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Username Post: can someone please explain in detail        (Topic#362421)
Posts: 357

Reg: 03-20-18
09-01-20 12:40 PM - Post#2802497    

how a battery works when in a system that is getting a charge. while at the same time there is a discharge.

Either I am a complete idiot or someone is seriously pulling my leg.

My understanding is in a system that contains a battery that has say an alternator connected as power source. anything you attach to that battery is pulling current from the battery NOT the alternator. Alternator is filling the battery as the battery is being discharged??

say pulling 100 amps from battery while alternator is running. Filling it after the 100 amps was pulled?

Is it taking the 100 amps out of the battery through its chemical process?

or is it taking the 100 amps from the alternator without impacting the chemistry process in the battery ?

So what the hell is actually happening to the battery when there is a draw and an input of power.

it defeats the purpose to charge from battery to charge another battery its a WASTE OF ENERGY. And requires replacement that much faster.

I am trying to avoid this in system I am trying to build. I do NOT want to be wasting a bunch of energy to daisy chain charge batteries of another battery.

I would really appreciate the help to get this straightened out in my mind.

and with terminology being indiscriminately interchanged by people who know better. is making it extremely difficult to teach myself what I NEED to know. to properly build a system that I can live with and not cost me my life due to incorrect build. .

So when someone says dc to dc to me that means any dc source on input. doesn't matter if its alternator, battery etc. just so long as the power being provide is dc current.

when someone says battery to dc that tells me the source can ONLY BE battery.


I already know that the initial discharge to get the engine going comes from the battery and the initial output of alternator charges up the lost energy.

lets use 80 amps for the draw. with an alternator that does 105 amps.

While the alt is running and there is a draw that does NOT exceed the alternator output. which is actually providing the power for the draw. the battery or the alternator?

when asking Questions its related to van I own: 1987 GMC 3/4 ton G2500 vandura, V6, No AC, standard length. 3speed trans(no overdrive).

Edited by VanLife on 09-01-20 01:01 PM. Reason for edit: No reason given.


Site Ambassador
Posts: 14711

Loc: Baltimore, MD
Reg: 11-27-04
09-01-20 02:22 PM - Post#2802504    
    In response to VanLife

I am by no means an expert in automotive electrical systems, but this is how I understand it. The battery is just stored energy to power things while the alternator is not working. The main thing you want it there for is to run the starter motor to get the motor running so that you can spin the alternator to get it working. Once the field is excited in the alternator, it is charging the battery and providing power for the electrical system. The battery is out of the picture as a power source as long as the draws on the system don’t exceed the amperage output of the alternator. You can remove the battery once the car is running and the alternator is working and the car will still run. That is why the old test to check for a bad alternator was to disconnect the battery with the engine running. If it stalled, the alternator isn’t working. With modern electronics in the systems these days, that is no longer a recommended test.

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Very Senior Member
Posts: 4548

Reg: 12-29-02
09-01-20 06:16 PM - Post#2802519    
    In response to VanLife

A fully charged battery is about 12.7V at the terminals. The alternator will source the power if it produces more than this voltage. As the alternator voltage drops to 12.7V or less the load will start to draw power from both the alternator and the battery. The exact amount of power drawn from each depends on the characteristics of the alternator, the battery and the wiring.

Your concerns are completely unfounded. You can draw the charging power from the vehicle battery/alternator system as long as the alternator can produce more current than the converter is drawing.

The typical 3-phase alternator with 6 diodes does not produce a very good DC output. It has a lot of ripple unless it's filtered. I would keep the battery in the circuit.

Why don't you just look for high output CS-144 alternators?

Also, if you're not going to run the engine over 3000rpm then double the size of the alternator pulley to spin it faster and get more idle output from it. Your belt will appreciate this too.

Ultra Senior Member
Posts: 11419
Reg: 03-25-05
09-01-20 07:09 PM - Post#2802523    
    In response to 65_Impala

  • Quote:
As the alternator voltage drops to 12.7V or less the load will start to draw power from both the alternator and the battery. The exact amount of power drawn from each depends on the characteristics of the alternator, the battery and the wiring.

As Wagonman said, the alternator should supply all the power needed for the system as well as keep the battery charged. The alternator voltage should never go below 12.7, indicating too much draw from the system. Besides starting the vehicle, as 65 Impala said, the battery is also needed to buffer electrical noise for modern electronics, but all power should come from the alternator.

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Very Senior Member
Posts: 4548

Reg: 12-29-02
09-02-20 06:32 AM - Post#2802541    
    In response to acardon

Should never and can't go below 12.7V are two different things. If you want to explain what powers the car then you need to cover when the alternator powers stuff and when the battery starts to power stuff.

Super Senior Member
Posts: 5694

Loc: paradise
Reg: 09-05-03
09-02-20 08:43 AM - Post#2802547    
    In response to VanLife

seems like you will need a big alternator. maybe a diesel truck size. automotive alternators charge 1 battery. truck alternators charge 3-4 batteries. you will also need a big wire, like 4gauge, coming off it to a junction block to feed out. on the diesel trucks they also use a big ground wire coming off the alternator going to the batteries. would be a good idea to add a ground junction block too.

70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails.

Posts: 357

Reg: 03-20-18
09-11-20 05:09 PM - Post#2803279    
    In response to 65_Impala

DC power says to go smaller on the alt pulley to increase the ratio. This is what increases RPM of the alternator. so in my current van with 8inch crank pulley and a 2.2inch alt pulley that is about 3.5 ratio.

The solution is to focus on the alternator set up to supply the needed power I just did not want the starter battery being drained so as to shorten the life of battery from having 100 amps being drawn for 2 hours.

The problem is the alt is too big "cs144 large case" oil pipe is too close to alt and can't be moved.

I am currently trying to get a newer van this vintage is not working in terms of being homeless. Resources and what you can do to repair is too limited.

Like not being able to do head gasket repair.

I also talking with DC power at seeing if They have pre made kit for installing 2 alternators that will work with GMC/chevy vans and which years.

The advantage of their alts is how they are built and they are OEM compatible and as far as I know beat CS144 in terms of idle output. Their 180amp does 130amps at idle.

2-270amp because they are the same they will work as one alt with no special setup.

This will work better to be able to supply 100 amps on continuous basis.

that is spread out over 24-40amp diodes between the two alts.

With the thermal triple protection it will back off the amps when it gets too hot to allow continuous use.

They said the 270 won't have a problem with 100 amp draw. I am adding redundancy so as not to burn it out premature.

If I can ever get the funds together.

when asking Questions its related to van I own: 1987 GMC 3/4 ton G2500 vandura, V6, No AC, standard length. 3speed trans(no overdrive).

Very Senior Member
Posts: 4548

Reg: 12-29-02
09-11-20 09:57 PM - Post#2803293    
    In response to VanLife

You're right, smaller pulley. But that will be harder on the belt.


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