Final version Appeared In Gas Engine Magazine April/May 2022
As the magnets in our old engine magnetos age, they very slowly lose magnetic field strength resulting in less spark energy. Age as well as excessive heat, radiation, strong external magnetic fields and shock cause some of the millions of tiny magnetic domains to become misoriented and therefore not contributing to the desired field. A brief discussion of these magnetic domains can be found in “Build your own magneto charger,” Gas Engine Magazine April/May 2020
The usual fix for a weak magnet is a recharge. A very strong magnetic field, properly oriented, is passed through the magnet. This strong field puts a torque on the misaligned domains, causing them to realign. Once realigned they contribute again to the overall magnets field. The usual source of the strong magnetic field used to recharge the magnet is a pair of big electromagnets, Figure 1.
Figure 1. Typical magnet charger schematic.
Although the usual source of the strong re-magnetizing field is two big electromagnets, any strong magnetic field will work equally well. A Star Wars magnetic propulsion system may come to mind but rare earth neodymium super magnets are much easier to come by and work very well for recharging a decayed magnet. Magnets would seem to be an ancient and mundane technology but efficient DC electric motors such as those in electric cars and drones have driven magnet technology beyond what was imaginable a few short years ago. Rare earth magnets are now readably available in all manner of size and shape at reasonable prices that are very strong. For example, it is not difficult to find magnets 2” square that require more than 100 pounds of force to pull them off a sheet of steel, making them stronger than the very best home-made magnet charger. Further they do not lose strength over time. Figure 2 replaces the big electromagnets in a charger with very strong rare earth magnets.
Figure 2. A rare earth magnet charger.
To recharge horseshoe magnets a pair of BYO88-N52 magnets were purchased from K&J Magnetics for $17.88 each. These magnets measure 2” X ½” X ½” and require more than 67 pounds to pull them off a 3/8” thick sheet of steel. The two magnets were placed on a 3/8” thick by 2 ½” wide by 12” long sheet of cold rolled steel, one magnet with north pole up and the other with south pole up. A word of caution here, these magnets are very strong and brittle. They will jump out of your hand and hit the steel bar with great force and can easily shatter. Figure 3 is an undesired $17.88 result.
Figure 3. Broken magnet.
The magnets will arrive stacked, with a plastic spacer between each magnet, as seen in Figure 4. Mark the top of the stack red and the bottom black. When the magnets are properly placed on the steel bar the red mark and black marks will both point up. The best way to keep them from jumping out of your hand seems to be to approach the steel bar from the side with the red mark up and slide the magnet on following with the other magnet, black mark up.
Figure 4. The magnet stack as they arrive.
As unbelievable as it may seem Figure 5 is a complete Bare Bones magnet charger.
Figure 5. A complete Bare Bones magnet charger.
The process of recharging is straightforward. First slid the two rare earth magnets together or apart to fit the magnet to be charged. Expect to find that task a bit difficult as the magnets cling to the steel tie bar with a force greater than 67 pounds. The magnet to be charged is then suspended over the charger on a string. The magnet will rotate such that its south pole is over the charger north pole. When oriented, set the magnet down on the super magnets, Figure 6. Another warning, to control setting the magnet down approach the charger with the magnet from the side. Beforehand, clamp the charger to a sturdy bench otherwise the charger will leap up and attach to the magnet.
Figure 6. Recharging a horseshoe magnet on the Bare Bone’s charger.
It is common to use a compass to determined the north and south poles of the charger and magnet to be charged but these super strong rare earth magnets will re-magnetize a compass if it gets too close. The author has several compasses that now point south, it seems better to suspend the magnet on a string.
The charger has the advantage of not consuming power and can therefore be left charging a magnet for hours or days, if desired, rather than seconds for an electromagnet charger. Data suggests, however, that 80% of the charging gain is achieved in 30 seconds and 100% is achieved in 2 minutes. A side note here. The author has measured several chargers and although many claim 20 to 30 seconds of cumulated charge time is sufficient, all measured chargers deliver a stronger magnet when left on for two minutes. However, on the Bare Bones charger there is no on/off switch, which makes for some difficulty in removing the magnet from the charger. The best way to get the magnet off the charger seems to be pushing on the side of the magnet until it tips then pulling upward, Figure 7.
Figure 7. Removing the magnet.
Several uncharged magnets were tested before and after recharge. A magnet’s magnetic field was first measured with a Tesla meter gapped at 0.050”. The Tesla meter gives the actual, true, no guessing, magnetic field strength. Each magnet was then placed on a magneto; the magneto’s energy output was measured at 50 and 400RPM. The test results, pre and post recharge, were compared. The $40 Bare Bones charger showed excellent results. There was substantial increase in the magnet’s field strength as well as the magneto output energy after recharging.
The horseshoe magnets above were recharged out of the magneto. Common folk lore and wives’ tales warn not to remove a magnet from the magneto. As loud as it’s proclaimed, it is in fact false. Several studies have measured magnet strength, over time, and found no degradation when removed from the magneto without a keeper. A keeper is a steel bar placed across the magnet poles. One study left magnets without keepers laying on a wood bench for 18 weeks, no degradation was measured. Magnets, both horseshoe and bar, may be removed from the magneto for a few hours or days without damaging their strength.
How good is the $40 Bare Bones magnet charger? The answer to that is, simply, how strong is the charger’s magnetic field that will be imposed on the magnet being charged. In the early days only engineering laboratories had the capability of measuring magnetic field strength directly. The strength of most electromagnets was estimated by measuring their Amp turns. The theory is that more turns of coil wire and more Amperes in those turns will increase the magnetic strength. This however is a relative measurement as it fails to consider core size and material as well as unintended air gaps and a host of other items. It is a bit like estimating that a 427 ci car engine is more powerful than a 396 ci engine while not knowing the compression ratios or valve and ignition timing. But technology marches on, today accurate Tesla meters that will directly measure the magnetic field strength can be found for less than $100. It is now a simple matter of measuring the magnetic field strength across a standard gap. The gold standard for home-built chargers is the John Rex model described in Gas Engine Magazine January 1989. Figure 8 is Redd Stanberry’s charger built to John’s specifications which produced a 733mT (0.733 Tesla) field.
Figure 8. Charger built to John Rex specifications.
Not as beefy as the Rex unit, Figure 9 is a home-built charger that produced a very good field of 560mT (0.56 Tesla).
Figure 9. A very good home-built charger.
The Bare Bones rare earth charger as shown in Figure 5,6 and 7 produced a field of 1110 mT (1.11 Tesla), easily outperforming every home-built charger that the author has measured over the years. It does not, however, outperform the commercial Weidenhoff Model 818 charger, Figure 10, that appeared in Gas Engine Magazine August/September 2020. The Weidenhoff, weighing 500 pounds and running on 120V mains produced 1600mT (1.6 Tesla). As a reference, most recharged horseshoe magnets will fall in the range of 150mT to 200mT under the same test conditions.
Figure 10. The Weidenhoff Model 818 charger.
If more than horseshoe magnets need to be recharged, the basic configuration of Figure 5 can be modified to a more universal form as in Figure 11. This configuration is identical to normal electromagnet chargers; the rare earth magnets do not need to be moved to accommodate different size magnets and pole pieces can be used when charging non horseshoe magnets or magnetos.
Figure 11. Universal rare earth magnet charger.
In order to planarize and level the deck the author used some plastic he had laying around the workshop, Figure 12. Any nonmagnetic material can be used here but avoid aluminum. Aluminum is generally nonmagnetic but in high fields becomes slightly magnetic.
Figure 12. Planarized universal charger.
As seen in Figure 13, the pole pieces can now be moved to accommodate other magnets and magnetos. Although the planarizing material can be any nonmagnetic material its height must be machined accurately as any air gap between the magnet and the movable pole piece will quickly degrade performance.
Figure 13. Conceptual universal charger.
Figure 14 is a completed subassembly. This assembly used the same size tie bar as in the Bare Bones charger, 3/8 X 2 ½ X 12. The plastic planarizing pieces are held in place with nonmagnetic brass screws, epoxy will likely work as well.
Figure 14. The universal charger subassembly.
The two cold rolled steel pole pieces, of different size to indicate the versatility, are added to finish the charger, Figure 15. Figure 16 adds other pole pieces to accommodate a Wico EK.
Figure 15. The completed universal charger.
Figure 16. Recharging a Wico EK.