A Primer on ATV Charging Systems
By: Gary L. Gustafson (Copyright 2006)
June 14, 2006

ATV charging systems have made enormous progress in the past decade, both in electrical power output quantity and quality. For a consumer to take maximum advantage of this power it is useful to have an understanding of how charging systems work, and why they have evolved the way they have. In this article we will review some basic principles of charging system operation and hopefully shatter a few long-standing myths that may be hampering you from enjoying peak performance from the charging system on your ATV.

Why magnetos are used on ATVs instead of Alternators

ATVs have been equipped since their inception with “magneto”-based charging systems. Simply put, Magneto charging systems make use of permanent magnets imbedded into the flywheel assembly. The assembly is located on the “mag” end of the crankshaft. These magnets are passed next to coils of wire assembled into a “stator”. Alternating current is generated in the coils. This is converted into DC power that is usable by the ATV—more on this later.

Belt-driven automotive-style alternators are capable of putting out substantially more electrical power, however there is a major technical obstacle that has prevented them from being integrated into an ATV application. Alternators require airflow to cool them, but most cannot tolerate the water and debris that ATVs are often ridden in. This has presented a major engineering obstacle to the use of alternators on ATVs.

Magnetos, on the other hand, are contained within the engine package and so they are protected from water and debris intrusion. Magnetos are usually designed to operate without outside airflow to cool them. The more robust magneto designs are designed to work at temperatures as high as 350 degrees F. Design features including engine coolant passages, oil baths, and integral fan blades are all used to keep magnetos operating within their specified temperature range. A magneto also allows for simple, compact integration of the charging and trigger coils for a CDI ignition. An alternator does not, thus magnetos have historically been used on ATVs.

Electrical demands of ATVs have grown steadily over the years—putting increased demand on charging systems and batteries. So far, it has made sense for OEMs to make improvements to magneto charging systems to meet these electrical demands rather than developing alternator systems. Updates that can be made to magneto-charging systems include—upgrading to rare-earth magnets, adding more charging coils and permanent magnets by making the flywheel a larger diameter, and designing charging systems with 3-phase as opposed to single-phase power output. All of these upgrades have an associated cost, and increased electrical output subjects the stator itself to more heat. But if the electrical loads warrant it, all these features are well worth the effort to implement.

If and when new technologies are implemented on ATVs that require a new type of charging system, these systems will no doubt come into being. Some alternators are beginning to be implemented on 4-stroke snowmobiles such as the Arctic Cat T-660 4-stroke models. One luxury of this increased output is the addition of electrical features such as heated seats. The use of an alternator is possible on snowmobiles because they are designed to have airflow through the hood, and they are not designed to be operated submerged in water as ATVs are. The truth is, the T-660 engine package was adapted from an on-highway application, so an alternator was selected by default over a magneto. Magneto systems continue to be improved to meet today’s ATV power requirements and will be prevalent on ATVs for some time to come.

Do big charging systems rob engine power?

In an ideal environment, 1 Horsepower equals 746 watts. Since no ATV charging system in existence to date has even reached a 746 watt output, “robbing” the engine of power is not much of an issue. What can be an issue in rare cases today is overloading the electrical system with accessories like high-powered lights. If the charging system is not designed to power these loads they can load the engine to the point of causing it to stall at idle. Inevitable increases in charging system output in the future may require that engine designers plan to give away some horsepower to be used as electrical power.

Charging system specifications—read the fine print

Buried deep in the spec sheets for most ATVs is the charging system output, measured in watts and sometimes specifying either a single-phase or 3-phase output. While most manufacturers would rather brag about horsepower, the charging system output should not be overlooked when considering which ATV to purchase—especially if you plan to use electrically powered accessories. In my experience benchmarking various ATV and snowmobile charging systems I found that published specs, if available, were pretty accurate. This is probably because these wattages have not yet become a point of competition between manufacturers. Whenever this begins to occur, you can expect published specs to be about as accurate as published specs for horsepower and weight are.

It is key to understand what RPM the charging system was measured at to compare charging system specs between manufacturers. The output of a magneto charging system varies greatly per engine RPM, with a typical curve looking like Figure B. The output of most ATV charging systems at slow idle (1,000-1,200 RPM) is only 1/3 to 1/5 of what it is at 5,000 RPM or higher. This is because the structure that the permanent magnets are embedded into is connected directly to the crankshaft without any gear or pulley ratio. At low RPM the magnets are passing so slowly past the coils that the average amount of power being generated at any given time is relatively low. If one manufacturer rates their charging system at 1,500 RPM, they will be at a huge disadvantage (on paper) compared to another manufacturer who rates theirs at 5,000 RPM—even though the first charging system may in reality be better! One other note that affects charging system outputs--the power output of ATV charging systems drop significantly as the stator warms up. They put out the most power when they are cold because the internal resistance of the stator wiring is at it’s lowest. There’s some trivia to impress your friends with!

Unfortunately, outside of what’s on the spec sheet there is no way for a consumer to measure the exact charging system output power on an ATV. The equipment required to accurately measure the output of the charging system is beyond what dealers have in-house. During my years at Polaris and Arctic Cat, I used a specially tooled “spinner” or “spin stand” that the magneto assembly was mounted to, plus a current shunt that converts current to voltage to measure the stator output independently of the vehicle electrical system. Bottom line--If you really want to get a feel for what the charging system does—there are some “seat of the pants” ways. One is to look at the physical diameter and the number of charging coils on the stator. More copper wire in the stator equals more charging capacity. Ask your dealer if he can refer you to anyone else who uses their ATV with similar electrical loads as yours. If someone is using an accessory sprayer or extra lights they can tell you if their charging system has been keeping up or if their ATV battery has been going dead occasionally. Another way to gauge the effectiveness of the charging system is to measure the ATVs “Break Even” RPM.

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