Battery drained?

Links to said sources, please. Only alternators with permanent magnets (in other words,not those used in automobiles) can self-excite. On all others, no matter how fast they are spun, they will not spontaneously start creating electrical energy.

Originally Posted by hornhospital

Downhiller, I agree with you 99% of the time, but on this you are dead wrong. You can spin an alternator as fast as you want, but until there's excitation voltage it will generate nothing. An alternator has no magnetic field without excitation voltage. No magnetic field= no electrical generation. Generators had permanent magnets and didn't need electrical current to start producing electricity, but alternators absolutely MUST have a magnetic field created by electricity in the windings before they'll do anything at all.

so youre saying the magnetic rotor that is built with a magnetic material isnt magnetic until it is powered? did you know there isnt anything wire wise on the shaft to the rotor? and if this is true from what you are saying, then there will be a dead short within the electrical system because the rotor would in theory be grounded.

not trying to be an ass here, but when the guys at youre trusted autozone, napa, etc just hook the alternator up to a belt and use a starter with a couple leads to a volt meter. really want to see something awesome, that proves me right. take the wires off the back of the alternator. leave everything else attached. start the engine with a volt meter across the leads on the alternator. youll see about 14 volt (depending on how strong the windings are).

Click the link that johnf posted and see what that says about the rotor of an alternator. It is NOT a magnet until it is energized by, guess what, the excitation voltage through it. And the test you got at Advance? Guess what the two wires to the alternator did before they started pulling current out of the spinning alternator? Yup...sent excitation voltage into it. Of course you'll get 14ish volts off a running alternator if you pull the wires. You already had it started with excitation voltage before you pulled the wires. An alternator's excitation voltage is self-sustaining once it's producing current.

I'm not trying to be an ass either, but I know alternators and electrical systems. I've been working on them for forty years.

Sent from my MB860 using Bimmer App

Originally Posted by hornhospital

Click the link that johnf posted and see what that says about the rotor of an alternator. It is NOT a magnet until it is energized by, guess what, the excitation voltage through it. And the test you got at Advance? Guess what the two wires to the alternator did before they started pulling current out of the spinning alternator? Yup...sent excitation voltage into it. Of course you'll get 14ish volts off a running alternator if you pull the wires. You already had it started with excitation voltage before you pulled the wires. An alternator's excitation voltage is self-sustaining once it's producing current.

I'm not trying to be an ass either, but I know alternators and electrical systems. I've been working on them for forty years.

Sent from my MB860 using Bimmer App

Well if you read the whole article on wikipedia regarding automotive alternators its says plain as day that they can self excite, the other "various sources" i used was to confirm that it was a true fact

Anyways I figured out my idle problem it turns out that when I was removing the air box I pushed a radiator hose into a cluster of wires and 1 of them came undone (poorly reattached harness) so a little electrical tape and wire strippers fixed that problem, but once again my battery is draining out the alternator is brand new and its still not keeping a charge to the battery so I'm back to checking wires again :/

I'm having this problem now =( something is draining my battery.. I'll figure it out tomorrow

agree. i'm without words.

Originally Posted by hornhospital

Click the link that johnf posted and see what that says about the rotor of an alternator. It is NOT a magnet until it is energized by, guess what, the excitation voltage through it. And the test you got at Advance? Guess what the two wires to the alternator did before they started pulling current out of the spinning alternator? Yup...sent excitation voltage into it. Of course you'll get 14ish volts off a running alternator if you pull the wires. You already had it started with excitation voltage before you pulled the wires. An alternator's excitation voltage is self-sustaining once it's producing current.

I'm not trying to be an ass either, but I know alternators and electrical systems. I've been working on them for forty years.

Sent from my MB860 using Bimmer App

but it also mentions alternators that self excite after a certain speed, so if they can self excite, that means there has to be a magnetic field to start with, correct?

Well i finally got my car back on the road again, i ended up replacing a worn alternator, a bad battery(it was also the wrong kind) the negative cable from the battery to the trunk, and the positive cable from the alternator to the starter

Why do some manufacturers put the battery in the trunk
Weight distribution ! C'mon is it that critical to distribute that little bit of weight to the back on a road car ?
The thick cables leading to and from the engine bay weigh nothing ? & then the losses...
& then some smart ass copies this layout, gets it all wrong and wonders why his engine now performs ****ty
The elctronics guy inside me scratches his head in bewilderment

Originally Posted by Nick323

Why do some manufacturers put the battery in the trunk
Weight distribution ! C'mon is it that critical to distribute that little bit of weight to the back on a road car ?
The thick cables leading to and from the engine bay weigh nothing ? & then the losses...
& then some smart ass copies this layout, gets it all wrong and wonders why his engine now performs ****ty
The elctronics guy inside me scratches his head in bewilderment

yes it is important. dont anger the gods.

on a side note, mercedes, vw, dodge, honda (hybrids), toyota (hybrids) either put them in the trunk or just behind the rear seat. go change a battery on a dodge magnum then come back and tell us the lovely time youve had with it

Originally Posted by Nick323

Why do some manufacturers put the battery in the trunk
Weight distribution ! C'mon is it that critical to distribute that little bit of weight to the back on a road car ?
The thick cables leading to and from the engine bay weigh nothing ? & then the losses...
& then some smart ass copies this layout, gets it all wrong and wonders why his engine now performs ****ty
The elctronics guy inside me scratches his head in bewilderment

It does help balance the load, keeps the battery away from engine heat and fumes and makes space in the engine bay. The thick cables going to the back are not so heavy.

Lots of cars have the battery under the rear seat or in the boot...trunk if you are in the USA

The only problem I have with this is that a charging battery, escpecially if it is an old un-sealed lead acid battery, can give off hydrogen fumes which could go "Boom" if your back seat passenger lights up a smoke.

Oh and as regards the issue of can you push start an E30 without a battery fitted....why does somebody not do a test for us and put us out of our missery??

I personally believe that the metal core of the rotor will pick up some magnetism over time and will become a "weak" permanent magnet and hence self-exitation can happen if you spin the alternator fast enough!

Originally Posted by Nick323

Why do some manufacturers put the battery in the trunk
Weight distribution ! C'mon is it that critical to distribute that little bit of weight to the back on a road car ?
The thick cables leading to and from the engine bay weigh nothing ? & then the losses...
& then some smart ass copies this layout, gets it all wrong and wonders why his engine now performs ****ty
The elctronics guy inside me scratches his head in bewilderment

Gotta admit though it makes wiring up a pair of 12in subs with a 1500 watt amp a little easier

Originally Posted by goat88

Well if you read the whole article on wikipedia regarding automotive alternators its says plain as day that they can self excite, the other "various sources" i used was to confirm that it was a true fact

Alternators used in automobiles are not self exciting. You CANNOT start a vehicle with an alternator without a excitation. hornhospital is 100% correct. In fact, i'm not sure, but I believe on many BMW's they have a minimum voltage requirement set to start, otherwise it won't even let you try.

I'd like to see a video of a car with an alternator and manual transmission being push started without a battery or some power source.

Sheesh, why not do an empirical test & blow up the ECU
I am not concerned about theory and how incorrect Wikepedia is this time (its dodo's like me who write junk on there)
What I worry about is how it translates to practice !
& practice is not to ever run your Alternator without a buffer (Battery) connected to it
Your Alternator generates some interesting Voltages before it goes through the Bridge & then regulator.
I used to take the Voltage after the bridge & use the Alternator as a welding machine
So if you run it without a Battery to smooth things out, it could cause some interesting smoke

In case you are curious, BMW's requires components at the generator output withstand +/-8V of ripple at 14.8V (a sinusoidal voltage swinging between 6.8 and 22.8V).

What you really don't want to happen is have the battery disconnect with the generator at full power. I don't think the E36 has a central suppressor to quench the resulting load dump.

Not sure if you figured out your problem yet but mine did the same exact thing come to find out a fuse had blown! Not sure why that killed it but after a little thought i figured out that the negative cable on my battery corroded completely through from the battery end to the bolt on the car it made it drain but would impede charging! I bought a 6 dollar replacement cable from walmart of coated copper and like magic! Never had a problem since!

This captain goes down with the ship