What is normal charging behavior?

1939mars

1939mars

Senior Member
I am still pretty new to A's and am trying to understand the charging system - specifically, what to expect the current to do as a function of rpm. This is partially because I have burned up 2 aftermarket and one refurbished ammeters.

I have run a jumper from the positive battery post to a bolt on the back of the generator and from the negative post to the front/battery terminal of the cutout. With a charged battery and the 3rd brush all the way up, I get a couple amps current at fast idle and it rises to about 8A at high rpm. I am measuring that at a loop of wire I put in place of the ammeter with a craftsman clamp on meter.

Battery is a fairly old optima 6V that measures about 6.5V when not under load. Voltage rises to about 8V at high rpm. Voltage drop across cutout at the high current is about half a volt.

Does 8A at driving speed sound at all normal under the rest of the conditions. I'm hesitant to drive it for fear of burning things up, but maybe this is normal behavior??
 
The Model A generator is crude compared to modern alternators. The Ford dealer in 1930 would measure the battery's electrolyte specific gravity when the cars came in for service and adjust the generator output accordingly.

8 Volts is a little high for a 6 volt battery and may cause your Optima battery to outgas. You could try burning the lights to limit the voltage but you may have to turn up the generator output. Back in the day, people would add a voltage regulator to the generator to make it voltage regulated like a more modern car. People now will install an alternator if they do a lot of cruising. A modern alternator is voltage regulated.

The way the Model A generator works, without getting into the technical details, is to limit the current as the rpm increases, but it is not exact so the current, and thus the voltage, is generally proportional to the rpm.

It sounds like you may have an electronic cutout, that has a diode inside. The diode will turn on and conduct current when the voltage difference is about half a volt. So there is always about half a volt drop across a diode when it is conducting.
 
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Thanks - should have said its still the points cutout. I have looked for the regulators but it looks like no one is making them anymore :(.
 
Just my $0.02 worth. My completely stock 31' is set-up to charge at 10 amps at road speed. That just about "zeros out" with the head lights and tail light, night driving... and I do not do much night driving. It has been setup like this for 50 years and have never had any battery problems. Now I will say that the car only gets driven a couple of hundred miles a year. I suppose that if one did a lot more daytime driving that charge rate may be too high.
Joe B
 
The ammeter measures only battery current from the generator, or to the load when the generator rpm decreases. Your observations seem fairly normal. 7.8 volts is the magic number to stay below for a 6 volt battery. You don't indicate what high rpm, but I suggest you don't reach that except during shifting. Check at normal driving speed and go from there.
 
Generators have kind of a sweet spot where they have peak efficiency in operation. Too little or too much rpm gets them outside the sweet spot so Ford used different diameters of pulleys over the years to get rpm in the efficiency zone as best as practicable.

While no two generators function exactly the same they are close enough to make the differences negligible for the most part. The three brush generator has only the one form of current control. The distance the 3rd brush is from the power brush is what limits the current regardless of the operating rpm. RPM will vary output a little but not enough to make the difference worth mentioning. The battery is the only limiting factor on voltage.

A properly functioning tubular generator should be capable of 15 amps. The powerhouse is cable of 20-amps. For daytime driving 4 to 6 amps is generally enough. For night time driving 10 to 12-amps. Folks were not fond of constantly adjusting the generator so you can imagine how that affected the batteries over time. Folks would complain that if you tried driving all night with the model A then the battery would get low on power and the lights would slowly dim out. They may have not bothered to adjust the generator brush. Most folks would have taken the car to a dealer or a mechanic to get that done. Fiddling with cars is not everyone's cup of tea. Adjusting the output should be done with a calibrated amp meter placed into the circuit in series. The dash gauge isn't all that accurate and especially with the cheap reproductions that are currently available.

I saw a post on a different web site that indicated that there may be another source for an electronic voltage regulator but it's still in a development stage so I don't know if it will make it to the market or not.

A properly adjusted generator set on daytime output will read generator output less whatever load is currently functional. If the coil draws 2-amps and the generator brush is set to 6-amps then the ammeter will indicate 4-amps until you stop at a stop sign. The brake light will drop the ammeter indication to reflect both the ignition and the brake light. Toot the horn or turn on the lights and it will go into the discharge range to reflect the difference in current flow.
 
A lot of currently available ammeters are of poor quality, probably made in China.

One problem I have often run across is that the nuts at the base of the studs on the back of the ammeter are not tight and the studs are not making good contact with the buss bar inside the ammeter. This will cause the ammeter to get hot and fail. Before you attach the wires, tighten these nuts and then install the wires.

Chris W.
 
If you're looking to add a band voltage regulator to your generator, contact Jim Peterson. I ran one of Jim's VR for years until I got a Russ Evans look-a-like generator/alternator. Sold the VR generator to a club member and he has 6 years of trouble free charging. I highly recommend Jim's band style voltage regulator!!!!!!

James Peterson
PO Box 912
Bend, OR 97709
541-389-0438
email: JPetersonBend@yahoo.com
 
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