Found This, hope it settles what's going on here.

If you closely inspect an alternator, you'll find it has vents on both the front and back side. Again, this aids in heat dissipation. A drive pulley is attached to the rotor shaft on the front of the alternator. When the engine is running, the crankshaft turns the drive belt, which in turn spins the pulley on the rotor shaft. In essence, the alternator transfers the mechanical energy from the engine into electrical power for the car's accessories.

In the early days, cars used generators rather than alternators to power the vehicle's electrical system and charge the battery. That's not the case anymore. As automotive technology evolved, so did the need for more power. Generators produce direct current, which travels in one direction, as opposed to the alternating current for the electricity in our houses, which periodically reverses directions. As Tesla proved in 1887, alternating current became more attractive as it generates higher voltage more efficiently, something necessary in contemporary automobiles. But car batteries can't use AC power since they produce DC power. As a result, the alternator's power output is fed through diodes, which convert the AC power to DC power.

 

Thanks 95 318SLT. Yes the battery is not the main power source, neither is the Alt. They back each other up.

If you don't have the alt you will drain the battery but it will last a while. I have drove around a Good part of a day once till I could get to autozone to replace it. I did keep all electronics off to help. But you will drain the battery at one point and may kill the battery due to over use/ heat ect.

Now if you run without the battery you will make it quite a while too, but you may over heat the alt due to overusing it or overload it and it not sustaining the correct amps. Also the computer will not have a datum for the voltage. It may think that the battery is to low and increase the alts power to compensate and can easily overheat or blow out some diodes, or blow the voltage regulator. Or it may think the battery it overcharged and drop the alt power to keep it from exploding the battery and drop the alt power all together.

You forget the alt is not running full power 100% all the time. Alts don't put out full power (at least what is it rated for) until higher RPMs. Above 1,500 to 2,000 depending engine and all. So above 2,000 yes at that time the alt is main power source, below that the battery is. The easy est way to see what I'm talking about (but is not the only time it happens is) to turn on your radio, lights, wipers, heater blower, and everything you got and you will see at Idle and lower RPMs it everything dims. That is due to the alt not supporting the load. Now the battery is the main power source. And it doesn't have to be fully loaded or even any other load besides the ignition for the battery to be the main power source.

Also the modern alts don't use fixed magnets they need the battery power to power up the electro magnetics, and the alt will drop the voltage every time it has to power it self up. http://auto.howstuffworks.com/alternator2.htm

If all you needed was the alt hybrids and electric cars would be self sufficient all the time expect for starting, and long Idles. They would be able to go on forever at freeway speeds. But with they can't at one point they will need to use use the battery and use all its energy.

Also there is alot more when dealing with AC which the alt produces and how it gets converted to DC and slag time in between and lots of variables as AC jumps up and down and switches polarity.

There is alot to this on both sides of our argument but we cant list them all. This is what I have learned in all of my classes Including automotive electronics. I am ASE certified master mechanic. But not my profession.

 

Yeah, the AC power in a house switches poles 60 times per second. You can see it if you hold an optical tachometer up to a light bulb in your house. As an interesting fact, your light bulbs turn off 60 times per second as the poles are changing.

Every single motor driven alternator and modern day generators use magnetic coils, and as the magnetic fields are forced past each other, it creates a potential difference and produces an electrical current. All magnets have 2 poles, so any alternator/generator using coils produce AC power.

 

That all makes sense to me. The only electrical education I have is what I learned in physics. There were a couple things you mentioned that I didn't think about, like the power production at idle. Thanks for clearing that up!

 

Ill quit after this one.
One quick example is think of it as an air compressor and tank. The compressor can be used without the tank but it will be under more stress as it has to suppport is self. Now add a tank and wow the stress is alot less. Thats why the battery is just as important.

 

Thats a really good example! I might have to use that someday if I'm ever trying to explain this again.

 

Ya I'm at work and I was about to use the air compressor, and it clicked like a light bulb. Thats why it took me a while to type that long replay. I can click away from work at the computer for a min, every now and then. You had correct info too. I'm sure for others this post will be very informative, or just boring to read!

 

Yeah, I hope it helps to inform people, cause we waaay overkilled the original question.

 

you guys missed one important issue, unhooking the battery with the engine running can cause a spike that can nuke the computer. i know 2 million people are gonna say i've done it many times without a problem,but the 3 people that nuked a computer are gonna freak. i'm just saying its not a wise move. and i am ASE certified heavy truck mechanic by trade for 24 yrs, have seen more electronics added to trucks than i like to see and seen things nuked very easily

 

+2.... i too am a HD truck tech