Is it possible for the battery to charge high enough soo that the ammeter not show charge? My ammeter does not show charge when the lights are on.

Does it show charge when the lights are off and the engine running along about 25 mph? If so, your generator is working. When you turn on the lights it is normal to show a discharge even with the engine running. That will present no problem if you only drive for short distances with the lights on from time to time. If you do a lot of driving with the lights on, you should set the generator to charge just enough to remain at 0 amps with them on. But it will be too much charge if you drive a lot in daylight. The solution would be to buy a fun projects voltage regulator. Then it would be more like a modern car and adjust to your needs. If you only drive during the daylight hours, you can set the generator for about 4 amps charge and leave it alone with the cutout or diode cutout. There, I've given you more information than you asked for.

The answer to your original question, is it should not show charge when the lights are on, but it should show charge when the lights are off.
Norm

Yes and no.

I do not have a generator, but here is my understanding based on what I have read and my experience with Model A's.

More likely that the generator is not turned up enough for night driving. There is a third brush that you can move. It determines the generator output. If you are going to do a lot of night driving, turn it up. Then turn it down for day time driving so you don't overcharge the battery.

Better gage is a voltmeter. The battery should be between 13.5 and 14 Volts when the engine is running above idle. Use a multimeter and hook it up anywhere that is convenient where you can measure voltage. Run the engine up to about 1500 rpm and measure the voltage. Block the wheels while you do this.

Neil

I think the answer to your question is no.

When a battery is being charged by a properly-operating alternator or "automatic" charger, when the battery's voltage reaches 6.3 or 12.6 the regulator cuts off the charger.

The older, non-automatic "trickle" chargers are built so that the internal resistance of the battery to accept any more 'juice' will lower the charger's output - but never to zero, hence you can boil a battery with them.

Larger chargers, as used in service stations, will easily boil a battery because they have such high capacity that the battery reaching full charge has little or no effect - they just keep on pumping.

The Model T's original generator and cutout will continue to put out at high engine speed, enough to boil the battery. But we generally don't run wide open for long at a time. That's why we adjust the third brush so the generator's output is just a tad more than we judge our need to be. It's very subjective, and we can easily get it too high or too low, causing battery boil or failure of headlights on a long night run.

The best simple solution seems to be the Fun Projects regulator which is designed to look just like a Model T cutout, but actually regulates the output of the generator according to the battery's need. The maximum rate of charge is still determined by the third brush, but the regulator can cut the output to zero when the battery is full.

Now back to your question. If your ammeter goes negative when you turn on your headlights with the engine not running, and if it stays at or near zero if you turn on the headlights with the engine running, you have a system that is working exactly correctly. It probably already has a regulator rather than a cutout.

What's happening is that you're driving along, the juice taken out of the battery to run the starter has been replaced, and the charging system has regulated down to so close to zero you can't see it on the ammeter. Then you turn on the headlights, the system sees a discharge, and ramps up the charge to compensate. That's the exact proper scenario, and if that's what you have, you are golden.

Or, suppose your generator is regulated to a maximum of 15 amps. Then you turn on the headlights which take 15 amps. Result: zero.

Does that explain it?

I was thinking 12 Volt battery. For a 6 Volt battery, the voltage while the engine is running above idle should be between 6.8 and 7 Volts.

Neil

Normal behaviour. The Model T has a constant current generator. Pretty much regardless of battery voltage, the generator produces a certain current as set by the 3rd brush. If the current being drawn off the electrical system happens to be the same amount, then the ammeter will show zero. In this situation the battery is neither being charged or discharged and the load is being powered entirely from the generator.
To answer the question, it does not necessarily mean the battery has charged "high enough". The battery could be anything from fully discharged to fully charged under these conditions.
The only way to determine charge status is with an accurate voltmeter connected directly to the battery terminals, or by using a hydrometer.

John is correct, a hydrometer is the gold standard for measuring battery status, but only after the acid is stired up by charging high enough to generate bubbles. Voltage will tell you if the battery is being charged and by how much, but you need to let the battery sit for a few hours to use voltage as a gage of how charged a battery is. The battery will have a higher voltage than at rest right after taken off charge and have a lower voltage than at rest right after taken off a load.

A really good site on battery technology is Battery University. Their article on charging lead-acid batteries is at http://www.batteryuniversity.com/partone-13.htm.

Getting back to Al's question, a modern voltage regulator would solve any question about battery charging, but would take away from the authenticity of the third brush. For Al's sake, the third brush controls how much current is in the field, thereby controlling the fields's magnetic strength, and thus the generator output. The location on the commutator where the maximum voltage is changes with rpm. This provides a natural current limiting feature. As the rpm increases, the location of maximum voltage on commutator shifts, the voltage to the field decreases, and the generator output is limited. Voltage regulation without a voltage regulator.

Neil