Bob Carabbio gave the following answer to a question on the www.ahooga.com message board, regarding setting the charging rate on the Model A. He was kind enough to allow me to reproduce it here.

The stock "A" charging system isn't aware in any way about the charge condition of the battery, it's basically an adjustable "constant current source" which naturally "rolls off" a bit at higher R.P.Ms. This is due to what used to be thought of as the "bending of the magnetic lines" at higher armature speeds, but actually is related to the inductive delay of the current build in the armature windings, and it's associated reduction in field excitation at the point that it's drawn off.

The generator IS INTENDED to be set to a value JUST ABOVE the average long-term current draw of the car in normal use.

Determination of this was done by the Ford dealer (or the owner) through the use of a hydrometer which determined (through measurement of the specific gravity of the electrolyte in the cells) the "state of charge" of the battery. A discharged battery essentially has water in the cells, while a charged battery is strongly acidic. The Ford Garage/owner would periodically test the specific gravity in the cells, and if it was observed to drop gradually, the charge rate would be increased until it tended to rise.

The specific gravity readings and their meanings are as follows -

1.280-1.300 Battery fully charged.
About 1.210 Battery about half discharged.
Below 1.150 Battery fully discharged.

These figures are given assuming the temperature of the solution is 60 deg. F. (15.5 deg. C.).

If the electrolyte temperature exceeds this, .002 must be added to hydrometer readings for each 5 deg F. ABOVE 60 deg. F to give the true specific gravity. Similarly .002 must be subtracted from hydrometer readings for every 5 deg. F. BELOW below 60 deg. F.

The readings for all cells should be approximately the same. If one cell gives a reading very different from the rest, it may be that acid has been lost by spilling or has leaked from this particular cell, the cell has become Sulphated, or shed plate materials may be causing a short circuit between the plates.

Ford had "Average values" for the charging rate in amps that the cars were set to - I recall 7.5 amps as being one of them with a boost in cold weather. They are listed in the "Ford Service Bulletins"

The difference is that modern systems -since the introduction of the voltage regulator - attempts (and succeeds generally)to keep the battery fully charged by MEASURING the condition of charge in the battery based on the voltage across the battery under charge, and attempting to regulate the voltage to something over 7.2 or so.

The 3rd brush system expected that the battery would be partially discharged during times of heavy current draw situations, but rebound under light current draw operation.

For a car driven long distances primarily in the daytime a low charging rate would suffice. For a car driven short distances primarily at night, a much heavier charging rate would be required.

Significant Loss of water was a sign of too high a charging rate.

A voltmeter would give some indication of battery charge, and a 6 volt battery showing something above 7.2 volts (with the car running and charging) would be close to fully charged.

Ford cautioned AGAINST the use of the dash ammeter as a measuring instrument. That goes double for the junk being sold today.

A Digital or analog multimeter capable of 10-15 amps in SERIES with the output lug on the cutout, and the wire removed therefrom would be the proper way to determine generator output.

The "A" generator is in danger of incineration above 13 amps since no provision for armature cooling was provided.