Alternator/battery discharge question

I have recently installed an electric fan setup in my 54 Chevy. It has a Powermaster one wire GM alternator on it. I assume it's a 100 amp model but don't know that for sure. I know that a fully charged battery will show about 13.2 volts. I have a fresh Optima battery in it.

So, here is my question. When running the fan, AC, etc. the voltage meter drops down to just over 12 volts. At what voltage are you actually discharging the battery? I haven't tried it with all of that stuff running plus headlights yet.

Am I ok or do i need to make some changes? I wont spend a lot of time idling around with this car but I might get stuck in traffic one day and don't want to have a problem.

 

Rock, I don`t think you`ll have a problem as long as there`s a toggle switch to operate the fan. The only time you should need it is in traffic. Your alternator should keep the battery at or above 12.75, but there may be times when it will fall a little lower. One thing to consider is the output voltage of the alternator, not just the amps it`s rated for. Also the hotter the alternator, the lower output, amps and volts.
Jeff

 

Is that at idle? does it go up if you rev the engine?

Generally anything lower than about 12.5 to 13 volts it's discharging the battery. If it only happens at idle, it's not a problem, usually. Generator systems usually discharge at idle, and they kept cars running for decades.

 

What he said. Also, the early one wire Powermasters don't charge well at idle, so check the battery voltage at 1500 rpm. You should see above 13 volts.

 

you don't need to run the fan all the time, get one of the inline thermostatic switches and set it up so it cycles when it's needed

 

Jeff, I'm absolutely going to do this. The way its set up, the fan comes on when the AC clutch is engaged. As you said, it doesn't need to be when your at highway speed. Thanks.

Did you get the Avanti yet?

 

sounds like u need a regulator if your running an alternator. voltage should change little. are u running an alternator or generator?

 

A regulator will reduce the charging voltage to a safe level when the load is not very high, but it won't increase the charging voltage when the load is high and the alternator is just not putting out enough current to supply everything.

 

Rock, out of curiosity, will the voltmeter read higher than 12 or so (with all the aforementioned accessories on) if you blip the throttle or rev it up to 2000 RPMs or so and let it come back down to idle? I recall an article in Car Craft a few years ago about some one-wire alternators having a turn-on (or cut-in) RPM point that must be achieved before the internal circuitry kicks on the voltage regulator. Not sure if this is the case here or not, but thought it was worth a mention.

 

The alternator should keep the voltage slightly higher than the battery voltage shown without the engine running or the battery is being discharged. You might consider changing pulleys to speed up the alternator. This might be a smaller pulley on the alternator or a larger crankshaft pulley which might require a smaller water pump pulley if the same pulley runs both. The AC clutch takes some current as do your engine cooling fans and the AC blower fan.

You could also ad a throttle solenoid that holds the idle speed higher when the AC is running.

 

I think you could sit in traffic for 3 whole days.

 

As Squirrel said a fully charged battery in a static sitting car should show 12.5 to 12.75 volts. That's with a decent volt meter across the top of the battery. With the engine revved up a bit and nothing on it should show 14.2 volts as that has been considered nominal charging voltage for years.

That said all alternators and all vehicles are different and you have to expect a tenth or two variation on different vehicles.

The gauge in the car could be off some due to resistance in the wiring or the gauge it's self.

The main thing is does it keep your battery up and handle your accessories? As long as you can run the stuff you have on the car and not have issues with the battery being discharged when you stop and go to start again you are good to go. If it doesn't keep the battery up when you are out cruising the gut for an hour or so then you have a problem.

Usually where a guy with charging issues will run into problems is when he is out on a cruise night running up and down a street Like Congress in Austin in traffic for a couple of hours straight with the sound system pumping and the ac going even though he has the windows down along with the headlights on. Then he stops to have a cold one and bs for a while and the car won't start because the battery has been drained below the level it will start the car.

 

It stays aboe 12 v, maybe 12.5 with everything on. At idle, it definitely drops below that but at around 1000 rpm, it's good to go.

 

I'm sure that would definitely kill my battery.