What happens to the power my car's alternator makes when the battery is full?

Fizzle and YMBR are right on.

Here is the deal in the alternator there are electromagnets. As you know you need magnets on an axis and magnets the outside body that rotate to generate electricity. The best way to think of the alternator is to imagine that one side of the necessary magnetic field is electromagnetically enhanced thereby giving your alternator variable power generating capacity. So lets say you are driving on a pleasant temperature day when the AC and Heat are off, its bright and sunny and you are on a divided highway and you do not feel the need for headlights, no rain so no wipers, you are talking with your companion so no need to blast the stereo and you have been driving for a while so the battery is full, in this case the alternator will only make the electricity necessary to keep the engine functions going, but say you stop for lunch and the weather turns nasty hot and muggy and rainy, now you have your lights, wipers, ac to dry the air heat to heat it back up, stereo going well then the alternator provides more juice n voltage to the electromagnetic enhancers to create more energy so those magnets on strong and are turning through the magnetic force there by generating more electricity.

I kept the discussion as non techie as possible but that is the gist of it. So to make sure every thing in the electrical generation system is working well you need a good functioning alternator and a good functioning battery and a good functioning voltage regulator at the alternator and for the cars electrical system as well. The two things control and activity of the alternator, one says ok then generate and the other then controls the electromagnet's to create the necessary power.

Interesting NOTE: Just as an aside, in some micro optimized vehicles, when you start the car in the morning the alternator does not start generating the full power necessary for a period of time because the engine is warming up and not running optimally, the brain tells the alternator to go ahead and which then increases the electromagnetic strength to full capacity etc etc.

Hope that helps, the alternator is really a fascinating pieces of electronic and mechanical wonder.

Best of Luck.

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That is what the voltage regulator is for. It is a simple breaker that opens when the battery is full. Some cars the alternator continues powering the car after the battery is full and other will run off the battery and then recharge when the battery falls under a certain voltage. Most are the former though

The alternator powers the car's electrical system and only makes as much current as is needed. In general if nothing's being used, nothing's being made even if it's spinning. It will have a voltage but not deliver any current.

that,s the job for the voltage regulator ! the regulator is suppose to keep the alternator from overcharging or undercharging the battery !

It only puts out what is needed, when full then it shuts off, or goes way down to only a couple of amps