Build Your Own Magneto Charger

By Staff
article image
by Peter Rooke
Recharging a Webster magneto on the homemade magnet charger. The blocks ensure a good connection with the magnets.

While a friend of mine had a magnet charger, I always felt a little uneasy about putting him to the trouble of ensuring that the battery that powered it was fully charged, and the fact he had to clear his work bench to make room to use it. Part of the satisfaction I gain from restoring engines arises from the turning of various bits of metal into a working piece of machinery, so I felt that to make a charger would be a new challenge, and that I would also learn something in the process. I knew little about electricity, apart from being able to change a plug, and knew even less about magnetism. Fortunately, I still met my old school physics teacher for a glass of beer most weeks, as we were both members of a local rifle club, more social members now with failing eyesight!

However, my hopes of an immediate flow of information just elicited the initial comment “yes, magnetism – a tricky subject,” and he started to talk about something else!

Next, I searched SmokStak and a few hours were well spent gathering numerous comments as well as printouts of articles detailing plans to build magnetizers, including a comprehensive one by John Rex printed in GEM in 1989, and a copy of an article in Dyke’s Automobile and Gasoline Engine Encyclopedia in 1918. There was also reference to a Dave Gingery design.

This was fine, but I was no further along as I now had three schemes, each one using different size cores, all of which were, according to numerous people, successful. The only common point that I was able to identify was that 20,000 ampere-turns of copper wire appeared to be the magic number to achieve the full charging of a magneto. Ampere-turns refer to the number of turns of wire around the core multiplied by the amperes that the length of wire draws.

 A commercial magnet charger.

The first decision was to identify the optimum core size before calculating the gauge and amount of wire to wind around this, along with the power source to achieve the desired result.

I was still in need of an “expert” so I contacted Martin Percy, the Help Desk contact for magnetos in Stationary Engine magazine here in the U.K. I hit gold dust, as Martin was only too willing to help. He knew the subject of magnetos inside and out, and had built his own charger. Furthermore, he had some information he would photocopy for me about a charger built by Warwick Bryce and featured in Stationary Engine in 1988. It soon became apparent that I needed to understand a little more about the difficult subject of magnetism before designing my own charger in order to be satisfied that it would meet my future needs. So again, I approached my old teacher, who subsequently proved most helpful.

While I could have simply described the charger I built and how I achieved it, this would not be much help to anyone else wanting to build a charger to a different specification, and also understand a small element of the theory.


My initial knowledge of magnetism was based on my school days when, as kids, we played with them. This knowledge was limited to the fact that a magnet had a north and south pole, other ferrous metals could be magnetized with the magnet, and that like poles on the magnet repelled while opposites attracted each other.

Magnet steel

 An early horseshoe magneto from an Amanco, with the retaining band removed.

Up to the 1920s, horseshoe magnets were made from the toughest steel then available, tungsten steel.

Since then, other strongly magnetic steels have been made, containing chromium and nickel up to the 1930s when compressed metal powders such as alnico and Alcomax were used. Alnico consists of aluminum, nickel, and cobalt, hence the name. These special high-energy magnetic materials are much more expensive than tungsten steel but they hold more than 20 times the magnetic energy.

The tungsten magnets had to be long and thin to prevent self de-magnetism, hence the large horseshoe magnets seen on the early magnetos, more compact magnets and magnetos being made when the new materials were available.

The downside to the later, non-tungsten, magnets is that they require a greater magnetic force to charge, but once they are fully charged, they create a stronger magnet.

Metals that can be strongly magnetized such as iron, nickel and cobalt are known as ferromagnetic, exhibiting a large attraction to magnetic fields and have a high ability to retain magnetic properties once the external field has been removed. They all get their strong magnetic properties through the presence of what is known as magnetic domains.

Magnetic domains

Within ferromagnetic materials, the atoms tend to have their own magnetic field, created by electrons that circle it. Small groups of atoms, consisting of more than a trillion, tend to align themselves in the same direction and these groups are known as domains.

Each domain has its own north and south pole, and in an un-magnetized state, the poles of the domains point in different, random directions, with the result being that little or no magnetic force is displayed as they cancel each other out.

Placing magnetic material in a strong external magnetic field or passing an electric current through it results in the domains starting to align themselves in the same direction, so the material starts to exhibit a stronger magnetic capability. The greater the magnetic force applied, the more domains will be aligned and the stronger the magnetic force. Once all the domains are aligned the material is said to have reached saturation and its magnetic properties will be at their maximum. They cannot be improved with the application of any more magnetic force.

With soft irons the domains align easily, whereas more magnetic power is required with hard materials. However, with soft irons the domains will be scrambled again when the external magnetic force is removed, whereas in harder metals a greater proportion is retained, making it a stronger magnet. This is why soft iron is used for the core of coils, allowing the rapid buildup and release of magnetism.

Magnetic force

The best way to describe a magnetic field is to imagine invisible lines of force parallel to each other entering the magnet at its south pole, travelling through it before exiting at the north pole, in a closed loop.

The magnetizing force of an electromagnet, which is in effect a magnet charger, is proportional to the number of turns of wire in its coil to the current flowing through this wire for 1 meter of coil; e.g. for a coil with a 1-meter-long core of 250 turns of wire drawing 50 amps, the magnetic force is 250 x 50 = 12,500 ampere-turns.

Magnetized material showing the domains aligned and the force lines.


The overall strength is known as the flux density of the magnetic forces and relates to the number of these flux lines in a magnet within a given area. The greater the density of the magnetic force the stronger the magnet, and this is measured by different units including the tesla, one tesla being about 30,000 times as powerful as the Earth’s magnetic field.

Unmagnetized material with domains pointing in different directions.

If the magnetizer will be used to charge modern rotating magnetos, which are best charged assembled, an allowance should be made for air gaps. An air gap can also mean a non-ferrous material like aluminum or other material used in the construction of the magnetos after the early horseshoe types.

For two air gaps as small as 1 mm each, the strength of the electromagnet (flux density) might fall by a factor of five or more, so the flux density will need to be five times higher to compensate. While the thought of 2mm in air gaps is impossible when re-charging a horseshoe magnet, if you look closely at the magnet, the surface is generally rough and uneven, and it is impossible to get a 100% metal-to-metal contact.

Having said all this, I must also repeat my comments earlier under magnetic domains that when charging a magnet there is a point when no matter what force is exerted on it, its strength will not increase once all the domains become saturated. Saturation occurs at around 1.6 tesla for a tungsten steel magnet and 2.0 tesla for composite materials.

Designing the charger

Webster AMM magneto with two tungsten magnets. The roughness in the surface of the magnets can be see

There are three aspects to the design of the magnet charger: the size of the core and frame, the length and thickness of the copper wire to be used as the windings, and the voltage to be used.

Before starting the design, a clear understanding is needed of the type of magneto to be charged. If purely small magnetos are to be charged with a modest cross-section to the magnets, then a small core of 1-1/2 inches will be sufficient.

Lucas, a major high-tension magneto manufacturer in the U.K., recommended the following design criteria in its workshop instructions in 1953, stating that this would be sufficient to saturate almost all commonly used magnetos:

• Core material — soft (preferably Swedish) iron.

• Core area – 9 square inches.

• Recommended core winding — 65,000 to 70,000 ampere-turns.

Another design consideration is how the magneto will fit between the coils, either by using various pole pieces on the top of the cores or by moving the coils themselves.

Wiring parallel and in series.

Core and frame

In deciding on the surface area of the coils, the magnets on the low-tension magnetos available were measured, as the prime intention was to continue in the restoration of low-tension ignition engines. However, the construction of the charger was to be a major project, not to be repeated, so to give added flexibility it had to be made to a specification that made it capable of charging a wide range of magnetos. Where there were two magnets joined and working together on a magneto, the combined measurement was taken.

The two plans considered for a magnet charger. 

To get the best results in recharging a magnet the source needs to have between two and two and a half times the surface area of the magnet so the magnetizing force from the charger is concentrated in the magnets. As can be seen from the table, the largest cross section was 2 square inches, giving rise to an optimum core area of 8 to 10 square inches. Therefore, 3.00-inch cores were selected for the coils, and to maintain this surface area of 9.4 inches the bottom of the frame would be made from 4-inch-by-2-1/2-inch iron.

The two plans considered for a magnet charger. 

The core of a charger should be able to reach magnetic saturation as easily as possible, so the best material to achieve this is Swedish magnet iron, which is extremely pure. This is not readily available, so the next best option is to find a steel that has a very low carbon content, such as C1010 which has a carbon content between 0.08 and 0.13%, in effect having the properties of iron. The more carbon there is in the steel, the more magnetic force is needed to reach saturation, and some of this magnetism will be retained when the magnetizing force is removed.

To get the best results the core needs to be as short as possible, but the design of an excessively short and fat core makes it difficult to fit a magneto between them so there has to be a compromise. The arms of the cores on this charger would be made from two pieces of iron 6 inches long, which, after allowing for the insulation rings at each end and the platform at the top, left around 5 inches of the core for the actual winding of the copper wire.

Power source

Flycutting the top of the base plate. 

Calculations for this charger were based on using a 12-volt battery, which is an easy source of power without the need to use a transformer to change mains voltage and a rectifier to smooth it out.

If a higher voltage is used, the amperage increases proportionally, thus increasing the amp-turns, so if necessary two batteries could be joined in parallel to obtain 24 volts. Another point to bear in mind when using batteries is that the voltage might drop to 10 volts when the battery is delivering peak current.

Copper wire

The bottom of the core after placing on the surface plate.  

Copper wire is sold by weight, on different sized spools. The resistance of the wire is measured in ohms, putting a value on how easily an electrical charge will travel down the wire, which for winding wire is generally expressed in ohms per 1,000 feet. With a thicker and low resistance wire, more charge will go down it, so the diameter of the wire chosen needs to be sufficient to allow the amperage necessary to obtain the desired level of magnetism.

As the resistance varies according to the length of wire used, the resistance of a coil in relation to the number of ampere-turns also depends on the diameter of the core. For a 1-inch-diameter coil the length of wire for one turn of the first layer is 3.1 inches; for a 2-inch core, 6.3 inches; and for a 3-inch core, 9.4 inches. Therefore, in calculating the ampere-turns of these different sized coils at 12 volts DC, a pattern emerges that the optimum wire thickness is less for the smaller diameter coil. There is also a point where the efficiency of the ampere-turns reduces rather than increases.

Skimming the top face before turning the lip to hold the upper insulation ring. 


The ampere-turns table at the bottom of Page 22 sets out a summary of the calculations for one of the two cores of a charger, the wire being wound over a 5-inch length of the core. This shows that doubling the number of turns does not necessarily mean a doubling of the ampere-turns, because of the resistance of the wire. In the case of the 3-inch core and the particular resistance of the 10-gauge wire used, it was found that the length of wire used increases proportionally to the turns, so there is no change in the ampere-turns.

It so happened that 4-kilogram (8.8-pound) spools of 10-gauge wire were available, each of which held 300 feet. This was convenient, as one spool of wire could be used per coil and there would not have to be any measuring or counting of the number of turns.

Before going any further, an important decision is how to wire up the two coils and the battery, whether in series or parallel.

Wiring in series doubles the resistance of the coils, thereby reducing the flow of current or amps. In parallel, the total resistance is half the value of one coil.

The resistance of the wire used in one coil is 0.318 ohm (300 feet of wire with a resistance of 1.06 ohm per 1,000 feet). This means that if the two coils were wired in series, the total resistance would be double, 0.636 ohm giving a current draw of only 18.9 amps (by the formula amps = voltage/resistance, which is 12 volts/0.636 ohm), giving 11,340 amp-turns.

Wiring the same two coils in parallel, the combined resistance is 0.159 ohm. The current is therefore 12 volts/0.159 ohm which equals 75.5 amps, giving 45,280 ampere-turns. This assumes a 12-volt voltage from the battery. If it reduces to 9, the ampere-turns reduce to 33,962.


When a current is passed through a copper wire, the wire starts to generate heat, the amount being related to the current in amperes and the resistance of the length of the wire. The shorter the length of wire the lower the resistance and the greater the heat generated. The resistance of the copper wire eventually selected was 1.06 ohms per 1,000 feet, therefore being 0.318 ohm for the 300 feet to be used in each coil.

• Power (watts) = amperes² x resistance of the wire.

• Power = 382 x 0.318 = 459 watts.

As can be seen, the coil will quickly heat up, and it is therefore imperative that as soon as peak magnetism is achieved it is switched off again, or else there is the risk that it will overheat. If thinner wire is used then the resistance reduces, and so will the heat, but this will also reduce the number of ampere-turns and the effectiveness of the charger.

Getting to work

Having decided on the basic dimensions of the cores and wire to be used, the next step was to draw up plans to get a clear idea of how the charger would look and order the materials.

My usual supplier of steel was able to source some “black steel,” which is similar to C1018 in that it has less than 0.2% carbon content. In reality this has properties similar to iron, although is not as good as pure magnet iron. Round black steel has a rough surface, so it needs to be ordered oversize so that it can be machined to a good finish.

I ordered a 12-inch length of 3.5-inch-diameter steel together with a 12-inch length of 4-by-2.5-inch steel that would be used for the base. In addition, an 18-inch length of 3.5-by-1.5-inch steel was ordered to cut up and machine to provide four pole pieces.

While 200 turns of 10 AWG wire per coil would in theory have generated near 45,000 ampere-turns, the current drawn would be high at over 100 amps. This could be reduced to 74 amps by increasing the number of turns to 300, still achieving the same number of amp-turns. In addition, a standard 4-kilogram (8.8-pound) coil from the supplier held just sufficient to wind 300 turns (if the starting diameter of the core is 3 inches). Furthermore, winding 3-inch diameter for 5 inches of the core results in the winding being nearly six layers, which meant it could start and finish near the bottom. The copper wire ordered was class H winding wire, with a dual polyester coating capable of withstanding high abrasion and temperatures.

Photo by Peter Rooke: The first layer wound on the core; a bottom insulating ring, showing the groove cut for the wire; the wound core.  

To help reduce arcing when the power was switched on and off, a 12-volt car starter solenoid was purchased, along with a simple press switch, some connecting wire (6 and 8 AWG) and some terminal screws. There will still be some arcing of these points, so rectifier diodes were needed to suppress it.

For a more professional device an ammeter can be added, which is very useful since it shows when peak amperes are flowing. This allows for the power to be switched off one second later, by which time the cores of the coils would be fully saturated.

Machining core and base

The 12-inch length of iron was cut in half using a power hacksaw before mounting one piece on the lathe. Ideally, one end should be supported by the fixed steady, but the core was too big for my steady. To get around this, the core was clamped tight in the chuck and then, at a slow speed, a hole was drilled to enable the tailstock center to be used when turning. This center hole would be drilled out later and threaded for the securing screws.

First, one end of the core was trued up as clean as possible using a combination of a fine cut and slow speed. Next, the body of the iron was turned smooth, holding just 0.500 inch of the core in the chuck, to remove the roughness before turning down to 3 inches.

Next, the core was reversed and again held in the 4-jaw chuck, then accurately centered using a dial gauge before the other end could be turned square and the roughness taken off the top 0.500 inch, which was left at just under 3.5 inches outside/diameter to support the plastic insulating ring (see photo).

Finally, the 0.274-inch holes were drilled in each end, ready to be tapped for the 0.313-inch set screws to be used to clamp the cores to the base and secure the pole pieces.

Having prepared the two cores, attention then turned to the base. Not having access to a surface grinder to obtain a smooth surface, the first step was to set the block on the milling table and then flycut it using a sharp, high-speed steel cutter at an extremely slow feed rate (see photo). Once satisfied with the quality of the surface, the 0.375-inch holes for the two securing bolts for the core were marked out and drilled, countersinking the underside.

The next step was to coat the bottom of the two cores with a thin coat of engineer’s blue before fitting them in place to check the fit (see photo). As discussed in the previous article, an air gap severely reduces the amount of magnetic force, so it’s ideal to have none. If you do not have a perfect fit then scrapers should be used.

The best way to do this is to thinly coat a surface plate in engineer’s blue (a piece of plate glass is a good substitute if you don’t have a surface plate).

Place the surface to be trued on the surface plate and if you have an uneven coverage of blue, scrape the high points and keep repeating the process until nearly 100% coverage is achieved.


Before starting to wind the cores, the insulation rings at each end and between the core and the wire needs to be completed.

First, the two insulating washers must be made from some form of insulating plastic. Take care to know the plastic and that it does not conduct electricity (as some do).

The top ring will rest against the shoulder left when machining the core, and can be held in place by epoxy cement. For the bottom ring, roughen the bottom 0.250 inch to help the epoxy cement bind completely.

To get a tidy winding, a groove can be cut in this ring the depth of the winding wire diameter so the beginning is not under pressure from the rest of the winding (see photo). If you decide to knurl this bit of the core for adhesive to bind to, ensure you do it before bluing and scraping as the knurling will raise an edge that might prevent an airtight fit.

Once the insulating rings are in place the core itself can be insulated. Something stronger than winding insulation tape is needed, and a good medium is a section of a cardboard file that can be glued to the core fitting between the insulating washers, with an overlap at its joint of at least 0.500 inch. To be safe, I used two thicknesses.

Other fittings

While machining the pole pieces, it was decided to make four so that there would be some flexibility to use different combinations when charging magnetos.

Their construction was a simple case of cutting the four blocks from the piece of 18-inch-long steel, marking out the steel to minimize the number of cuts and waste of material.

After sawing, the sides of the blocks were flycut to both square and clean up the faces. (The use of a surface grinder would certainly have been a quicker and easier option!)

The slots were then milled for the securing screws, cutting the slots on different sides of each pair to make them more versatile.

In a similar way to fitting the cores to the base, each block was scraped to ensure no air gaps and a 100% fit with each other and the cores.

The base of the charger would be bolted to a 1-inch-thick piece of timber to provide a fixing point for the box containing the wiring. Alternatively, some steel box section could be used to provide a larger base and some stabilization for the narrow iron base.

Winding wire

Before starting to wind the cores, it is important to remember that the windings need to follow the correct direction in order for the two cores to work together and not cancel each other out. The right hand grip rule applies: let your fingers point in the direction of the current and the thumb points to the north pole, so one core needs winding in a clockwise direction, the other counterclockwise.

As mentioned earlier, the winding wire was ordered on two 4-kilogram (8.8-pound) spools so that no measuring or counting was required when winding. If you are using 10-gauge wire, it needs a bit of tension in order to seat properly, and of course the easiest way to wind it is to use the lathe, after setting the screw pitch to the thickness of the wire, in this case 10 TPI for wire 0.100 inch in diameter.

The wire was tensioned by passing it through a nylon sleeve clamped in the lathe boring bar tool holder. For this operation it is better if there are two people; one controlling the lathe jogging switch, the other ensuring the wire is feeding correctly.

If you don’t have access to a lathe, then it is possible to make a crank handle-driven winding machine.

Tape a 12-inch tail to the insulating washer and wind the first two or three turns. Then use some rapid cure epoxy to hold it in position before starting to wind the first layer. Once the first layer has been completed it is best to give it a coat of rapid fix epoxy and leave it to dry, thus stopping the wire from moving when the next layer is wound. Reverse the lathe and then wind the remainder of the spool, leaving another 12-inch tail. Fortunately, the calculations were correct and the sixth layer finished near the bottom of the core. The final tail was marked with tape to identify it before wrapping some cloth trip around the coils and covering with epoxy resin for protection, which was later sanded down and covered with plastic electrical tape.

After the first core had been completed the second core was wound, taking care to ensure that the direction of the wire was reversed.

Back Electro-Motive Force

Back EMF is a voltage that occurs when power to a coil is shut off and the magnetic field of the coil collapses. The collapsing magnetic field produces a Back EMF that will try to keep the current from dropping to zero. Even using a starter solenoid will not stop the contacts from arcing when this occurs, and they could soon become damaged.

To prevent this, it is necessary to use a rectifier diode linked across each coil. This diode will have to be of sufficient rating in terms of amps and volts to take the surge, which could be 1,000 volts or more. The low resistance of the diode will short circuit the Back EMF and protect the switch.

To cope with the potential Back EMF for the design chosen (two coils of 75 amps each), 1,400-volt diodes were acquired, which were more than sufficient to neutralize it.

Wiring up

To finish each of the coils, some 8 AWG wire was soldered to the end of the winding wire after first carefully scraping off its plastic insulation (see photo). The soldered joint was then covered using heat shrink insulation before mounting the coil on the lathe for the last time to wind a couple of layers of insulation tape (see photo).

Once the coils were completed, the temporary bolts centerdrilled for turning were removed, and the joining faces on the base were cleaned then lightly oiled before they were securely bolted in place.

Car jump cables were used to connect the battery to the charger. To make it easy to use the large alligator clips, two pieces of 1-inch brass were turned with a groove before cross drilling with the 0.188-inch hole for the 6 AWG cable to be used to connect the power source for distribution to the coils (see photo). Finally, 2-by-
0.156-inch threads were tapped in both the top and the bottom of the brass in order to mount it to insulating plastic and act as hold-tight screws for the cable. After assembly, the positive terminal was marked with red paint to help ensure that the connection to the battery was always correct.

The car solenoid was a bulky item, so a plastic box big enough to house it with the wiring, diodes and meter (if fitted) was the next purchase. The solenoid was purchased on eBay and came without a wiring diagram, so a few minutes had to be spent to identify the correct wiring sequence so that it would operate.

Terminal strip can be used to joint the various components. Alternatively, some homemade terminal blocks can be used with 0.250-inch threaded rods, onto which crimp terminals fitted to the wires can be bolted.

To activate the started solenoid, a simple 12-volt press switch can be used, switching off as soon as pressure is released.

The wiring was completed in stages, first the lugs for the battery, then the switch and solenoid. Once this appeared to work correctly the first coil was connected, along with a diode so the polarity of the coil could be checked using a magnetic compass. The press switch was then pressed for a couple of seconds while holding the compass near to it, but not too close in case the compass polarity was changed. To be cautious, this first coil was then disconnected and the second coil connected and checked. Power was only applied for a couple of seconds to stop the coils from overheating.

The two coils were then hooked up and the ammeter connected, following the wiring instructions that came with it.

Charging magnetos

After testing the connections, the best way to test the charger is to recharge a magneto.

It is easier to recharge the magnets of a magneto when they are assembled, as this saves the need to place keepers across the poles to retain the magnetism after removing the magnets from the charger.

The first and essential step in recharging a magneto’s magnet is to correctly identify the north/south polarity, then mark it on the magnet with a piece of chalk. Incorrectly placing a magneto on a charger can result in the loss of all magnetism and make it extremely difficult to recharge it again to its full potential.

To find the polarity of the magnets, a compass can be used. Alternatively, suspend the magneto above the charger, using some cord so that it is free to turn. Switch on the charger and the magneto should align itself, using the principle of opposites attract; the north pole of the magneto needs to be placed on the south pole of the charger.

When setting the magneto on the charger, use the best combination of the pole pieces in order to concentrate the magnetic field in the magneto magnets. In cases involving the charging of more modern high-tension magnetos with alloy cases, it might be necessary to make special shaped pole pieces in order to focus the magnetic field where it is needed.

It takes only a second or two for the charger to work, and there is no need for any other black art practices, such as tapping the magnets as suggested in some of the very old articles on charging magnetos. If you have an ammeter then you can soon tell when the peak amps are being drawn; wait a further second or two and then release the switch. The cores of the charger will have been saturated and passed the magnetism to the magneto. If you feel the need, the charging process can be repeated.

When operating the charger it was noticed that the small car battery being used only delivered the current at 9 volts at around 50 amps rather than the calculated figure of 12 volts at over 70 amps. This could be caused by poor contact through the alligator clips, and fitting bolts on battery cable lugs to the jump leads would probably improve the connection. At 50 amps the amp-turns were still over 30,000 – more than sufficient for the older-style magnetos. If more amp-turns were needed then a more powerful battery setup could be used.

The magnet charger has proven to be a very useful addition to the workshop, and I gained a number of friends as a result.

Weighing in at more than 100 pounds, including the pole pieces but without the battery, this is not something that can be taken to engine shows without building a suitable trolley.

Contact Peter Rooke at: Hardigate House, Hardigate Road, Cropwell Butler, Notts, England, NG12 3AH; •

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