I got to thinking while boosting my lawnmower’s battery. It is my understanding that the electrons leave the battery from the negative pole and enter back in via the positive pole to close the circuit. I just never understood why the 2 negative terminals were connected to each other and not to the opposite charge when performing a “boost”. If the dead battery needs energy, why not plug the good battery Negative to the bad batteriy Positive to “feed it”?
I can’t make sense of two batteries both having their negative going to one another…
My nose is bleeding. Help. Thanks!
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Electricity can be kind of confusing like that. We have so many analogies for it but none of them are fully accurate. What you need to know for batteries is that we are taking electrical energy and storing it as potential energy in a reversible chemical reaction. Charge the battery we aren’t adding electrons, we are giving energy to one side of the battery which causes reaction to go to its energized state. Think of it like winding up a spring.
So you’re taking the energized end and (the positive terminal) and using it to give energy to the depleted positive terminal of the dead battery. The negative terminal is just there to give the energy something to safely and stably flow towards. That’s why you can connect the black negative cable to a ground wire and not the negative terminal of the batter and still have the same effect.
Think of the positive and negative terminals as long metal rods. Current will flow along any connection between these two rods, be it a starter motor, light, or whatever. If your car’s battery is dead, it cannot produce enough current to run the loads. Attaching another battery between the other ends of the rods not only supplies current to run the loads, but also begins charging the dead battery.
Never reverse polariry!
When a battery is doing work, it’s using chemical reaction to create energy, to push electrons out the negative end and suck electrons into the positive. When your battery is weak or has no charge, that means the chemical energy has been used up and it can’t push and pull anymore electrons around.
Feeding a strong battery’s negative to the weak negative pushes electrons back into it and reverses the chemical reaction, feeding energy back into the system (like blowing air into a deflated balloon). Connecting the strong negative to the weak positive won’t do anything, because the positive plates are already filled up with electrons. It’s the negative plates that need the electrons.
When charging a battery, electrons must always be fed in the opposite direction of how they move when the battery is doing work.
With a car battery, there’s two separate things going on: on the one hand, you need power from the battery to start the engine; on the other hand, once the engine is running, the alternator produces electric power with which you can charge the battery.
When you boost a battery, you’re not trying to charge the dead battery with the charged battery. You’re borrowing the charged battery to get the engine going, so the engine/alternator can get to work charging the dead battery as normal. For that to work, the two poles on the charged battery need to connect to the car’s electrics at the same points as the dead battery’s poles (so it can be used to get the car running), while the dead battery is still connected at those same points (so it can charge as normal)
You’re not putting energy into the dead battery, you’re using the good battery to start the engine. Think of it as taking the dead battery out and replacing it with the good one.
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