Technical Alternator Charging Amperage

I have questioned this on many occasions. I am not an electrical wizard.

I have a customer with an off topic early 70s car that wants to install a 250 AMP alternator. This one is pretty extreme in AMPS in my opinion. He says the current alternator is not keeping up with the demand.

Never installed an alternator with those kinds of charging numbers before on an old car.

Car has a boom box, radio amp and other electrical shit and even an extra battery to operate all this shit in the trunk. So, the alternator is charging two batteries all the time while running.

What does that charging AMP capability of the alternator do, to all the mostly original electrical/wiring system in the car, that was originally designed for 63 AMPS?

I think it's a bad idea on stuff that was never designed to have that much current.

Will this end up burning the car up?

Bill

 

The battery is your current control; the regulator will let the alternator charge according to the voltage demand on the battery. As long as you have the correct size battery cables for the amperage output and the wire going to the alternator is big enough gauge to carry the current (amperage) you should be good, also a good complete ground circuit.
Maybe @Crazy Steve can weigh in on this also.

 

Electricity is strange stuff....you can "force" voltage onto things. But you can't "force" amps onto things. Things can only "draw" the amps they require.

So having an alternator that is capable of providing 250 amps, to a car that originally only needed 63 amps, won't hurt anything. The tricky part is the battery...as mentioned, it will "draw" as much current as it needs to get it's charge back, if it's been run down.

Just make sure the wiring to all loads is big enough (wire AWG), and all the loads have a fuse/breaker that is sized to protect that AWG wire.

and make sure the connections between batteries and alternator can handle however much current the alternator will provide to recharge them. I don't know what that number is, but it's probably around 50 amps?

 

Use welding cable, highly doubt he will ever use all 250A, also the alternator will not last long at peak. So #2 would probably suffice, however you need to know what the system will draw normally to keep the battery voltage up.

Find out what the normal charging rate of the alternator is.

Will the batts be running tandem or will one be an isolated 'house' battery for the tunes only?

Batts should also be of the same age/size if running in tandem. An older weaker battery can become a constant draw on the newer/healthy battery.

Verify the capacitor is properly sized for the amp.

And most important, make sure all the grounds for the system are correctly sized and bonded to the hull. Including the engine grounds they will need to be resized to support the higher draw/charging capacity of the alternator.

 

The key here IMO is the intended use. Is this for driving down the road, or is the owner going to run the audio system off the batteries for extended periods of time? You could end up with a monster charging rate if the latter usage is intended with two batteries, particularly if they are deep cycle. I would ask the battery manufacture tech line what the possible maximum charging rates would be. Even so, if for during driving conditions it would be mandatory IMO to perform a load calculation to determine the size needed for the alternator-to-batteries connection wire.

I suspect a 250 amp alternator is probably overkill, load calcs will likely tell you what you actually need.

 

Having two batteries has nothing to do with the amperage output capacity of the alternator. My '85 F350 diesel pickup has two batteries and the stock 70 amp alternator, along with very minimal electrical equipment (lights, radio, heater). As others have said, the voltage regulator will determine how much current to send back to the batteries during charging and how much is required to run all the other electrical equipment. It is the other electrical equipment that must be taken into account. If the potential draw when everything is turned on exceeds the alternators output capacity, then the system will draw additional current from the batteries and they will discharge.

Add everything up and see were you end up.
Headlights, 15 amps.
Radiator Fan, 30 amps.
HVAC, 10 amps.
Air Compressor, 30 amps.
Giant ass Stereo system: Unknown, but probably lots of amps.

Is a 250 amp alternator needed? maybe if he is welding pipeline too....

 

OK, I am CHICKEN here. I don't want to say what I think, but will be a politician. I have always thought , with a dead, or very low battery, the alternator could put out it's maximum trying to charge the battery. If this is the case, 250 amps , even for a short time, would require a LARGE wire.
Well, crap, I said what I think anyway.

Ben

 

The alternator has to put out enough amps to replace the amps the accessories are drawing out of the battery (s) For once posts 2 through 7 all hit things pretty spot on with no nonsense.
If you think on that battery setup as being like a 70's Motor home where you had the main battery for the running gear of the chassis including engine, head and tail lights an anything involved in going down the road and then you have your "house batteries" that run the RV part of the rig and are isolated from the main battery with a continuous duty solenoid in the line to the house batteries from the alternator that shuts the connection off when you shut the engine off. If you have ever played your radio with the engine off long enough you probably had a too low to start the engine battery when you decided to start it. Cole Herse has a pretty decent continuous duty solenoid but the car may already have one or something in the circuit that will do that.

Engine side wise it shouldn't over charge the battery or affect the circuits on that side as there should be no big load on that side.
In this case I would size the cable from alternator to sound system battery for the max output of the alternator. Looking on line it is a challenge to figure out the max amps that each component might draw. If the alternator puts out a max of 250 amps that is all the cable is going to have go through it even if the request is higher.