The Why and How of Recharging Magneto Magnets

John Rex explains some common misconceptions about magnetos and how they can lose their charge and offers advice for magneto maintenance.


| November 1986



A heavy duty 12 volt portable magneto charger made by the author

A heavy duty 12 volt portable magneto charger made by the author (or use at local engine shows).

"How often do magnetos need recharging?" is a question frequently asked by antique engine enthusiasts. The answer has to do with the permanence of the magnet on the magneto. Magnets are mysterious to us because the magnetic field cannot be detected by any of the five senses, yet it does exist and has many powers. It can attract metal objects, convert mechanical energy to electrical energy, and vice versa, and even alter the normal characteristics of materials.

It is a common fallacy that a permanent magnet expends internal energy to create electricity from a magneto or generator. This fallacy leads to the belief that after repeated use the magnetism will all be used up and hence the magnet must be recharged. The energy to create the spark comes not from the magnet, but from the mechanical energy required to drive the magneto. The magnet merely acts to convert this mechanical energy to electrical energy.

Confidence in the permanency of permanent magnets is substantiated by evidence of the many applications they have been put to over the years. For example, magnets are present in compasses but they never wear out or need recharging. The continued accuracy of some of the most exacting scientific electrical measuring instruments such as the familiar speedometer also depends upon a permanent magnet remaining constant.

Most magnetos on hit and miss engines used magnets that were made of quench hardened steel alloyed with chrome, cobalt, or tungsten. Magnets of this material were made from rolled stock by forming or cutting into the shape of a horseshoe or long bar. Immediately after quench-hardening, the material was rather unstable metallurgically and considerable change in magnetism could occur if magnetized during this transition period. Fortunately most manufacturers allowed for proper aging after quenching and before magnetizing. There is probably some reduction in magnetism over the decades; however, it is minimal and probably only results in about one third loss at the most. Such a loss would hardly prevent a well-designed magneto from functioning adequately.

Owners of antique engines sometimes report that their magneto has mysteriously lost its magnetism. In most cases, the fact that the magnetism has been lost cannot be questioned; however, the 'mysterious' aspect can be disputed. There is nothing mysterious about it. If a piece of material can be magnetized, it is likewise capable of being demagnetized. There are definite and logical reasons why magnets lose their magnetism. The most common cause for these losses is that the magnets have been subjected to demagnetizing forces. 

Investigation into how this happens usually uncovers at least one of the following: