What powers the car when running, the ALT or the Battery?

S

SIZEMOREMK

Junior Member
I have always been under the impression that the battery runs the car, and the alternator charges the battery. Of course if you unhook the battery, the car will still run, though I have been told this is not good for an alternator... I have also always heard that charging a dead battery is also bad for an alternator.

My curiosity began when reviewing my logs showing voltage down close to 12.5 at WOT.

Voltage at idle was closer to 13.5-13.8ish. I was looking for closer to 14 at idle without lights on and such.

I am also still working through an idle issue with my shop, where the car wants to idle a little lower than I would like. My headlights will also dim here and there at a stoplight.

We have also recently discovered that the used IW damper that they sold to me as an 18% is actually a 10% (shop is swapping on a smaller blower pulley to make up the difference on their dime). This means that the 3.2" metco alternator pulley that was spose to be matched up with the 18% damper was probably not spinning the alt fast enough at idle and/or very light cruise.

So there are two potential issues: Idle voltage, and WOT voltage.

Idle voltage, will hopefully be corrected by replacing a stock sized alt pulley.

So then the potential WOT voltage would still be of concern. This concern will then be exacerbated when I add a water/meth kit into the mix for yet another decent current draw.

As I was discussing the issue with my builder, he said that the alternator runs the car, and the battery would have no bearing on powering the car at WOT...

I also called up a local auto-electric shop that has rebuilt alternators for me in the past to discuss the options of rebuilding and/or upgrading my alternator to a higher output unit. He says that the higher output alternator was likely not necessary and that mine is probably not putting out what it should...
He says that the alternator charges the battery and the battery runs the car.

So which is it? I'm gonna vote for the battery... No way those little wires coming from the alternator are gonna supply all that juice.
 
The latter, the alternator charges the battery , and the battery runs the car.
 
All the electricity your car uses is generated by the alternator. The battery serves two major functions: storing energy to start the engine and acting as a damper/buffer for the alternator.

Anytime a heavy electrical load turns on, ie high beams, fan, window motor, the total electrical load changes, and the alternator will need to work harder to compensate for this, but without a battery it would need to do it instantly and rapidly every time you hit a switch, which is why disconnecting your battery while running can be bad. With the battery in place, every time the load changes, the battery absorbs the rapid change and tapers out the load to a more consistent amount so the alternator can do its thing.

The thing is that pretty much everything you've described is pretty normal for a Marauder. 13.5-13.8 at idle is pretty normal, upwards of 14 when crusing no load is normal. The PCM will shutoff the alternator at WOT to maximize power, but it will only allow so low of a voltage before it turns it back on, which again means your 12.5 at WOT is normal.

Our alternators just kinda suck, it would be nice if we could fit the 2v ones cause they rock, but our engines just do not allot that much room, if there was a good way around it we'd all be doing it. There are a few companies that sell rebuild alternators that are specified to higher outputs, but their success at fixing the issues you have are limited.

Battery tests are fast and free just about anywhere. Go get your battery tested, if it needs one put one in it.
 
My car has dimmed the lights at stop lights and my gauges "danced to the music" since new no matter the alt. or battery used.

I call it character.


Builder Of Badassery
 
justbob said:
My car has dimmed the lights at stop lights and my gauges "danced to the music" since new no matter the alt. or battery used.

I call it character.


Builder Of Badassery
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I call it a car without the big 3 done, lololol
 
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Your builder.... may be able to build a good motor, but he doesn't know anything about electrical systems. Your battery(as others have said) powers the car, the alternator charges it.
 
I know the Marauder alternator is quite shallow but I have a police alternator you can have just cover shipping. It is a 200AMP and I will show pics and dimensions if you're interested. It has about 13K miles on it when the car rolled.
 
You don't change the alternator pulley because it's clutched. Computer sees WOT and allows the alternator pulley to free spin for several seconds, at that point in time voltage will drop to whatever the surface charge voltage is on the battery usually 12.5-12.8 volts.

For NA people who swapped in the Steeda underdrive pulleys only the crank (smaller than stock) and water pump (larger than stock) ones are changed. Most have to bump up their Drive Idle Rpms from the stock 600-650 range to 800-900 rpms because the serpentine belt is spinning the alternator pulley slower than the minimum rpms needed for full 13.8-14.4 volt output.

I have mine at 850 and if I'm stopped at a light with the front and rear defrosters on my voltage drops to 12.8-13.0. If I only have just one of those things on then no issues even with other accessories on.
 
Logizyme said:
All the electricity your car uses is generated by the alternator. The battery serves two major functions: storing energy to start the engine and acting as a damper/buffer for the alternator.

Anytime a heavy electrical load turns on, ie high beams, fan, window motor, the total electrical load changes, and the alternator will need to work harder to compensate for this, but without a battery it would need to do it instantly and rapidly every time you hit a switch, which is why disconnecting your battery while running can be bad. With the battery in place, every time the load changes, the battery absorbs the rapid change and tapers out the load to a more consistent amount so the alternator can do its thing.
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I'm quoting this because it's the most accurate answer I've read in a long time.

To paraphrase, the answer to the OP's question is "they both do". If you disconnect the battery, the alternator generally CAN supply all the energy needed, but is not designed to do that and will eventually overheat. The alternator's output also depends on the available mechanical energy input. If you try to run the car at idle with all the accessories on, it will likely not have the capacity and the car will die.

If you disconnect the alternator, the battery is more than capable of supplying the needed energy by itself, but with no means to resupply that energy, it will eventually deplete and the car will again die.

So it isn't a clear-cut case of one or the other "running the car".
 
"We have also recently discovered that the used IW damper that they sold to me as an 18% is actually a 10% (shop is swapping on a smaller blower pulley to make up the difference on their dime)."

I don't know anything about your build*, but if it were my build I would not accept a "smaller blower pulley".

I would get my Marauder out of that shop.

*Assuming the 18% is a fit.
 
SIZEMOREMK said:
I have always been under the impression that the battery runs the car,

I am also still working through an idle issue with my shop, where the car wants to idle a little lower than I would like. My headlights will also dim here and there at a stoplight.

So there are two potential issues: Idle voltage, and WOT voltage.

Idle voltage, will hopefully be corrected by replacing a stock sized alt pulley.
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You could also get a better alternator. I went to a 160AMP DB unit, and have plenty of V (13.9-14.x) at 750 idle with an underdriven crank pulley.

SIZEMOREMK said:
So then the potential WOT voltage would still be of concern. This concern will then be exacerbated when I add a water/meth kit into the mix for yet another decent current draw.
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To complicate things the stock tune for MM's will actaully TURN OFF the alternator at WOT, I did not like this and had it changed in my tune so the ALT stays on.

Get a DB electrical alt if you still have 10 year old factory unit, change your SCT tune to have the alternator stay on at WOT for all your goodies, meth etc.

I wonder how Marto sets up his tunes. I bet he could help you.
 
All the hype on here about DB electrical. Here's my story- their alt. lasted about 18 months and their starter caught an internal fire and started to melt my wiring in 13 months. That's my experience.


Builder Of Badassery
 
ctrlraven said:
You don't change the alternator pulley because it's clutched. Computer sees WOT and allows the alternator pulley to free spin for several seconds
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That's not exactly true. The pulley clutch is more like a one way ratchet. It's only purpose is to prevent belt chirp when you shift gears caused by the inertia of the alternators spinning mass by allowing the alternator to over spin the belt. The clutch has nothing to do with the alternator turning off.

What is really happening is the computer is telling your alternator to stop generating electricity. Generating electricity creates magnetic friction. Back in highschool, we did this experiment where we had a hand crank generator. You could spin it very easily when it wasn't wired to anything. Then we connected it to a 20 watt light bulb and it was harder to crank. Then we connected it to a 150 watt light bulb and it took 2 of us turning the crank together to even get it to move.
 
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