The Model T Ford Battery Charging System by Ron Patterson

The Model T Ford battery charging capability is a subsystem of the Ford FA Starting and Lighting System designed by Mr. Fred Allison at the Ford Motor Company electrical engineering department and introduced in late 1918 for use on Model T Ford cars and trucks.

Many people have trouble with the charging system on their Model T and need help repairing it. Here is an electrical (not physical) schematic diagram and technical description of how it works.

Above is a Model T Ford generator rebuilt by Ron Patterson equipped with a Fun Projects voltage regulator in place of the factory cutout.



When the engine is stopped and the generator is in its quiescent (inactive) state, as shown above, the four field winding pole shoes maintain a residual alternating N-S-N-S magnetic polarity from their last use. These weak magnetic lines of force from the pole shoes are impressed upon the generator armature windings, but no current is produced because the armature is not rotating.

When the engine is started, the generator armature begins to turn producing a small voltage in the armature windings; a portion of this voltage is picked off by the third brush and fed to the field windings increasing pole shoe magnetism and hence armature voltage. This armature, third brush to field winding feedback loop continues to increase the generator output voltage to full operating level and is referred to as “building up”. The inability of the generator to “build up” is a common generator failure and has several causes. The armature, third brush and field winding circuit controls the output current of the generator when operating normally. I.E.  The position of the third brush controls the field winding pole shoe magnetic strength impressed on the armature windings and hence the output current of the generator to the terminal bolt.

The armature output brush is wired to the generator terminal bolt where the cutout is connected.

The cutout has two windings; a voltage and a current winding.

The “build up” described above continues until the voltage reaches 7.2 volts (the normal charging voltage of a 6 volt battery) in the cutout voltage winding which operates the spring loaded relay closing the cutout contacts connecting the generator to the battery. At this point the generator charging current is flowing to the battery via the cutout current winding holding the cutout armature contacts closed and charging the battery. At this point both cutout windings have current flowing in the same direction.

As long as the generator continues to produce current to the battery the cutout contacts will remain closed and the ammeter will show the charging rate as set by the third brush in the generator.

As the engine slows down or is stopped (or turned off) the generator charging rate decreases to a point where current is flowing from the battery to the generator reversing the current flow in the current winding which forces open the cutout contacts disconnecting the battery from the generator. This is why the cutout is called a “reverse current cutout”

This connecting and disconnecting the generator to the battery process in the cutout goes on continually depending upon the speed of the engine while driving. There is heavy current flow at the cutout contact points and much arcing occurs during connection and disconnection. This arcing directly affects the long term reliability of the cutout.

A word is in order about setting the generator charging rate. In the Model T era cars were driven at slow speeds on short trips. To keep the battery charged Ford recommended setting the charging rate at about 12 amps. But today we drive our Model T’s at faster speeds for longer periods. With the charging rate set so high the battery is continually being over charged with the generator operating at or near its design capability.

The best solution to this problem is to replace the reverse current cutout with a FunProjects voltage regulator which modulates the output of the generator to the extent the battery actually needs charging thereby operating the generator at a reduced charging rate, prevent overcharging the battery and exceeding the generators limited capability. The generator charging rate is correctly set when installing the voltage regulator.

WARNING: The Model T generator can be ruined easily if the engine is operated with the generator disconnected from the battery, either due to disconnected wires or a faulty or missing cutout.

The following is a description of “why” the Model T Ford generator destroys itself when the battery is disconnected. I described the Model T generator “building up” process in a previously published article entitled “Model T Ford Battery Charging System”.

When the cutout fails to operate or there is otherwise no battery connected to the battery the armature and third brush” building up” process continues uncontrollably. The armature voltage increases to 20+ volts and there is no load on the armature output to pull off the current being produced. A great amount of heat is produced in the armature and field windings and it becomes a race to see which fails first stopping the building up process. The field windings can simply burn out or the solder in the commutator segments where the winding connects melts and is found thrown around the inside of the generator case.  In either case the feedback loop fails and the generator ceases to function.

This is one of the biggest downside risks of the third brush regulation generator design.

I would like to thank Bob Cascisa for preparing the simplified electrical diagram of the Model T Ford charging system for this article. It is a composite of various diagrams found on the internet and modified by Bob for use in this article.

Editor’s note – Ron Patterson is the acknowledged authority on the subject of Model T Ford Ignition Coils, their operation, history, and repair. He will be forever known affectionately as “The Coilman”. Ron has personally rebuilt thousands of Model T coils, generators, starters, and hand cranked coil testers. We thank Ron for all of his contributions to the Model T Ford. Our Model T’s all run better due to Ron’s efforts.

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