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MG TD TF 1500 - Generator Voltage Output

The October issue of the Southeastern MG T Register includes the following in the "A Few Words from the Navigator's Seat": "I had to limp up to LaGrange with our MGA's alternator pegging my voltmeter at 16v+ at idle"

After replacing both my ginny (not an alternator but they both make electricity from something spinning)and my regulator box I installed an aftermarket voltmeter to read the voltage at the A2 (I think - anyway, it's the terminal connected directly to the ginny output wire). The voltmeter indicates that the regulator is working because at low RPMs it reads zero to very little and above about 1,100 RPM it jumps to 14v+. My concern, however, it that at higher RPMs (4,000 - 4,500) it will go up to 16v - 18v.

Should it be doing this? Is voltage that high damaging my battery or something else? Should I run with my headlights on?


J K Chapin

Correction, that either the F or D terminal that the voltmeter is connected to. Can I lower the highspeed voltage output by adjusting the screw for the contact on the regulator relay so it will open more easily (i.e., with less coil voltage)?


J K Chapin

The function of the regulator, on a generator or dynamo is NOT to control voltage, but to control current. It does this by vibrating a relay the controls current into the field coil of the generator. This relay vibrates quickly and most analog meters will not respond to the vibrations. Digital meters, most of which do not read true RMS voltage may be very confused.

For a battery to charge the voltage, that the generator supplies, must exceed the voltage at the batteries terminals. This, however is temperature sensitive and on hot days will rise.

Nominally you will need more than 13.5 volts.

If you AMMETER is working and does not indicate excessive current then everything is OK. Its the current that charges the battery and not the voltage.

Think of current as water flowing out of a faucet. Think of voltage as the pressure in the pipe. Your generator is designed to deliver a certain number of gallons per minute. It needs some pressure to do that. The regulator adjusts the pressure by (very) quickly turning the facet handle on and off.

An alternator, OTOH, is controlled by the voltage it puts out. Its regulator is entirely different. (It also generates AC which is then converted to DC.)
That is why our cars have ammeters and modern, alternator cars have voltmeters.

As long as the generator is not exceeding its design current I would not worry about the voltage you read.

Jim B.
JA Benjamin

If you're checking the voltage at the battery terminals and it's over 15, it's too much. Normally the regulator set point should be in the neighborhood of 14.5 volts. A temperature sensitive regulator will be higher voltage on a cold day and lower when the under hood temperature increases.

A constant current smart charger may have a higher voltage but the automobile generator or alternator is simply a constant voltage system.
J E Carroll

I would not be happy with 18V in my car! It's enough to burn out lights prematurely and boil batteries. Makes you headlights nice and bright, though.

The volt meter is connected between the D and E terminals. I understand (now) about it maybe not responding fast enough to see the regulator cut out and "thinking" the voltage is staying too high. I'll connect across the battery terminals and see if it reads differently. The ammeter does show + charge most of the time but it's not pegged so maybe it's doing OK. The lights do not seem to be excessively bright.

Thanks for the input.

J K Chapin

Again, its not voltage that boils out the battery, its current.

If the voltmeter is not RMS, then on a generator system it may be reading peak voltage. You need to rely on average RMS voltage measurements.

When I checked the construction of the Detroit voltmeters I found some that were actually bimetallic arms that relied on heating to bend an arm that moved the pointer. Thats real RMS.

Most modern multimeters are peak reading. Some do have a position that says RMS.

With a generator or dynamo if the current is under control you are good.

Moving the meter to the battery terminals is a proper approach. The battery acts like a large capacitor. Perhaps as large as 1/4 of a Farad. However, be sure of your meter.

Jim B.
JA Benjamin

That is sort of like saying its not speed that causes injury in accidents, but impact. True, perhaps, but it takes speed to create the impact. Likewise, if you don't have the voltage, you won't have the current. So for all practical purposes we can ignore the distinction, and quantify and worry about whichever one we are best equipped to measure.

Given the parameters of the typical auto electric system, generator or alternator, it is just about impossible to get too little amperage without the volts being low, or too much amperage without the volts being too high.


The voltage regulator does accomplish its task by regulating the field current and ultimately limits the generator output current so as to not damage it.

RMS on a DC circuit? The output may not be 100% clean DC and although it may have a ripple, itís hardly a sine wave. With the battery and other loads it looks pretty clean with a Ďscope.

My Fluke meter was a true RMS meter, it cost me dearly and I was sad the day it disappeared. The much cheaper replacement actually measures, on AC, an average rather than RMS and applies a correction factor to display, for all practical purposes, an RMS value. I don't think any multi meters read peak, unless it has a selection, for if you checked your home outlets they would show around 170 VAC instead of 120.

The DC generator is actually an AC generator as well, the segments of the commutator effectively rectify it although not as nicely as the 3 phase AC is rectified by the diodes in what we call an alternator.

Itís true that current charges a battery but the voltage supplied must be higher than the batteryís to overcome what appears to be resistance. Too low a voltage, no charge, too high, too much. If youíre trying to charge at a specific current then the voltage must be variable. What we have in our cars is a constant voltage system, more or less as itís temperature compensated, so the voltage set point has to be one that keeps the battery happy.

I think we have amp meters on old cars because it was the fashion. They donít really tell you much about the systemís health. Airplanes with similar generator systems were quite likely to have volt meters or combination volt/load meters where they always read the system voltage and when a button was pushed would read the generator load, the shunt being placed in the generator output line to read load on it rather than placed to read battery charge. If the voltage is right the battery will take care of itself.

J E Carroll


The kindest thing you could do to protect your battery and associated components would be to convert to a solid state voltage regulator. The performance characteristics of the solid state regulator that I constructed from information on the web merits the time spent and is totally housed within the original fitment regulator housing.

I know my battery is being charged at the correct float voltage because I adjusted the regulator components to create the correct value and I no longer have variability due to relay contacts arcing or chattering.

G Evans

This thread was discussed between 11/11/2013 and 12/11/2013

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