MTFCA Forum

If I set my '27 generator with stock cutout rate high enough to hold the ammeter at zero to 1 amp charge at road speed with the lights on, it shows about 7 amps charge with the lights off. If I turn the lights on with the engine stopped the ammeter shows about 7 amps discharge, which seems too high to me, so I question the ammeter accuracy. It is accurate at O. Question: If I add a switch to allow inserting a resistor, such as one or more ignition resistors, into the wire between the cutout and the battery to reduce the battery charge rate at road speed with a charged battery and with the lights off, will that allow the generator to damage itself? Is there a maximum recommended voltage, measured at the generator output terminal with the engine running at road speed? I'd think anything up to 8 volts would be OK. I'd like to limit the charge rate with a charged battery and lights off to about 1 amp. PS: The engine runs MUCH better on BAT with the higher charge rate than it did with a lower rate or no generator, but I normally run on MAG.

The way you have your charge rate set, appears to be about perfect. I would not change a thing. Seven amps discharge with engine stopped and lights on does not seem high to me. My lights draw 9 - 10 amps.

If you're so worried about amperage draw from the bulbs just change over to LED bulbs and you'll be satisfied. I'm not promoting any supplier, it's all your choice.

You don't need the resistor. The charge rate can be set by moving the third brush (that's the one at the top). If you move it toward the engine it will charge more and if you move it away from the engine it will charge less. You need to loosen the nut to move the brush and then tighten the nut when you get it set.
It depends a lot on how much you drive the T how to set the rate. If you drive it all day long on tours, you might decrease it to about 3 amps. If you drive short distances only once or twice a month leave it as it is. In fact you could do as many have done in times past. On a long tour you drive with lights on during the day or at least after you drive a few miles to recharge from starting. If you drive mostly after dark, then you might want to set it up to where it charges zero with the lights on.
Anyway, I think it is set about right for the way most of us drive our T's
Norm

If I reduce the charge rate the amount of discharge with the lights on will increase in proportion. Turning the lights on in the daytime gives a reading very close to 0 on the ammeter at engine speeds of 1200 RPM and up. So the rate is never really right either day or night. If I'm doing the math right, the current draw at around 7-8 amps equals about 1/10 horsepower, assuming about 8 amps at 8 volts. Adding a resistor to ground to burn off the current would do the same as running the lights, with the same disadvantages. However, adding resistance between the generator and the load would reduce the current draw, much like using lower wattage bulbs. But at some point, with enough resistance in the circuit, wouldn't the generator would run away with itself at higher RPM?

You dont need to burn off electricity. Run a ground wire from the output terminal with a switch. The switch turns off the generators output.
No horse power loss, it rotates freely.
As far as 7 amps output, thats ok if you run a lot @ night. I personally like a charge @ 4 amps. Thats plenty for medium trips with a lot of electric starts. Nothing wrong with running a small net loss @ night as long as the battery stays charged with your day driving. Running a constant 7 amps during the day is harder on the battery & possibly the generator.

Adding a resistor between the generator and the load would do nothing at all. The generator is a current device. If you put a resistor in series, the generator will just increase voltage to maintain the current output that the third brush is set at. How much driving do you do with the lights on? I would think that setting the charge rate of 3 or 4 amps would be good for most daytime driving and a discharge of 3 or 4 amps while driving with the lights on would not be a problem unless you were driving several hours that way.

I believe there is a tendency to complicate "battery charging" unnecessarily. There is always worry about over charging or draining the battery when those issues can be easily regulated by turning the lights on or off. The major draw on the battery is the starter and that loss is easily replaced. Setting the 3rd brush to cover the needs of the lighting system or a bit less is about all that is necessary. Keep the battery charged is then just a matter of turning lights on or off. Cold Crank specification only applies to starting in cold weather and not how long you can keep the lights on. The specification for available power, longevity, is measured in ampere hours Ah. So a 100Ah battery will provide 7amps for about 14 hours (100Ah/7A).
Now some battery specs give a C20 value in Ah. What that means is the number of Amps it can provide for 20 hours. An Optima 6v Red Top has a C20 of 50Ah or to say it differently the battery simple Ah is 100Ah (20 x 50). Many Group 1 lead acid battery are about 200Ah.
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No matter what solution you apply you will still not know if your battery is fully charged. The only real way to test it is to record the voltage without a load if its within spec then measure with a load and note the difference. Much simpler to put a battery minder on at that shuts off automatically.

I get pretty good lighting, for a T, with the setting I have,which is very close to holding 0 amps with the lights on. The ignition definitely works better on battery with the higher charge rate. I don't like running for longer periods with the lights off and the charge rate showing 7-8 amps with a charged battery. 1/10 hp, plus or minus, isn't much, but I'd rather use it to to move the car. Adding resistance in series would make the generator think the battery was charged, even if it wasn't. It looks like a switched shunt to ground connected at the generator output terminal would be the best solution. That would eliminate parasitic electrical load and eliminate any need to remember to turn the lights on or off. When driving at night, dim headlites would prompt me to "turn on" the generator, and if I forgot to turn it off, the battery would not be discharged, assuming the cutout relay functioned. I'll need to add some kind of dash light so I can see the ammeter at night. A 12 volt idiot light connected between the gen output and ground would indicate whether the generator was switched off or on when the engine was running.

TXGOAT2 wrote: ↑

Thu Jun 30, 2022 2:30 pm

I get pretty good lighting, for a T, with the setting I have,which is very close to holding 0 amps with the lights on. The ignition definitely works better on battery with the higher charge rate. I don't like running for longer periods with the lights off and the charge rate showing 7-8 amps with a charged battery. 1/10 hp, plus or minus, isn't much, but I'd rather use it to to move the car. Adding resistance in series would make the generator think the battery was charged, even if it wasn't. It looks like a switched shunt to ground connected at the generator output terminal would be the best solution. That would eliminate parasitic electrical load and eliminate any need to remember to turn the lights on or off. When driving at night, dim headlites would prompt me to "turn on" the generator, and if I forgot to turn it off, the battery would not be discharged, assuming the cutout relay functioned. I'll need to add some kind of dash light so I can see the ammeter at night. A 12 volt idiot light connected between the gen output and ground would indicate whether the generator was switched off or on when the engine was running.

Nobody here suggested that the resistor was a good idea, but you seem bent on going that route... enjoy your T.

What? Read again.
What I said, based on advice given here:
"It looks like a switched shunt to ground connected at the generator output terminal would be the best solution. That would eliminate parasitic electrical load and eliminate any need to remember to turn the lights on or off. When driving at night, dim headlites would prompt me to "turn on" the generator, and if I forgot to turn it off, the battery would not be discharged, assuming the cutout relay functioned. I'll need to add some kind of dash light so I can see the ammeter at night. A 12 volt idiot light connected between the gen output and ground would indicate whether the generator was switched off or on when the engine was running."

TXGOAT2 wrote: ↑

Thu Jun 30, 2022 4:02 pm

What? Read again.
What I said, based on advice given here:
"It looks like a switched shunt to ground connected at the generator output terminal would be the best solution. That would eliminate parasitic electrical load and eliminate any need to remember to turn the lights on or off. When driving at night, dim headlites would prompt me to "turn on" the generator, and if I forgot to turn it off, the battery would not be discharged, assuming the cutout relay functioned. I'll need to add some kind of dash light so I can see the ammeter at night. A 12 volt idiot light connected between the gen output and ground would indicate whether the generator was switched off or on when the engine was running."

Apologies. Somehow, I thought I still read "resistor" in your last posting. Maybe when you said "shunt", I thought "shunt resistor"? Anyway, I'll still say, "Enjoy your T"

You definitely need guidance from Ron Patterson on generator output and load. Do what you want and good luck

A resistor put in series for the field can be used to adjust output — some of the higher priced cars used the headlight switch to also switch in a resistance in the field circuit when the headlights were off

If I ever take the generator apart, I intend to put a field terminal on it. In the meantime....

Go for it ... and let us know your results

Putting resistance between the cutout and battery will cause the generator voltage to increase as it tries to restore the same amount of charge current it produced before. Not only might this damage the windings in the generator, but assuming a mechanical cutout, the voltage coil might burn out. The generator is a constant current device. A shunt resistor can be used between the generator terminal and ground to divert a certain amount of charge current, but it's inefficient. At least once commercially made device did just this: A more efficent control is a resistor or rheostat in series with the field coil. (Setting the charge current to 25Ah as suggested in that article is way too high)
Best option is to simply set the charge current to 5A and leave it. A conventional wet lead acid battery won't be damaged by that amount of current.
And let's get rid of this fallacy that the battery must always be charging when the headlights are on - there's 50 to 100Ah of capacity in the battery. If generator charge current is 5A, and headlights draw 7A, then only 2A is pulled from the battery, and there's a lot of hours before the battery is discharged. Who, in a Model T, drives for hours upon hours, night after night?

The Bradleystat does about what turning on the lights does. Is it correct that grounding the output terminal completey disables the generator? If so, adding a switched ground connected to the output terminal of the generator would work for me. I assume the generator would retain enough residual magnetism to ramp up without further attention once the ground switch was opened. I also assume that closing the ground switch with the ammeter indicating 0 charge or a discharge would not result in fireworks.

The grounding switch certainly works; it's the principle behind the various electronic voltage regulators. Since the generator's field coils are powered from the output terminal, they are unable to produce any magnetic field (above that from residual magnetism) with the output terminal shorted to ground.

However, the grounding switch should only be used with solid state diode cutouts. The reason being is that the mechanical cutout actually provides a direct connection between the generator and battery when in operation. If the grounding switch is closed while the engine is running above idle, the battery is effectively being short circuited to ground, for the short period it takes for the cutout contacts open. If one remembered to only close the ground switch with the engine off or only at idle (when the cutout is still open), then it could be used this way.
I remember one car in my club with a grounding switch and mechanical cutout - the owner complained of blowing the battery fuse when the switch was operated whilst driving.
In the case of the diode cutout, the diode is immediately reverse biassed when the grounding switch is closed, so the switch never sees the battery current.

I have a regular cutout that works well, so I'd have to be careful grounding the generator. Engine stopped or at slow idle ought to be safe. //// Would adding a 12 volt coil primary ( or 2 of them) in series with the ground switch allow grounding the generator output terminal with a standard cutout at road speed without wrecking something?

TXGOAT2 wrote: ↑

Fri Jul 01, 2022 10:32 pm

I have a regular cutout that works well, so I'd have to be careful grounding the generator. Engine stopped or at slow idle ought to be safe. //// Would adding a 12 volt coil primary ( or 2 of them) in series with the ground switch allow grounding the generator output terminal with a standard cutout at road speed without wrecking something?

Dont have to be "careful". Nothing gets wrecked. Ground connection @ any speed. The cutout sees no voltage & opens.
You are trying to complicate the simplest thing.

John H cautions against closing a ground switch when the conventional cutout is closed, since it forces the cutout contacts to open with battery current applied. With a solid state diode, which I don't have, there is no problem grounding the generator at will. I'm speculating that adding an inductor to the grounding circuit would allow the cutout to disengage with little or no current across the contacts, much as it would do during normal service.

I get it now. Makes sense. I normally run a diode conversion cutout.
I have been running a heavy duty period aftermarket mechanical cutout with no problems with a kill switch & idiot light.
I will keep in mind the grounding out possible damage @ a hi rpm use.