zking786

I have a technical question about the alternator and the battery charge. When you strart up your vehicle on a low battery, eventually (pretty quickly with the S2000) the battery will reach full charge. The charge generated by the alternator recharges the battery. When you have a full battery, however, what happens to the charge that the alternator produces. Is it simply pumped into the fully charged battery or where does it go? Seems like a tremendous inefficiency because performance doesn't seem to be influenced much by battery charge. In all situations, the alternator cranks out charge (irrespective of battery charge and applicance energy draw). Would appreciate it if someone with more experience or more automotive knowledge would check my conclusions. I have very limited knowledge in this area and would appreciate any assistance you guys can provide.
Thanks!

HommedeAil

I have an Electrical Engineering degree and have worked in the motors and generators field for over 27 years. Perhaps I qualify.

When a generator (or alternator) is spun, it creates a voltage. Obviously, it takes some force to spin that generator. The amount of force that it takes is directly proportional not to the amount of voltage produced but to the amount of current delivered to the load (in this case, the battery).

Thus, when the car is first started and the alternator begins to produce energy to charge the battery, the alternator may need to deliver full rated current let's call it 60 amperes at 14 volts, for argument's sake. This equates to roughly 1-1/4 horsepower that the engine needs to deliver to the alternator so that the alternator can, in turn, deliver the 14 volts at 60 amperes.

You are correct in that some time later (the amount of that time depending upon the initial state of charge of the battery) the amount of current drawn by the battery will begin to taper off as the battery approaches fully charged. Note that it will never taper to zero since as long as the engine is running, there are devices that are drawing current from the battery, i.e., the radio, the lights, the heater/AC fan, the ignition system, etc.

For argument's sake, let's assume that the total consumption of all of these devices adds up to 10 amperes. The alternator would now require only about one-eight of a horsepower to deliver the 10 amperes at 14 volts. Keep in mind that some of the car's electrical devices can consume a surprising amount of current. For example, if you were cruising along, at night, in the rain with your lights, air conditioner, wipers and radio all going, you may be demanding close to the full output of the alternator.

Does this answer your question?

steven975

yes, you are kind of right.

I think the alternator is rated at 90 amps at 12V (1080W). That's more than enough to run the motor and accessories.

The battery is really there to temporarily hold charge to start the car. That's really it. You are right; there is an inefficiency to running an alternator on a full battery; no matter how much demand there is for power, the alternator still spins at the same resistance. All in all, it really isn't much. I think a power steering pump saps like 5HP, and an alternator much less.


Hybrid cars do solve some of these inefficiencies. With Hybrid systems (with the exception of the "hybrid" siverado, which is not a hybrid at all), the electric motor both starts the gas motor and acts as a generator. So, when the car is cruising (and not using the electric motor for motivation) the excess energy is put back into the batteries, and that energy can then be used to run the car, saving gas.

zking786

Thanks for the response. I have a couple of questions to clarify what you are saying. Firstly, isn't the motor always turning the alternator? Because the alternator is basically a motor which is being turned to generate current aren't all 60 amperes being cranked out all the time. Or is the alternator some how variable -- the force to turn it decreases (thereby decreasing output amps and decreasing draw from the motor). What your saying seems to make sense, but I don't see how turning the alternator becomes easier when the battery becomes fully charged. Am I mistaken or does the alternator always produce the 60 amperes you discussed. If not, can you clarify how it is regulated so as to decrease the force it takes to turn the alternator's motor. Hope you understand what I mean

zking786

As steven975 discusses, since the alternator is always (I think) delivering the same resistance, it should always generate the same power--irrespective of the consumption or draw from the vehicle. So isn't this an inefficiency when the vehicle is charged?

zking786

I just want to confirm that the resistance is not variavle and that the alternator will always deliver a set amount of power, irrespective of the vehicle's power draw.
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HommedeAil

Yes, zking, I understand precisely what you mean.

First, to address the other well-intentioned responders, when the alternator is called upon to deliver zero current (i.e., the battery is fully charged and there are no accessories drawing current, the amount of force that it takes to spin the alternator is virtually zero. Try it. Go to a parts shop, take an alternator out of a box and spin it with your fingers. It spins pretty easily and will continue to spin on its own for awhile. Now, how much horsepower do you think you can deliver with two fingertips?

Now, zking, the regulating factor is current, not voltage. The alternator will put out a constant 14 or so volts from idle all the way up to redline but the amount of current delivered is determined by how much the battery demands. As the battery gets closer and closer to fully charged, it demands less and less current and the amount of energy that it takes to spin the alternator gets proportionately less and less.

Got it?

zking786

How? The only way one could expect to "regulate" the alternator is by decreasing the force it requires to spin it. In order to do this, one would need to be able to add or remove coils which would alter the resistance.

I can understand engine speed affecting alternator output (faster engine turns, faster alternator turnes --> more charge generated). But I can't understand how the feedback loop your describing would work. That is, how is it that the alternator changes its resistance to maximize efficiency?

CrazyPhuD

Really listen to the expert.... HommedeAil is correct.

HP consumed is dependent upon the load required from the Alternator. If there is no load on the Alternator then there would be no power loss in an ideal case. In reality there is always going to be some loss due even in a no load case but it will be minor. Look at how an alternator works(or a generator) check howstuffworks, it should be there and will help.

Oh an FYI the alternator IS NOT more than enough to run both the motor and accessories. I know this because my battery lead popped off while driving and my stereo would brown out(i.e. cut out) when starting from a stop because the power demands from the engine alone were enough not provide enough power to run the stereo. The only thing running at the time was the car and the stereo. You might be surprised about the power demands of the engine under load.

SheDrivesIt

The variable mechanical resistance to spinning the alternator is caused by a variation in the magnetic field generated by the current being drawn from the alternator. As the current demand decreases, the magnetic field changes so that there is less and less mechanical resistance to spinning the alternator.