Rechargable Cell +/- Convention with Respect to Car Battery

[Rickybobby856]

It appears that car batteries have the opposite +/- terminal convention than what one might expect.

With respect to rechargeable cells (such as car batteries), when in discharge mode the negative terminal is the anode and electrons flow out of the battery at this point, as seen in this picture:

I am fairly certain that electrons flow from the car battery’s positive terminal when in discharge mode (versus being charged by the alternator). However following other battery +/- convention, the positive terminal of a car battery should really be labeled as the negative terminal. As seen in the diagram below for jumping a car, electrons are flowing form the positive terminal of the live battery (on the left) to the dead battery (on the right) to the engine starter. Why isn’t the car battery terminal where electrons are leaving labelled the negative terminal, like all other battery convention?

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[aldol16]

Great question! I actually commend you on thinking about jumping a car because not only do most people not even know how to do it correctly but 99.9% of people don't actually know what they're doing when they're jumping the car in terms of electricity and physics. So let me go through what happens in a good car and what happens when your battery dies. I hope it'll elucidate everything - this is the way I understand it and I don't pretend to be a physicist, so it could be completely wrong. So in a good car, you have a battery. But what's the battery for? The battery supplies current to the starter, which is of course what starts the car. That's all it does. So when you try to start the car, the electric current flows out of the positive terminal, through the load, and to the negative terminal. This supply of current provides the electricity to the starter. Electrons, of course are flowing the opposite direction. Everything's good.

But now your battery dies. So when you go to start the car, the current no longer flows. So you ask your buddy to jump start the car for you. What's happening there? All your buddy is doing is bypassing your dead battery. He's not charging your battery. I repeat, he's not charging your battery - your alternator does that. Remember, your battery is just to start the car. After the engine is started, the alternator provides the current required to power your car and the battery is no longer needed. So when your buddy's car is running, the battery is basically not doing anything. Now remember your battery is dead so it's basically a piece of junk metal (with toxic chemicals inside). By connecting the positive terminal of your dead battery with the positive terminal of his car, you're bypassing your dead battery. You have two batteries in parallel with each other. Your actual physical positive terminal is just an easy chunk of metal to attach the clips to. You could theoretically attach the red clip anywhere on the circuit prior to the load (the starter). And then you attach the black clip to the negative terminal of your buddy's car and ground it somewhere on your car to complete the circuit. So now when you start your car, the current is flowing from the positive terminal of your buddy's car, through the positive terminal of your dead battery which again is just a piece of metal, through the load, to the negative terminal of your dead battery which is the ground and connected to your chassis, through the jumper cable which is connected to some metal surface of your car, and back into the negative terminal of the good battery.

Electrons, of course, move the other way. To illustrate, consider it like this. You can think of the chassis as an electron source for all of this. The electrons move from there via the negative terminal of your dead battery through the load to the positive terminal of your battery to the positive terminal of the good battery to the negative terminal of the good battery back through the cable to the chassis of your car and thus completing the circuit. This bypasses whatever is between the positive and negative terminals of your dead battery, which is what caused it to die.

So usually if your battery isn't holding charge (i.e. you're not just leaving your lights on, etc. when the car and thus alternator is off), the issue is what's between the positive and negative terminals of your battery, i.e. inside your battery. And so you'll need a new one. It's not fixed just because your buddy jumped your car.

Note of caution: never attach the red to black and black to red. This will kill both batteries - sometimes violently.

 

[Rickybobby856]

Thank you for the help. That all does make sense. At first it seemed a little unintuitive that electrons would flow from the negative terminal through the car chassis before getting to the load (i.e. the starter) and then eventually returning to the positive terminal (whether that be your battery when it is working or your buddy's battery when he is jumping your car). I was originally thinking that electrons were flowing from the positive terminal to the load then through your chassis then eventually to the negative terminal. That is where my confusion arose, as that would imply a backward convention compared to the typically galvanic cell convention (electrons flowing from the negative terminal to the positive terminal). I guess it doesn't really matter that electrons have to travel through the chassis before reaching the load, as all incurred resistance is additive because everything is in series (the resistance of the load and the resistance imparted by the wiring/chassis).