Alternators - What they are, How to Test them

[JackG]

Originally Posted by MNBoxster

This means, the Alternator does not provide Current directly to any Accessory of the Car, it only supplies current to the Battery, which in turn provides the current to whichever Accessory is running. Happy Motoring!...Jim'99

That was a good, basic primer for the non-technical. A good read for anyone on the forum.

One minor point... the quoted text above isn't quite correct. The battery has two jobs in life... 1) to start the car, and 2) to act as a capacitor to smooth the ripple in the output of the alternator. The alternator provides power to run the car, and it's accessories, while the motor is running. There is no device to assure that the output of the alternator is only flowing into the battery, and not directly into the rest of the car.

Think of it this way... you've left your lights on, and the battery is so dead it just barely is able to start the car. Now that it is started however, you can turn on your lights, roll the windows down, crank the stereo, and blow the horn... all at the same time! That's because the alternator is powering the car, not the battery. The battery is nearly dead, and is being charged by the alternator with part of it's current output, while the rest of the alternator output runs the car.

Hope that helps!

Jack

[MNBoxster]

Originally Posted by JackG

That was a good, basic primer for the non-technical. A good read for anyone on the forum.

Thank you for your kind words

One minor point... the quoted text above isn't quite correct. The battery has two jobs in life... 1) to start the car, and 2) to act as a capacitor to smooth the ripple in the output of the alternator. The alternator provides power to run the car, and it's accessories, while the motor is running. There is no device to assure that the output of the alternator is only flowing into the battery, and not directly into the rest of the car.

I'm not sure I agree with you totally here. Your example is of course a theoretical extrapolation, which may confuse some people.

Depending upon how far you want to carry your example, it is not a mistatement that it isn't the Alternator at all, but the Sun which powers the electrical components of the Car. Solar Energy allows the Plants to photosynthesize and grow. These Plants die and succumb to millions of years of geologic heat and pressure turning to Oil, which is refined into Gasoline, which is converted to Mechanical Energy by the Engine, which is converted to Electrical Energy by the Alternator and so on...

Of course the actual electricity used to fire the Spark Plugs, Stereo, etc. originates in the Alternator. No argument here at all.

But, it is supplied to the Accessory by the Battery, not the Alternator. The reason that it can happen with an almost dead Battery is that the Battery will only store unused energy. If there is a demand, it will store only that excess energy which is not demanded. You correctly state that the Battery acts as a Capacitor, which is really just another sort of Battery (a storage device). If the current did not first flow through the Battery, the current would still be 'rippled' and therefore unusable by most of the electrical appliances.

In fact, there is a means by which the Alternator supplies current only the Battery, it's the wiring. Almost all cars have the Alternator connected via cable, straight to the positive (+) terminal of the Battery (I say almost simply because I have not seen the schematics for every car, but I suspect they all are the same). There is usually no Junction Block, fuse or any other device between the Alternator and the Battery except the Positive (+) Battery Cable. Conversely, there is no other path to any Accessory from the Alternator other than through the Battery (except of course through Ground, which is common).

In the days of Generators, a Voltage Regulator with an electro-mechanical Armature literally disconnected the path to the Battery, isolating it from rest of the electrical appliances (wiring harness) once a sufficient voltage was achieved indicating that the Battery was fully charged. But with an Alternator, it is the Alternator itself which is isolated from the rest of the electrics by the Battery.

Think of it this way... you've left your lights on, and the battery is so dead it just barely is able to start the car. Now that it is started however, you can turn on your lights, roll the windows down, crank the stereo, and blow the horn... all at the same time! That's because the alternator is powering the car, not the battery. The battery is nearly dead, and is being charged by the alternator with part of it's current output, while the rest of the alternator output runs the car.

Hope that helps!

Jack

I hope this helps...

Happy Motoring!...Jim'99

[JackG]

I think I see your confusion, MN... the method of wiring makes you think that the current flows through the battery. This is simply not the case.

As long as the current doesn't blow a fuse or fusible link, they can be discounted. So then, a simple diagram would have the alternator, battery, and the rest of the car (the load) all in parallel circuit. The potential available on the positive lead is the same potential available for everything... both the battery and the car and its accessories. If the electrons are needed by the headlights, they most certainly do not flow first from the alternator, into the battery, then out to the headlights. The vast majority travel directly from the alternator to the headlights, with a tiny percentage being supplied by the battery to fill in the ripple.

One last thought... many cars have the battery remotely located from the engine compartment, such as a boxster. The load presented to the alternator by the engine and accessories is closer, via wiring, to the alternator than to the battery. The power doesn't first flow down the longer cable, "through" the battery, then back to the rest of the car. Electrically, it just doesn't happen the way you describe. Those electrons just aren't that smart.

Jack
BTW... I guess I should explain... I'm an EE. Electricity is my "thing".

[MNBoxster]

@JackG

Hi,

Dooooh.... I don't know what I was thinking, wasn't thinking would be more accurate... Thanks for setting things straight. I know what you're saying, I didn't misunderstand it, I misread it. Guess my multi-tasking skills ain't what they used to be.

I am not an EE, I'm an AE, BS and MS in Aeronautical Engineering with a BS in Materials Science and former Navy Certified Test Pilot. So while electricity ain't exactly my thing, it's always been sorta a 1st or 2nd cousin... I defer to your expertise. Now if you wanna discuss how MW Plasma can mitigate the Boundary Layer in Supersonic Airflow, I'm your man!

I totally forgot about the cutoff relay to the Battery and such. But, rather than add to the confusion, again I'll say thanks for the clarification.

Happy Motoring!...Jim'99

[QporscheQ]

"BTW... I guess I should explain... I'm an EE. Electricity is my "thing"

EEs of the world unite!