The Model T Ford Battery and Charging System.

                                                            Part 1 - Know your battery.

The first thing the Model T owner wants to know when presented with a flat battery is whether it is the battery or the charging system at fault.
In order to narrow down the cause of the problem, it helps to first have an understanding of the battery and what to expect from it.

The lead acid battery.
To use the correct terminology, a “battery” is formed by more than one “cell”  being connected together. A lead acid cell produces 2 volts, regardless of the number, or size, of plates it has. Thus, to obtain 6 volts, three cells are required, or six for a 12V battery.
In each cell, two sets of lead plates are immersed in a mixture of sulphuric acid and water, otherwise known as “electrolyte”. To charge the cell, external current is connected to the plates.
Without going into detail of the chemical reaction that occurs, it is sufficient to say that when the battery is charged, the acid is stronger than when the battery is discharged.
As the battery is discharged, the plates become coated with sulphate which comes from the acid. This reduces back to lead when the battery is charged again.

Internal parts of a typical 2 volt lead acid cell.

Because the acid strength is dependent on the chemical condition of the plates, this can be used to determine the level of charge. Typically, the specific gravity of the acid is about 1.25 when charged, and 1.11 when discharged. This means the acid has 1.25 and 1.11 times the density of water respectively.
To measure this, a hydrometer is used; the level at which the internal float sits provides the specific gravity reading. Hydrometers are becoming obsolete, so far as the average motorist is concerned, partly because they are messy to use, and partly because of the increase in “maintenance free” or otherwise, sealed batteries. With the high accuracy of low cost volt meters now available, it is more convenient to measure the battery voltage to ascertain charge level.
The voltage of a lead acid cell is 2.2 -2.3V upon completion of charge. This rapidly falls to around 2.1V and remains there until the cell has current drawn from it. At 1.8V, the cell is at a very low charge. These figures are multiplied by three or six, for 6V and 12V batteries respectively.

 The Hydrometer is an inexpensive instrument for testing charge level.

Battery capacity.
Every battery has an amp hour capacity, which is often specified at a 20 hour rate. A typical Model T size 6 volt battery has about 80 amp hours.
This means that one could draw 4 Amps for 20 hours before the battery is discharged. The capacity increases for a lesser current drain, and decreases for greater. This is why it is not possible to run the starter motor for more than a few minutes, even though the calculation would suggest 40 minutes cranking time with 150A starter current. Knowing the battery capacity is useful to determine how long it can be used for, before being discharged. Pertinent to this are the Model T’s which have electric lights, or ignition, run from a battery, but no generator. A common question is, “how long can the lights be used for, before the battery goes flat”? First thing is to know the current draw with lights and ignition on. Assuming this is found to be 10A, and the car has a 100Ah battery, then 10 hours would be the theoretical maximum. Because of the 20 hour rate, the time will actually be less, and if the battery is old or in poor condition it will be less again. Nevertheless, it gives a rough idea of what to expect. Typical experience suggests about half the running time to that calculated.
What about “Cold Cranking Amps”? This is an indication of the ability to maintain voltage under the high load of a starter motor. It is expressed in hundreds of amps, and is how much current the battery can actually provide instantaneously, rather than how much it can store.

Types of battery.
Lead acid batteries also come in forms other than the familiar type with removable filler caps, which has been around for over a century.
So called “Maintenance Free” batteries were once popular. These are really just ordinary wet cell batteries without filler caps.  More than the normal amount of electrolyte is added to the cells, hoping that by the time it has evaporated, the owner thinks an adequate life has been had from it. Some enterprising motorists did in fact discover filler caps underneath moulding in the battery top with some types, or simply made holes in the tops of others, to inject water.
In recent years, the Sealed Lead Acid type has become popular. These go under such brand names as “Optima” or “Orbital”. The generic name of “Gel Cell” is often used for the smaller non automotive types. The operating principles are the same as the wet cells, but the electrolyte is in paste form. This means the battery can be used in any position without spillage. Small SLA batteries are popular with owners of non electric T’s for ignition coils and low powered lighting, because they can be stored unobtrusively under the seat without acid fumes and spills.
However, the cells are not intentionally vented to the atmosphere. This means hydrogen production must be kept at a minimum. Because of this, charging requirements are far more stringent, and normal car battery chargers are not suitable for the smaller batteries due to the likely risk of overcharge.

Allowing a lead acid battery to get below about 1.8V per cell will cause irreparable damage. When this happens, excess sulphate builds up on the lead plates, insulating them from the electrolyte. A classic scenario is the flat battery because the lights were left on all day. Just one bout of this causes irreversible plate sulphation. Even though a charge the next day may make it appear that all is well again, the capacity is likely to be reduced. The battery will now no longer have its full life expectancy and soon the owner may find the need to occasionally use a mains powered battery charger, when previously the car’s own charging system was sufficient.
Self discharge causes just as much damage. All batteries discharge by themselves, even if not connected to anything. Many a Model T has suffered the death of a battery from self discharge through lack of use. Unfortunately, merely charging the battery the night before a drive, after the car has sat for a few months, will not reverse the deterioration. Fortunately, it is possible to keep the battery fully charged at all times, and this will be explained further on.
Note also, there are instances of a faulty electrical system being responsible for a high rate of discharge, leading one to think the battery is faulty. Particularly prone to this are cars with modified charging systems, which with incorrect wiring may well charge the battery, but still draw current when everything is turned off. A very simple test for the possibility of this fault is to connect an automotive test lamp between one of the battery terminals and associated battery post. With everything switched off, any leakage will be indicated by the test lamp. Those proficient with multimeters can do the test with more accuracy, but starting on the 10 amp DC range and reducing the range as necessary.
Unfortunately, the Model T dashboard ammeter is not sensitive enough to show low levels of leakage.

                                                                                    Part 2 - Battery Charging and Maintenance.

Correct charging.
It may be a surprise to know that a good quality lead acid battery can last 10 or more years if it has always been correctly charged and maintained from new, and never allowed to discharge.
Lead acid batteries are the simplest of all types to charge.
All that has to be done is to apply current to the battery until each cell has come up to 2.3V (i.e. 6.9V for a 6V battery), or the specific gravity is 1.25. Failing to stop charging at this point with normal charge currents will cause excess gassing and loss of electrolyte. This is most important to avoid for sealed batteries. Unfortunately, the standard Model T charging system is very much of a compromise and does not terminate charging at all. This will be examined later to see how the undesirable effects can be minimised.

Charge Current.
What current is the best to charge at? Over the last century of battery technology, it has been found that a good rule of thumb is about a tenth of the battery capacity. So, it’s no coincidence that the typical low cost car battery charger is rated at 4-6A, for the common 50Ah 12V type of battery. Those with 6V batteries can usually charge at around 8-10A. If you’ve followed everything so far, you can probably guess that the time taken to charge a totally flat battery at this rate will be 10 hours. In practice, because the battery is not 100% efficient, up to14 hours are recommended.
A battery can be charged much more rapidly with higher current, but this should only be done in an emergency, as the high current can buckle the plates, overheat the electrolyte, and cause excess gassing. The charging must be terminated immediately the charge is sufficient. Even so, this kind of treatment is detrimental to its life.
Of course, a battery can be charged at a lower rate but will simply take longer.
It is important to note that the gas vented from a charging battery is hydrogen, and that explosions have occurred. Usually, this is when battery chargers or other items are connected or disconnected from a furiously bubbling battery. This is why the instructions for jumper leads often state to make the negative connection to somewhere on the chassis away from the battery. Chargers should be turned off at the mains before connecting or disconnecting from the battery.

Types of Chargers.
In practice what does all this mean for the Model T owner who needs to charge the battery from the mains? The basic low cost battery charger is quite suitable here. This is the kind of charger that puts out about 4A and includes a simple ammeter. With it one also needs a hydrometer and/or digital volt meter to check the charge – this can be done every four hours or so. The key point is to not leave the charger on for more than necessary. A time switch is a good idea if the charger has to be left unattended.
Another type of charger has an automatic function whereby the charge current is reduced to a low level after the battery reaches full charge. Basically, the charger senses the battery voltage and when it reaches about 13.8V (full charge for a 12V battery), switches to a so called “trickle charge” which is usually less than 1A. This is better than the first type in that no harm will come to the battery if the charger is left on for a few days. An improved variation of this is the “Fully Automatic” type which can be left on indefinitely as the trickle current is low enough not to overcharge the battery.
Some chargers have a LED bar graph voltmeter to show the charge level. Beware that these types do not shut off when full charge is shown, unless the specifications say otherwise.
While the “Optima” kind of sealed car battery is rugged enough to be charged with conventional chargers, the smaller “Gel Cell” types are not. The high current will damage the internals and the electrolyte will dry out. Special chargers are available which should be used.

Battery Maintainers.
Becoming popular among vintage car owners, these solve the problem of battery self discharge. The maintainer is connected to the battery whenever the car is garaged and keeps it at 6.9 or 13.8V. Thus the battery does not discharge, and because the voltage is fixed, it cannot overcharge. The ideal state of equilibrium is reached, eliminating the need to constantly check water level or charge state. The battery is always ready for use at full charge. Unlike chargers, maintainers are a precision unit and provide only enough current to offset self discharge. To charge a flat battery off one would take several days, if not longer. The Model T owner should therefore keep a conventional charger, as well as a maintainer.

Charging a 6V battery from a 12V charger.
While 6V battery chargers are available, it is possible to charge a 6V battery from a 12V charger. All that is required is one or two headlamp bulbs. These serve to limit the charge current, and absorb the extra 6V. Start by connecting the 12V battery charger to the 6V battery with one bulb in series with one of the connections. The bulb will light to show charging is taking place. If more charging current is required, connect an extra bulb across the first. It does not matter if a 6 or 12V bulb is used because the difference in voltage between the charger and battery will be around 6V.

 It is quite practical to use a 12V charger with a 6V battery as shown above.

The basic kind of charger is preferable here, as types that monitor the voltage will be confused and may shut off too early. It is not possible to use a 12V maintainer with a 6V battery this way.

Pills and Potions.
Since cars were first fitted with lead acid batteries, all sorts of additives have been made available to the gullible motorist, in order to rescue an unusable battery. Generally, these are intended to break down the excess sulphate. In recent years electronic devices have become available which pulse a high voltage through the battery to do the same thing. Results appear to be quite variable, and it can take quite some time to achieve the desired effect. Whether or not these methods are successful, it is highly doubtful that full performance can ever be restored.

Testing a battery.
Usually, a battery only gets tested when something is amiss – the battery has been charged but the lights are still dim, etc. The first thing to do is check the electrolyte. At least 50% of problems are because of electrolyte being below the tops of the plates. Distilled water should be used to top up.
The next thing to do with an apparently faulty battery is charge it off a mains powered charger, so as to eliminate the possibility of a faulty electrical system in the car. If the ammeter on the charger is showing very low current, this is indicative of one or more faulty cells. One can wait a few hours to see if charge current increases, but it is likely the battery is ruined. Sometimes, it is noticed that by touching the side of the battery, that one cell is warmer than the others – this is also a sign of failure.
At the end of the charge period, each cell should be checked with the hydrometer. A cell with low specific gravity while all the others are high also indicates problems.
The simplest test is then to run the battery into a load after a full charge. For this, the car’s headlights are ideal. Measure the voltage at the actual battery posts, with the lights on. Do not measure the voltage anywhere else, as wiring resistance will give a false reading. If it drops below 6 or 12V the battery is faulty.

Taken off the maintainer before this reading was taken, 6.7V shows the battery to be in good condition.

With a faulty battery, the voltage often drops in multiples of two; each loss of two volts indicating a defective cell.

                                                                  Part 3 – The Ford Generator.

With commercial pressure forcing Ford to eventually adopt the electric starter, a lead acid battery had to be included, as the current drawn by a starter motor is too high for any kind of dry battery. A means of recharging the battery was required, so a generator driven by the car engine was the obvious solution.

Generator Principles.

 Essential parts of a charging system using a 3rd brush generator.

As is well known, a generator works on the principle of an electrical conductor (the armature windings) rotating in a magnetic field (developed by the field coils fixed to the generator body). The strength of this magnetic field, in turn, controls the generator output current. Thus, by controlling the current through the field coils, the charge rate can be set to a particular current.
This is the purpose of the third brush. Without going into the complexities of what really happens, it is sufficient to know that by sliding the 3rd brush towards the insulated brush, field coil current increases, and therefore the battery charge rate. A simple way of explaining this is to think of the voltage gradient across the armature, starting at the grounded brush (0 volts), and going towards the insulated brush (6 volts). By connecting to the armature at an intermediate point via the 3rd brush, it can be seen how the current fed into the field coil can be varied.

No Voltage Regulation.
The generator fitted to Fords between 1919 and 1938 is of the 3rd brush type.
This kind was chosen because it fits in with the “simple but practical” theme. It does not require an external current regulator, as do two-brush type generators. Over a wide range of driving speed, the charge remains reasonably constant. However, there is no voltage regulation. What this means is that once the battery has reached full charge, (about 7 volts), it continues to be charged at the full rate, which shortens battery life.
The concept of voltage regulation was well known at the time, and in fact many other cars of the T era did use it. A voltage regulator works by reducing field coil current when the battery reaches full charge. However, Ford, and others more interested in economy, decided they could ignore it.
The idea was simply to set the charge current to the lowest point before a flat battery is likely. The problem now is that when high currents are drawn (e.g. headlights), the battery won’t fully recharge. Alternatively, if the charge current is set high enough to allow the headlights to be used, without discharging the battery, the battery will be overcharged when the headlights are not used.
A compromise has to be reached in this situation, but 10 to 12 amps as mentioned in the Ford Service Manual, is too high for Australian driving conditions in the modern day.

Charge Current.
What should one set the charge current to? Firstly, the generator maximum power is 100 watts. This translates to 16A for 6V, or 8A for 12V, before excessive wear or burn out results.
Ford’s idea of setting charge current is based on being able to run the headlights continuously without having a flat battery. However, it is enough to damage a fully charged battery on a long drive when the headlights are not used – which typical of Model T driving today. As the standard electrical system draws little current, the charge rate does not need to be very high at all. What of the huge current drawn by the electric starter? While it may be around 150 amps, remember it is for a very short period. To explain this, consider one minute of electric cranking. Drawing 150A for one minute is the same as drawing 2.5A (equivalent to one brake lamp) for one hour. Put in perspective like this, it is clear the overall energy consumption is actually very low.
It is for this reason one hears of non generator cars being able to tour for a week on one battery charge.
Over a long period of time, I have found 5A to be the ideal charge current for a 6 volt system. This accommodates occasional use of the headlights and starter, as well as the ignition coils full time. 5A is not so high as to cause excess electrolyte loss. Typically, the battery might need topping up once or twice a year. My first Model T battery provided ten years of service, and never went flat.
For those with 12V systems, the charge rate should be roughly halved; about 3A.

Running on no load.
As the armature spins and produces current, the field coils become more energised, causing the armature to output more current, which energises the field coils even more. It’s obvious that a runaway condition exists, and the generator will soon burn out, unless the output voltage is limited. As there is no voltage regulator, the only thing to do this is the loading by the battery. If for any reason, the battery becomes disconnected whilst the engine is running at speed, there is a real risk of a burnt out generator.
Not only that, any accessories in use will be damaged from excessive voltage. Such a scenario does happen, and is often the result of a faulty cut-out, battery isolation switch, loose terminals, or even a faulty battery. This is the likely fault if light bulbs start mysteriously burning out at a rapid rate.
There is also the situation where some T owners wish to run an unconnected generator because there is no battery or charging system in the car.

The generator terminal must be earthed when running into no load. The cut out need not be present.

This can be done quite safely simply by earthing the generator terminal. Some may wonder, “Won’t shorting out the generator overload it?” The answer is no, because the armature is prevented from creating any voltage, which then prevents the field coils producing a magnetic field.

Typical Generator Faults.
Apart from the usual faults that occur with any brush type motor or generator, such as worn bearings, worn brushes or scored commutators, there are some common faults pertinent to the Model T generator:

• Unless the generator has been reconditioned with a new brush plate installed, any attempt to adjust the 3rd brush is likely to destroy an original brush plate. The original material used for this tends to crumble where the brush is tightened. The result is a short circuit to ground, and is one of the most common causes of no generator output. There is an excellent fibreglass reproduction which overcomes this problem.
• Despite years of being soaked in oil, the windings appear to be quite reliable, except where charge current has been excessive. Crumbling insulation and charred windings means a trip to a rewinder, and if one has ideas of using 12V, the generator should be rewound to suit.
• When removing the back of the generator, note exactly where the field coil wires connect to the brush plate, before any further disassembly. Note the crossover of wires. If these should be connected in reverse, the generator can be set up in the normal way, but causes a rather high discharge current when an attempt is made to set the third brush in the car with the engine running. The reason is that this reverses the direction of rotation.

Flashing the Generator.
If a generator is in good condition but fails to produce an output, it may need to be “flashed”. Going back to the principles of operation, it may be wondered, how can the armature produce output to energise the field coils which are needed to make the armature produce output in the first place?
The answer is that there is a small amount of residual magnetism, in the iron core of the field winding. It is not much, but enough to start the process.
Where does this residual magnetism come from? When the generator is manufactured, it is “flashed”.

If a generator has lost its residual magnetism, flashing will restore it.

For the Model T, the simplest way to do this is simply bridge the cut out terminals for a split second. This feeds battery current into the generator windings and remagnetises the core. Flashing is also required if a generator from a positive earth system is to be changed to negative earth.
The residual magnetism may be lost if the generator has not been used for many years. If flashing is regularly required, it would indicate commutator or winding problems – the losses being too high to allow the residual magnetism to start the process.

Setting up the Generator.
After all the details given so far, actual setting of the generator is very easy.
If the generator has been dismantled or worked on, it is necessary to first set the brush plate. First the generator is removed from the car. The third brush is then lifted from its holder so as not to make contact. Then, the four brush plate screws are loosened just enough so the brush plate can be rotated. Now, with a set of jumper leads, connect a 6 or 12V battery. The generator will now run as a motor (incidentally, a quick go/no go test). Move the brush plate in one direction, then the other. At a certain point, the motoring stops and then reverses direction. It’s at this point where it doesn’t move, that the brush plate needs to be tightened. Now return the 3rd brush to its original position, and again the generator will run as a motor.
Reinstall in the car, start the engine, and run at a fast idle. The third brush can now be adjusted by loosening it and sliding it across the brush plate until the desired current is obtained. Retighten without crushing the brush plate, and adjustment is done. No further adjustment of the brush plate is required, even if the current is readjusted, unless internal work is done to the generator.

                                                              Part 4 – The Model T Cut Out

Often a source of confusion, cut outs and regulators are two different things. As described previously, the standard Model T incorporates a cut out only.
What is its purpose and why is it necessary?

The Cut Out Function.
If the generator were to be connected directly to the battery, it would certainly provide a charge in the normal way, so long as the engine was rotating, at or above a fast idle. However, when the engine slows down or stops, the battery current would now flow back into the generator, and try to run it as a motor. This is obviously undesirable, because not only will the battery discharge, but the generator will sit in a stalled condition, and its windings will soon overheat.
The immediate solution to this is to have a switch to connect the generator to the battery when it runs above a certain speed, and disconnects it when it slows down.  A simple switch operated by the driver, could indeed perform this function. However, the possibility of not remembering to use the switch at the right time makes this idea impractical. The switch needs to be automatic.

How it works.

The cut out is rather similar to a horn relay in internal appearance and operation. One difference is that there are two windings on the core instead of one. When current is passed through windings around the core, a magnetic field is developed which attracts the contacts together, completing the battery to generator circuit.
Expanding on this, we will just examine the voltage coil first. Note that it connects directly across the generator output. Now imagine what happens as the generator is brought up to speed. The magnetic field starts to build up in the core, and when strong enough, the contacts close.
At this point it can be seen that current can now flow to the battery, via the contacts and the current coil.
The current coil is only a few turns of heavy gauge wire, so it has no effect on the charge current. It does, however, develop a magnetic field of its own. This adds to that already developed by the voltage coil, and the contacts remain closed. The battery thus starts charging.
So far, we have accomplished the task of automatically “turning the switch on”.
Operation becomes slightly more complicated when the generator slows down and we want the battery to disconnect again. What happens now?
Keep in mind that the contacts are still closed, meaning the battery is connected to the generator. But this also means the battery is connected to the voltage coil, which will keep the contacts closed, and now the battery will discharge through the generator and voltage coil.
Indeed it would, except this is where the current coil comes into play. Consider that when the generator stops, the current now flows from the battery into the generator. The current has reversed direction, therefore reversing the magnetic field around the core. Being in the opposite direction, it cancels out the voltage coil field, and now the contacts open. The battery is now isolated from the generator and cannot discharge.

Cut out Adjustment.
The only adjustment is the voltage at which the contacts close. If it is too low, the battery will discharge through the generator when its output is too low to provide a charge.
If it is too high, the battery will not start charging except when engine speed is higher than normal.
The cut out is thus set to close when the generator voltage has reached the level just above that of a fully charged battery; i.e. about 8V.
The adjustment is most easily done using a variable power supply connected across the voltage coil, and setting the contact tension so the contacts close just as the supply reaches 8V. Failing the availability of a variable power supply, the generator itself can be used.  An analogue voltmeter connected across the generator output can be used to measure closing voltage.

Cut out Faults.
The construction methods used for the original cut out means that those using them really need to do a rebuild. Remember, a cut out that does not close can result in a burnt out generator.
The problem is the internal electrical connections rely on rivets, which loosen over time. Additionally, the insulating material used crumbles away, making a short circuit possible.
 A piece of blank fibreglass printed circuit board is ideal for replacing the insulators, but any similar high temperature insulating material can be used.
All riveted connections need to have short lengths of wire soldered across them.

Inside a reproduction Ford cut out. Failure of any of the internal parts, or connections thereto, can result in a damaged generator.

Sticking contacts are likely to show signs of pitting, so by means of a file these can be repaired. Sticking points will show up as a discharge reading on the ammeter with everything in the car switched off.

Diode Cut out.
In view of potential problems with mechanical cut outs, some owners replace the internals with an electronic diode. A diode will easily fit in space formerly occupied by the relay, but it will need proper cooling. With a charge current of 5A, heat dissipation will be around 5 watts with a silicon diode. A Schottky diode has a lower voltage drop and will thus dissipate less heat.
The type of diode is not critical, except it should have a high current rating. For reliability, something in the region of 25A or more should be used. Diodes extracted from modern car alternators are ideal. A disadvantage of solid state diodes is that if they fail, they usually do so in the form of a short circuit. If this happens, the symptoms are the same as stuck points with a mechanical cut out.

Ground switch.
An idea made popular by an old “Tinkering Tips” article, was to install a switch to earth the generator terminal, when charging wasn’t required. A supposed beneficial side effect was that, with the generator not providing any power, a certain amount of horsepower would be saved. However, we must realise that with many such articles, “tinkering” was an apt description, and were without any scientific or methodical basis.  At a speed of 60km/h and a charge current of 6A, the power consumed by the generator is only 0.18hp. To put things in perspective, one horsepower is 746W, which equates to over 100A charge current! For those that still want to try the idea, note that it should only be used with diode type cut outs. This is because when in operation, a mechanical cut out provides a direct connection to the battery. Grounding the generator when the cut out contacts are closed will result in a destructively high current flow before they open.  As long as the grounding position was only selected with the engine at idle speed, or switched off, one could get away with the scheme with mechanical cut outs.

Voltage regulators.
As explained previously, the cut out has no regulation feature. That is, it does not terminate the charging when the battery is full. While the technology to deal with this was available at the time, Ford chose not to use it until the late 1930’s. Some T owners therefore, have adapted regulators from other cars to use with the Model T generator. This is quite in order, but it does require that the field winding be brought out to a separate terminal on the generator. However, given the appearance of the Fun Projects regulator in recent years, such a modification is scarcely worthwhile.

Fun Projects Voltage Regulator.
This modern accessory is highly recommended for those who wish to overcome the limitations of the stock charging system. It means one can charge at 10A (at 6V) without overcharging the battery. Essentially, it contains a diode cut out, and also an electronic grounding switch, which cuts off the generator output when the battery is charged. Generator wear is less, because it’s operating only for a short period, rather than continuously. It eliminates problems with mechanical cut outs, and most importantly, eliminates battery overcharge. In effect, a modern charging system is gained with no modifications to appearance or wiring. The unit is simply a drop in replacement for the original cut out. It is available in 6 or 12V from