Building a heavy duty magnet charger

Make it at home

| January/February 1989

John using the charger

John using the charger at a local New England engine show.

"Could you send plans and drawings for the magneto charger in your article?" This was the most frequent inquiry I had after my November 1986 GEM article on magneto recharging. This article has drawings and instructions for making and using that charger. Size and expense make the charger most practical for club or group use. The expense can then be divided among the group.

Construction of magneto chargers is not a new subject for GEM. Chargers have been described in the following issues: Jan-Feb '78, pp. 26, 27; Mar-Apr '78, pg. 7; May-June '77 shows pole pieces for charging flywheels, rotors, etc. Those units were not sufficiently heavy to charge many magnets while installed on the magneto. Remember that charging the magnets while installed on the magneto is essential. Chargers with sufficient energy have been pictured in GEM, but no construction details were given.

The charger described here is designed to operate from a 12-volt automotive battery, has a core area of seven square inches and provides 20,000 ampere turns of energy. It is sufficient to saturate and charge most fully assembled magnetos used on antique engines. It's important to state that increasing charger size beyond the point where magnet saturation occurs does not improve magneto performance (there's no harm either).

Plan on spending at least $200 to $300 if all the materials have to be purchased. If you have a source of scrap iron, machine shop facilities and can come up with some surplus magnet wire, the costs can be drastically reduced. The charger weighs about 150 pounds and is mounted on an inexpensive 2 wheel hand truck. This second generation design features ease of construction, improved operation, bottom mounted coil terminals and diode protection to prevent excessive switch sparking and coil failure.

The design
The main criteria for any charger design is core area, number of wire turns, and amount of current flowing through the coils. Core area must be large enough to saturate the magneto core without becoming saturated itself. Magnetomotive force (product of amps times turns of wire) must be large enough to bring magneto magnets to saturation.

This charger uses 3-inch diameter soft iron core pieces 6 inches long, each wound with approximately 500 turns of 10 gauge copper magnet wire. 20,000 ampere turns of magnetomotive force is provided with a 12 volt battery supplying about 40 amps. The coils are connected in parallel and protected with diode arc suppressors. A knife switch is used to turn the charger on and off.