THE MODEL "A" BATTERY & GENERATING SYSTEM - PART I

Subsystem components - Batteries, Generators, and Cut-Outs

The Model "A" DC generator and cut-out have been extensively covered in the past as individual components. This series of articles discusses those components and the storage battery inter-relations as a system. It presents the general performance requirements and some tests performed on a diode cut-out and generator voltage regulator available from suppliers to correct deficiencies/ improve the system performance.

When automobile reliability progressed so they could be driven often, intermittently, and at night, the electrical system requirements increased substantially. The advent of the electric starter further placed demands on the system.

There are three major subsystem components:

• Electrical Energy Storage - the lead acid storage battery
• Electrical Energy Replenishment - the DC generator
• Interconnection Control - a relay switch to connect battery and generator

The storage battery must initially provide power for starting and ignition and lights as desired. The generator must replenish the starting current drain in a short driving time so the car can be restarted as often as desired. It must also continuously supply energy (current) to power the ignition system, the lights (when used), and maintain the battery at full charge. The relay and switch must connect the generator to the battery when the generator can supply power to the external circuit and disconnect the two when it cannot, to precent battery discharge.

The rechargeable lead-acid battery has been in existence since before 1900. The individual cells are inherently low voltage-high current ones. Electric motors and generators are normally higher voltage and lower current devices. The auto industry compromised early on three cell batteries. We will return to the other electrical system requirements.

During the engine start cycle, the starter motor/ battery current fluctuates through a range of about 150 to 250 amperes as the engine cranks through intake, compression, and exhaust strokes ¹. The average value is about 200 amperes. A typical cold start time may be as long as 15 seconds of time. The battery energy discharge may be expressed as 15/60 x 200 or 50 ampere-minutes.

It is desirable to replace the energy in, say, 5 minutes of driving time. That would require a minimum of 50/5 or 10 amperes of charging current if the battery recharge was 100% perfect. If the start time were reduced or the driving recharge time extended, the recharge current would be reduced proportionally.

The DC generator must also supply a maximum current capacity of about 4 amperes to sustain the ignition system, if the starting recharge is ignored. To sustain the lighting load, current capacity of about 10 amperes must also be added, for a total load of 14 amperes. If the engine starting recharge time were extended, a current of 6 amperes might be apportioned. In any event, at least 1 or 2 amperes might also be added to assure some constant battery recharge/ float current.

The generator maximum current output thus is approximately 4 plus 10 plus 6 or 20 amperes total. The Model "A" generator maximum current is so rated at 8 volts output.

The next system requirement is to interconnect the generator and battery when the generator output voltage exceeds the battery voltage, to charge and supply power to the electrical system. The battery must be disconnected when its voltage is lower than the battery voltage else current will flow from the battery and "motor" the generator, discharging the battery. The generator voltage is dependent upon the engine RPM since it is belt driven.

The device controlling the connection/ disconnection is properly called the "Reverse Current Cut-Out." This relay and its contacts connect the generator to the battery and electrical system when the generator voltage initially exceeds about 6.5 volts and disconnects when the battery voltage causes a current of about 2 amperes to discharge (reverse current) into the generator. This occurs when the generator output voltage is just below the battery voltage².

If the relay contact points "stick" or the release is not completed, the reverse (battery discharge) current "locks up" the relay and discharges the battery at about 10 amperes current. To correct this type of failure, a solid state diode cut-out (electrical equivalent of a check valve) has been used and is available from several sources ³.

The Model "A" direct current generator is the type known as "Third-Brush," patented (No.9364) in England by W.B. Sayers in 1886. Originally designed to be used at constant speed, it was modified for the variable speed auto applications by placement of the field winding. The third brush supplies current to the field winding ⁴.

When connected to a storage battery, the generator tends to operate as a constant current machine, independent of its speed. This means it tries to deliver a pre-set current to the battery and any other load from its minimum charging RPM to the maximum operating RPM. The output voltage changes to accomplish this constant current.

The maximum current the generator can supply is determined by the pre-adjusted position of the sliding third brush. It is a simple DC generator and only requires the reverse-current cut-out to connect/ disconnect from the battery and other loads.

As the battery charges, and generator output remains constant, this characteristic overcharges the battery resulting in gassing, heat, and evaporation of electrolyte, and is very undesirable to the battery. If the generator current setting is low, the battery is under charged, also undesirable for restarting/ operating the vehicle.

The third brush generator thus either overcharges or undercharges the battery with the variable speed and fluctuating loads (lights, etc.) of the auto.

A further undesirable attribute of the third brush system is the rapid rise in output voltage (trying to keep the output current constant) if a high resistance or open circuit occurs in the charging circuit. The resulting high voltage usually burns out lights and damages the generator windings.

With the exception of Ford, most of the automobile manufacturers adopted some form of automatic output voltage regulation to correct these deficiencies. They all controlled the current in the field winding to maintain a preset and constant output voltage which is most desirable for battery and lighting considerations ⁵.

For a completely or partially discharged battery, the initial charge current (at constant voltage charging) is high and falls off to lower values as the battery accepts charge. The battery terminal voltage rises to approach a limiting value. The time rate at which the current decreases is related to the battery state of charge, temperature, past history, etc. Typical unregulated, standard Model "A" battery charge performance is shown in Fig.1.

The peak current shifts with RPM and the third brush setting. A regulated generator should hold constant output voltage for all speeds after some minimum start charge RPM. Thus, never under or over charging the battery as lights or other loads change.

A commercially available voltage regulator modification for the standard Model "A" generator is shown in Fig.2.

This regulator, mounted on its band, replaces the band normally installed on the aft end of the generator. It is made for positive ground use and designed to supply a constant output voltage to the electrical system. The usual Reverse Current Cut-Out or solid state replacement can be used with the regulator.

The Sep/Dec 1993 and Jan/Feb 1994 Model A Trader contained a two part article titled "Generators & Regulators - A Mini-Course", written by Ron Lekse. The articles titled "Generator Theory and Operation" and "Cut-Outs and Regulators," discussed the two devices and can be read for additional understanding⁶.

Test results of the standard "A" reverse-current cut-out, unregulated generator, and the "regulated" modification of Fig.2 will be the subject of a following article. The storage battery charge characteristics will also be presented.

¹ G.W. Vinal Sc.D., Storage Batteries, 4th Ed.
² Paul Moller, "Model 'A' Ford Generator Cut-Out," Skinned Knuckles, April 1981.
³ Don Ball, "A Solid State Generator Cut-Out,"Skinned Knuckles, April 1981.
⁴ Langsdorf, Principles of Direct-Current Machines, 1931.
⁵ B.M. Leece, "Generation of Current for Automotive Apparatus," Journal, Society of Automotive Engineers, June 1928, pg 658
⁶ Copies are available from MODEL A TRADER, 1247 Argonne Rd., South Euclid, OH 44121 for $2.00 ppd.

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