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How your dirt bike's electrical system works

The juice must flow!

by Martin Hackworth

Photos: Honda, Martin Hackworth


Any web search for "motorcycle electrical system" will yield a number of results and many are good resources. My goal here is not to merely regurgitate this information, but to put it in terms that I hope will be useful to those setting up a bike for adventure/enduro rides like the Tour of Idaho.  I'd like to to help the reader answer the questions "Should I upgrade to a high-output aftermarket stator or have my stock stator rewound?" and/or "What kind of battery should I use?" with good information.

Not like acquiring solid information is ever an issue on the old Interwebs. I hear that you can trust everything that you read there.

So let's get after it. Most dirt bikes currently in use as adventure/enduro machines have an electrical system containing all of the following: 1) a battery, 2) an alternator (of which the stator is a component), 3) a rectifier/regulator, 4) a spark plug/ignition system, 5) wires, fuses and auxiliary components. Wires and fuses don't require a lot of explaining so let's take a closer look at just the first four.

A battery is a device that stores energy via electrically charged particles. Capacitors do essentially the same thing as batteries but their charging and discharging characteristics are different. A battery charges and discharges slowly. A capacitor may be charged and discharged much more rapidly. Batteries are better for repeated use between charging cycles - i.e., you can use rechargeable batteries many times before they need recharging, and non-rechargeable batteries many times before they run out of power. Capacitors are better if you want a big jolt of energy all at once and then have the ability to recharge the capacitor before it's used again.

Before lithium-ion batteries became popular one used to occasionally encounter large capacitors used in the place of a battery on race bikes that required lots of lighting.

There are a couple of different types of motorcycle batteries that are generally available. The first is a lithium-ion (or lithium-ferrous) battery. The second is a lead-acid battery which comes in several variants: flooded, sealed AGM (absorbent glass mat) and sealed gel. 

Lithium-ion batteries have been around conceptually for about a century (1912) but it took advances in chemistry and materials science to make them practical - a process that is still evolving. Lithium-ion batteries have some very useful features for dirt bikes - extremely high energy density coupled with very low weight. But lithium-ion batteries have some disadvantages as well - intrinsically short lifetimes (in my experience - one season) and frailty in cold (below freezing) weather. The shade tree mechanic rule for cold weather operation is to warm a lithium-ion battery in a cold motorcycle by first turning on and off the lights a few times for a few seconds each time prior to starting the bike. This works, but in general Li-Fe batteries don't like being charged in below freezing temperatures. 

You'll also need a lithium battery specific charger to safely charge a lithium-ion battery when not being charged by a bike's charging system.

Because lithium-ion batteries have high energy densities (check out the cranking and storage specs - which tend to be very impressive) they do have the potential to seriously overheat and possibly even ignite if mistreated. Is it likely? No. Is it possible? Yes. There's simply a lot of energy crammed into a small space which is kind of the same definition as a bomb.

Vendors who sell lithium-ion batteries tend make light of this - even mocking the assertion on some of their websites. But these particular folks are probably not hawking batteries on the Interwebs because they aced Physics. On the other hand a safety feature in the favor of lithium-ion batteries is the absence of corrosive liquids that may get loose. All in all it's probably a wash.

Lead-acid batteries are the workhorses of electro-mechanical systems. They're inexpensive, robust and work in a variety of conditions. Lead-acid batteries are ubiquitous in automobile, truck and motorcycle applications. The downsides to lead-acid batteries are that they are heavy for a given size (both the lead and electrolytes are dense), that they must be kept upright to prevent acid leakage and that they should be used in ventilated areas to prevent the accumulation of hydrogen gas produced during charging. Lead acid batteries also require maintenance by occasionally adding distilled water and maintaining a proper level of charge.

AGM and gel batteries are in a category know as valve-regulated lead-acid batteries (VRLA). These are commonly referred to as maintenance-free batteries because they are sealed and adding water isn't necessary (or even possible) once they are sealed. AGM batteries are far more common that gel batteries for motorcycle applications. Some AGMs are activated at the factory and then sealed while others are shipped with an electrolyte solution that you pour into the battery and then seal permanently. AGM batteries can handle higher charging and discharging rates while gel batteries are better in situations involving high temperatures. "Maintenance free" is a bit of a misnomer as all VRLA batteries must maintain a proper level of charge.

I use batteries best-suited to the specific motorcycle and application. For my street bikes I use exclusively factory activated AGM batteries. Each year I drop a new Li-Fe battery into my KTM300 XCW. For my dual sport CRF450X I use a standard (non factory activated) AGM.

Dirt bike batteries are typically small in size and relatively light in weight when compared to their automotive counterparts. Their small size is a requirement of being generally mounted in tight spaces. Light weight is important because dirt bike batteries are located fairly high up on most bikes and it is desirable not only to keep the overall weight of the bike down but to minimize the roll moment of inertia about the axis connecting the front and rear contact patches. Extremely lightweight Li-Fe batteries are popular for precisely these reasons.

In addition to providing starting power for electric start bikes, many electrical components found on dirt bikes require a source of 12-volt direct current (DC) and these may run directly off the battery. In many instances the battery also serves as a buffer between spikes and drops in charging system power and components downstream. 

Motorcycle batteries are designed to be continuously charged while the motorcycle is in use. Charging systems in motorcycles, while similar to those in automobiles, do have some significant differences. Both automobiles and motorcycles use alternators as the base component in their charging systems. Alternators use the motion of a coil of wire with respect to a magnetic field to produce an alternating (AC) current - a process known as electromagnetic induction. A stator is a component of an alternator - the circular portion of the alternator that remains stationary while the rotor (or flywheel) rotates in or around it.

In an automobile, where batteries are physically large and have the ability to produce a lot of current, the coils of the alternator contain electromagnets (magnets powered  by current as opposed to permanent ferrous magnets) energized by the battery. This design, which incorporates two sets of electromagnets and rotors, increases the amount of magnetic flux created as both sets of rotors and coils interact. The advantage of this is a much more efficient alternator that charges very effectively even at low rpm. In an automobile the lights, wipers and all other electrical systems will run just fine at idle all day and all night. The only disadvantages to this design are increased size, weight and complexity. In automotive and truck applications this isn't an issue. In a motorcycle it is. 

Many dirt bikes don't have a battery. Many of the ones that do will run just fine with a dead or nearly dead battery because dirt bike alternators contain ferromagnets instead of electromagnets and don't require a battery to work. Dirt bike alternators make do with the energy acquired through the movement of coils of wire with respect to permanent magnetic fields created by the ferromagnets. A dirt bike alternator consists of coils or wire wrapped around spools arranged in a circular pattern - the stator - and a rotor with permanent ferromagnets. As the rotor spins the changing magnetic flux through the coils in the stator produce a potential difference and subsequently a current. This arrangement is similar to something you might have made in a physics lab experiment on electromagnetic induction in high school or college.

Dirt bike alternators are much lighter and simpler in design than their automotive cousins because they have to be. But the price for this is inefficiency at low engine speeds. You can hear a dirt bike cooling fan speed up or down as engine speeds increase or decrease. Incandescent lights will get brighter or dimmer depending on engine rpm. Some of this voltage fluctuation is buffered by the battery for components that run off the battery but only within limits.

There are a couple of different coils configurations on a typical dirt bike stator. The difference between them is in the coil winding and type of wire used. Some of the coils are wound to produce relatively high currents at low voltages to charge the battery, run lights and other ancillary systems. Others (source coils) are wound to produce higher voltages (12x or more) at lower currents to run the ignition system.
Some dirt bike stators also have a pickup coil that detects crank angle and sends this data data to the CDI (capacitor discharge ignition) system to control ignition timing.

A CDI is a part of the ignition system often used in dirt bikes to augment or supplant the source coils. Source coils have relatively slow inductive response rates which is a fancy way of saying that it takes them a relatively long time to create the high voltage required by a spark plug. By energizing a capacitor, which is able to charge and discharge very rapidly, with the source coil, the battery or some combination of the two, it's possible to create spark voltages (12 - 20 kilovolts) much more rapidly which enables the engine to run at a higher rpm.

The rectifier converts the alternating current native to alternators into direct current. Some lights and most all ignition systems will run with alternating current while batteries, other lights and most auxiliary components require direct current. The rectifier assembly works by inverting half of the alternating current waveform and buffering the ripple that's left over.

The voltage regulator keeps the voltage going to the battery somewhere between 13.5 and 14.5 volts which is what's required to effectively charge a 12 volt battery. It also keeps the voltage where needs to be for other components. Voltage regulators, by design, create a lot of heat and are typically mounted in an open place with cooling fins or some other mechanism for heat dissipation.

Most stock dirt bike stators put out between 30 and 50 watts. A high output stator puts out double this or more. A watt is a unit of power or energy per unit time. For you techo types this corresponds to the power in an electrical circuit with one volt potential difference and one amp of current. Light bulbs are typically rated by their wattage (between 20 and 100 watts). As an interesting aside the sun has an output equivalent to a 10^26 watt incandescent light bulb. 

On any dirt bike the system that uses the most electrical power is lighting. Nothing else is close unless you have heated grips - in which case you should quit reading this, stop riding dirt bikes and take up checkers, or hanging out at Starbucks, or creating lovely floral arrangements or some other cozy activity that any wimp may enjoy.    

Allow me to digress for a moment. For the better part of a decade I owned a sound, stage and lighting company. I spent many afternoons before an evening show hastily wiring my rig into live boxes in crappy venues all over the Intermountain West (the good venues are all set up for this with dedicated connections at the breaker box). What's relevant about this here is that lighting was the issue. You can actually run a pretty hefty PA off a single 120 volt outlet as long as it's on an otherwise good circuit. The same applies to dirt bikes.

Back in the days of incandescent and HID lighting, high-output stators, typically 100 watts or more, were required to run bright lights. Even with a 100 watt stator one had to be running flat out to keep a high-lumen incandescent system at full brightness. HID systems were much brighter than incandescent systems for about the same power but unlike incandescent systems would not work at low voltages (a seriously inconvenient feature for the unwary). HID systems are a type of arc light and require voltages above a threshold to arc. When the voltage falls below this threshold HID's don't dim, they flicker then go out. The threshold voltage for HID lights is like an on/off switch.

Due the the high power requirements of very bright HID systems you can fall below the threshold voltage just by rolling off the throttle a little - even at high speeds. There's nothing like going from 10,000 lumens to utter darkness in a few milliseconds at 80 mph. Don't ask me how I know.

With the advent of efficient, bright, low-powered LED lights all of this has changed. Trail Tech, for instance, makes a nearly 5000 lumen LED headlight (about half as bright as daylight) that requires only 27 watts of power. Most common LED dirt bike headlights require even less. Dirt bikes typically come from the factory equipped with  incandescent bulbs that require 30 watts or more. So you can switch to LED lights and still run a stock stator with power to spare.

LED's, like HID's, have cutoff voltages below which they will not work - but those cutoffs are much lower. LED's also tend to be available in higher color temperatures which increase visibility for a given rate of power consumption. LED's are the schizz-nizzle. You have to be running a lot of LED lights, GPS units, fans and other gizmos to need a high-output stator these days unless you are riding at slower speeds with a lot of power-consuming components running all of the time.

Bear in mind that dirt bike stators are inefficient at low engine speeds by virtue of their design. It's an inherent feature. You won't get the 100 watt output of a 100 watt stator at trail speeds unless you are revving your engine to the moon. You'll get more than you would from a stock stator but not as much as you might think.

What you will absolutely get from a high-output stator is heat and lots of it - especially at higher engine speeds. Some claim that this is a myth, that it is not possible to overheat a stator by any mechanism other than Joule heating under heavy loads, that you can't ever go wrong with too much stator, just too little. This is incorrect and I have a boxload of well-made, briefly-used, completely toasted high output stators to prove it. Without making this article longer than the Magna Carta let me try to explain.

As it turns out it's possible to overheat a stator with either too much output for the load or too little output for the load. The exact reasons for this are complex but the simple version goes something like this.

For a given stator, voltage and current vary harmonically (sines and cosines are involved) with the angle between the rotor and coils. It's possible to analyze this mathematically but it's not particularly easy. Ohm's law, the simple version anyway, doesn't apply here as you might think. Loads may be resistive, inductive or capacitive, voltage and current are cyclic. I taught Engineering Physics at a University for 25 years and the analysis of inductive circuits took two entire weeks out of a 16 week course every semester. And this was to students who had a good grasp of calculus and electromagnetism.

A simpler way to think about what a motorcycle stator is doing is to recognize that its power output rises significantly with engine revs. With power comes heat. The bike's voltage regulator prevents things from getting overloaded downstream (which is why they can get very hot), but it doesn't do much to prevent the stator itself from generating a lot of heat when it's nearing it's maximum power output. When you connect loads to a stator you draw down the voltage it's producing for a given rotational speed and transfer the power from the stator to the load. This helps keep it running cooler.

You can overdo this and apply too much load, but that's difficult to do with high output stators. 

If you are not careful with a high-output stator running at high revs you'll eventually cook the left side of your engine and the stator assembly. If you're lucky you'll just melt the auxiliary coils. If you are unlucky you'll melt the source coils as well. If this happens you'll be able to use the opportunity to figure out how fast your friends respond to "come get me" messages from your PLB - unless using a dirt bike as a wheel barrow is your Zen.  

All of this being said there are situations, e.g., lots of lights and accessories, in which an upgrade to a high-output stator makes perfect sense. If you decide to upgrade make sure that you also upgrade the voltage regulator along with the stator. Also consider running something that uses a few watts most of the time as a power-soak. My dual sport CRF450X has a 100 watt stator. I keep all of my lights running all of the time at anything above trail speeds and set my Trail Tech fan to come on at the minimum temperature to soak up excessive power and the resultant heat. 

An alternative to off the shelf high-output stators is to have your stock stator rewound. This allows you to choose wire, windings and a host of other custom features that can result in a stator that's just right. I've had a few of these that were done by folks who knew what they were doing and they worked great. The only disadvantage of this is that you won't have a spare stator on hand if you cook your fancy one.

Well that's about it for this mighty lengthy treatise. After making you read all of it I'm sorry not to be able to provide a single definitive answer to the question posed way back in the beginning - to upgrade or not upgrade one's charging system and/or battery.
There is no single correct answer to this. Instead there is a continuum of circumstances with "stock is best" at one end and "high output stator is best" or Li-Fe is best at the other. A host of factors come into play along the way: your bike, your accessories, typical engine speeds how much attention you want to spend on monitoring heat. .

If you ride a WR250 (which I hope that you do not) the stock stator puts out enough juice to power a space shuttle. No need to ever upgrade. If you ride a typical dirt bike with a 35 - 50 watt stock stator, a low-powered LED headlight and typical aftermarket accessories you probably do not need to upgrade either. But if you are planning on running 10,000 lumens of lighting and mondo accessories you should upgrade, but be aware of the provisos above. .

I generally carry a NORCO Genius GB40 or G1500 as a backup charger that comes in very handy when I spend a lot of time at trail speeds where a stator is not be able keep the battery charged. Not only will these start your bike dozens of times and recharge the battery, but you can run a bunch of accessories off of them as well. These are spendy but worth every penny. 

Baja Designs has a pretty useful chart that shows the stock vs.modified stator output available for it's products. 
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