Simple enough question - until you think more about it…
How does the car engine continually recharge the (conventional) car battery when the engine is running? First, what is physically happening to generate a current off the engine. And, second…how can the wiring allow current to flow back to the battery when current is supposedly flowing to the spark plugs, right? (And, there are only two wires off the battery.)
Yes, I do dare ask what seems so simple and well-accepted as obvious…at first.
The current is generated by an alternator that is coupled via a belt and pulley to the engine’s crankshaft. More than you ever wanted to know about alternators here: Alternator - Wikipedia
This one is quite simple. The positive terminal of the alternator is connected to the positive terminal of the battery and the spark plugs. So the alternator charges the battery and fires the spark plugs at the same time. If for some reason the alternator fails, current for the spark plugs is drawn from the battery alone. I will try to upload a simple schematic.
And of course the current from the battery/alternator passes through coils and/or a distributor before it reaches the spark plugs but i’ve ommited them.
In simple terms, the battery is only there to start the engine (and run accessories like the radio and headlights when the engine is off). When the engine is running, it acts as a generator, creating the sparks and restoring to the battery whatever charge it lost in the start.
The alternative to having a battery is using a crank to hand-turn the engine until it is “primed” and can cycle on its own. Batteries is better.
The battery is simply a storage device. It stores enough electrical energy to start the car. Once the car is running the alternator supplies all the energy necessary to run the vehicle and recharge the battery.
How does the battery get recharged if there are only two cables? Fairly simple really it has to do with the voltage. Electricity will flow from a higher voltage to a lower one. As the state of charge goes down, the battery voltage goes down. A fully charged battery has about 12.6 -12.72V. A battery that is 75% charged has about 12.4V. To recharge a battery it takes a little more voltage than the battery puts out. It takes about .2V per cell to overcome the resistance inside the battery. Six cells in a 12V car battery means that you need about 1.2V more than battery voltage to start the charging process. The alternator puts out at least 13.92V (12.72V of a fully charged battery + 1.2V) Some Alternators charge at a higher voltage. The voltage regulator inside the alternator controls the rate of charge. On modern cars they are mostly all microprocessor controlled as to charge rate and voltage.
So what happens when you are driving down the road and you turn on your headlights? You are increasing the electrical load on the system. The load will be taken up by the battery, and a moment later the alternator will ramp up it charge rate and take car of the load.
