I realize there are sophisticated computer controls in today’s automobiles, so let’s keep it simple and use an old Jeep CJ as an example. The thing sometimes powers the lights, wipers, water pumps, radio and charges the battery, but once the battery is completely charged, and all those accessories are turned off, the alternator is still churning out the same output. Where does it go?
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The alternator only outputs current if the battery voltage is below the “fully charged” threshold. When the battery is charged is simply stops outputting current, which also means it’s not putting load on the engine anymore.
The alternator doesn’t churn out electricity in that scenario.
The center rotating piece is an electromagnet, not a permanent magnet. There is a regulator, either built into the alternator or external and connected to it via wiring, which monitors the voltage output of the alternator and and adjust the charge to the electromagnet to increase or decrease production. This is also how it avoids overproducing during constant but lower than max draw (running the engine but not every light and fan, for example). Since it’s controlling the voltage this is also how alys produce a reliable voltage (usually about 14-14.5v) instead of spinning up to something higher and damaging all the nominally 12 or 24 volt input devices connected to it.
On top of the other two answers that are ok, I must add that once the battery has been fully charged the alternator will provide the electric power required by the car systems (stereo, lights, spark plugs, and so on)
The amount of electricity being consumed or called for causes more resistance on the alternator. Think of it like driving a car uphill, when there’s not much demand for electricity (like a car on level ground or down hill) there isn’t nearly as much force required to make it move.
So if the battery is topped off and the charging circuit no longer applying power, it requires less energy from the cars mechanical systems as a result.
Some energy is still needed for the car electrical systems, radio, headlights, fans, wipers, etc. and the alternator will usually easily accommodate this. Though in special situations such as for ambulances or fire trucks, they may have a greatly oversized alternator (or more than one) for the additional electrical requirements.
The 12 V battery is a reservoir of electrical power. This is why when an alternator fails, usually the car will continue running for at least another few miles before things start shutting off and failing.
It’s because it can run on that limited battery reserve for the electrical requirements, just as you can use your headlights or radio when the car engine is an actually running.
So it’s not so much a question of “where does the power go?”, but just that the power isn’t generated in the first place if it’s not really needed.
Now hybrid cars are a little different with the regenerative breaking, the idea there is you use the motors to generate power to charge the battery and that resistance acts as a brake.
So you can slow the car down without actually applying your wheel brake pads, by just letting the car make electricity from the forward momentum.
In that case when your battery happens to be full, and you use the regenerative braking (it’s automatic just by applying the brakes), it dumps the energy into resistors which get hot, and the cooling system dispels the heat.
That way the behavior (and braking characteristics) of the car does not drastically change depending on the charge and condition of the battery.
Electric only goes where it’s needed, if there’s no load it isn’t generated at all, just a spinning rotor. The more demand put on it, the more it will generate using more of the engines power to drive it, up to the 500 watts or so the alternator can output.
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