe me, you don't want to pop the ECU on a modern car.

 

[Moosedrool]

I'm always forgetting how to jump-start a car, so that I have to look it up on the internet each time someone asks me to jump-start their car for them. I think the best way to remember how to jump-start a car is to understand how it works, what the rationale is for doing what we're doing.

So this is how I think it works:

In an electrical circuit, electrons flow from the negative terminal of a battery to the positive terminal.

This is how you connect batteries when jump-starting a car:

1. Connect jumper cable to positive (red) terminal on dead battery.
2. Connect jumper cable to positive (red) terminal on good battery.
3. Connect jumper cable to negative (black) terminal on good battery.
4. Connect jumper cable to engine block (not battery) of car with dead battery.

I've tried to figure out why this works and this is what I came up with:

The trick is to see the positive terminal of the dead battery not as part of the battery, but as the end of the electrical circuitry of the dead car before the circuitry re-enters the dead battery. Call the end of this circuitry (X); this happens to correspond to the positive terminal of the dead battery. Then:

• Electrons move from the negative terminal of the good battery to the engine block of the car with the dead battery.
• This causes electrons to move into the good battery though its positive terminal.
• This causes electrons to move out of (X).
• This causes electrons to move through the electrical circuitry of the dead car. These electrons don't enter the dead battery through its positive terminal, but enter the good battery though its positive terminal.

Hey presto, the dead car has current moving through its circuitry as though its battery were working.

In effect, we are by-passing the dead battery when jump-starting a car. The fact that we connect a jumper cable to its positive terminal has nothing to do with the dead battery itself; it is merely a way of moving current through the electrical circuitry of the dead car.

Please critique my explanation.

You must work for an insurance company or something.

This is the circuit:

 

[agentrfr]

The negative terminal of the battery is the ground for all circuitry of the car. You connect the cables in the order you do to keep the sparks that form as far away from anything that can easily melt (I.e you want it to spark away from the bare wires leading into the battery of the dead car, so you grond the voltage rise of the fresh battery to the engine block or car frame, where the sparks are less likely to melt wires on the battery terminals of either car).

In terms of the circuit, you are just putting the fresh battery in parallel with the dead battery. Current will flow from the fresh battery both to get the starter to run as well as through the dead battery (like a battery charger for your phone). There is no bypassing of the dead battery , infact it makes its own internal circuit with the fresh battery. This causes a lot of strain on the fresh battery, which is why you have to rev the engine of the fresh car while you try start the dead car - the fresh car's battery usually needs help with the whole ordeal. Once the dead car's alternator is charging the dead battery, then you are both okay (assuming the dead car's driver doesn't stall it 5 meters up the road )

 

[Richardxtc]

Just because its simpler doesn't mean it's correct.

That's the fastest way to not only melt the jumper cables, but also the components in the car.

And believe me, you don't want to pop the ECU on a modern car.

hmmmm never seen jumpers melt and i have jumped a lot of times, i have a golf hahahahaha

 

[zippy]

Do what most women do. Phone the AA and show some flesh if you aren't a member.

/runs away

 

[grok]

Do what most women do. Phone the AA and show some flesh if you aren't a member.

/runs away

Or get a new battery like I did this morning. R1500 later ouch!

 

[Alton Turner Blackwood]

I have to get jumped at least a couple of times a month as I always seem to forget that Amps don't like to be on when the car isn't idling