Wasted alternator generated energy

[Gerinski]

Sorry if this has been discussed before, I'm not a regular of the site and I have not found anything in a first quick search, if so just direct me to the relevant threads.
Many millions of cars are driving every day with their alternators turning. Unless I'm wrong, once the battery is fully charged the alternator keeps turning but any electricity produced is simply wasted.
If this is correct, I can guess that considering all the internal combustion engine vehicles driving around each day in the whole world, the amount of wasted energy may be very significant.
Has there been any calculation of how much energy is being produced but wasted by this fact?
Is there any project by which that wasted energy could be harnessed and used?
Thanks!

 

[Averagesupernova]

Unless I'm wrong, once the battery is fully charged the alternator keeps turning but any electricity produced is simply wasted.

Do you really know what you are saying here? Do you understand electricity well enough to be able to say that?

 

[Gerinski]

Do you really know what you are saying here? Do you understand electricity well enough to be able to say that?

No I don't, I'm just a layman with limited technical education (I have the diploma of automotive mechanics/electrics, but I never worked on applied mechanics or electrics, my career took a different path).

 

[russ_watters]

I'll just put a finer point on it: an alternator with no electrical load on it creates no mechanical load on the engine. So if the battery is fully charged, the load on the car's engine drops. No wasted power.

 

[SteamKing]

Once the battery is charged, the car's voltage regulator temporarily cuts off the alternator until the battery needs charging again.

 

[Gerinski]

I'll just put a finer point on it: an alternator with no electrical load on it creates no mechanical load on the engine. So if the battery is fully charged, the load on the car's engine drops. No wasted power.

But the drive belt is still turning the alternator shaft around, doesn't it?
That's friction at the very least. How does the engine 'know' that the battery is already fully charged? doesn't it keep turning the alternator exactly the same as when the battery was half-charged?

 

[SteamKing]

That's also why the voltage regulator is in a car's charging circuit. That's how the car 'knows' when the battery needs charging by the alternator. If the regulator cuts off the exciter current to the alternator, then there is no magnetic field created and no electricity generated by the alternator while it is turning.

See this article: http://en.wikipedia.org/wiki/Alternator

Scroll down to the section they have on automotive alternators.

 

[russ_watters]

But the drive belt is still turning the alternator shaft around, doesn't it?
That's friction at the very least.

Yes, but friction is a very small part of the loss in an alternator and in any case isn't recoverable so it doesn't relate to your question.

How does the engine 'know' that the battery is already fully charged?

Overcharge protection notwithstanding, it doesn't know anything. The alternator is simply harder to turn when it is charging the battery (or otherwise loaded) than when it isn't.

...doesn't it keep turning the alternator exactly the same as when the battery was half-charged?

Nope. The electrical load determines the torque.

 

[Gerinski]

Thanks, it's clear now.