How to Test Your Motorcycle Charging System — A Comprehensive Guide

This is a quick (but comprehensive guide on how to test the motorcycle charging system of any motorcycle — whether you’re buying it, or it’s your own.

Electrical “gremlins” are scary for most people — but they don’t need to be.

A charging system for a motorcycle is actually pretty simple. Once you know how the motorcycle charging system works and what can go wrong with it, it’s easy to diagnose faults.

And because faults aren’t uncommon, knowing how to fix a charging system can mean the difference between getting 200km to home, or being stranded out in the middle of nowhere.

It can also mean avoiding buying a dud, or knocking $500 off the asking price of a motorcycle.

Testing Motorcycle Charging Systems in a Nutshell

In a nutshell, there are just five major electrical components in a motorcycle’s charging system (that fail regularly).

These are the

1. Alternator (or stator coil)
2. Regulator/rectifier
3. Battery
4. The cables between them
5. Fuses and switches

When something isn’t working with your motorcycle’s charging system, it’s one of these parts that has failed. Figuring out which one has failed is the important part.

A lot of times, when you ask something about a motorcycle charging system on forums, people will say “dead battery” and cite examples of when they bought a battery and it was dead from the shop. I’m sure this happens and sometimes they might be right. But don’t buy a battery unless you’re sure it’s the culprit. Better, if it’s dead, know what killed it. It’s better to not just “throw spaghetti at the wall”. It’s a waste of spaghetti.

Luckily, the charging system of nearly every motorcycle is the same. There’s really very little that changes.

The major things I’ve seen change on modern motorcycles is that sometimes the stator coil isn’t driven directly off the engine, and is instead in a different unit (the alternator) driven by a belt, just like in a car. I’ve seen this most recently in BMW R1200* engines, and I wrote up a guide to changing the alternator belt here too.

Symptoms of a bad motorcycle charging system

There are a number of major symptoms of a bad motorcycle charging system. Any one of these can mean something in your charging system is broken.

1. Motorcycle won’t start. This one’s the big, obvious one. When it turns over super slowly or you just hear a “click” noise, then something in the motorcycle charging system is kaput.
2. Spluttering as you drive. When voltage is low, it’s hard to keep the motorcycle running.
3. Lights light up and dim as you rev. This shouldn’t happen — your motorcycle should keep the lights constant (unless it’s a truly ancient motorcycle).
4. Battery warning light comes on. Oh, so you have a fancy, late-model motorcycle do you? Actually battery lights are pretty common — my 14-year-old 2006 BMW R1200S has one (which came on).

There are other symptoms too, but those are the main ones.

What can go wrong with the charging system?

Basically, when your motorcycle isn’t charging (or it turns off), it means one of the components above has failed!

Failures of a component in the motorcycle charging system happen in a number of different, all fairly common ways:

• Regulator/rectifier overheats and dies. The Reg/rec lives a long and difficult life. The whole time the motorcycle is on it’s sucking in current from the alternator, trimming it back to the right voltage, and converting it to DC so your battery can charge. What happens to the excess current? It just turns it into heat. That’s why the reg/rec has a huge heatsink on it (it gets hot) and sits in airflow. All this gets too much after a while, and after about 50,000 km (or about 1,000 hours of operation) it’s pretty reasonable for them to die.
• Battery gets old. Batteries have a shelf life. Once they get old, or if they’re not properly maintained, they die. Modern batteries do last longer.
• Battery gets fried by other components. If your reg/rec isn’t regulating voltage properly it might be sending too much voltage into the motorcycle’s battery. Motorcycle batteries operate between about 12 and 14.5V — any more than this puts a lot of stress on them. A fried reg/rec means you might see voltages of over 20V when revving high which is bad news for the battery.
• Stator coil has died. Or if you have a separate alternator, your alternator might have died — or the belt might have broken (j’accuse, BMW!)
• Loose wires. Lots of times, people don’t realise that they can’t just use a screwdriver to screw down the battery terminal wires. You have to use a wrench and give it at least 5 Nm of torque (i.e. hand-tight).

Equipment you need for testing a motorcycle charging system

You need a bit of equipment. People are often shy the multimeter, to start with.

• A multimeter. Any household one will do. But I like this good quality (but affordable) Fluke automotive multimeter. This is a combination of a voltage meter, a resistance meter, and continuity tester (all things you need).
• A battery charger. Your motorcycle has to be fully charged before you can do the tests. You can use a Battery Tender (that 0.75A “Junior” one will charge a motorcycle battery overnight), and also use it to maintain your battery.

Besides that, all you need are regular tools, like the ones in your toolkit. You’re going to need to get at the battery and to unplug a few things. I’d just say you need

• Needle-nose pliers to undo a few clips
• Allen keys to undo (and re-do) bolts and stuff
• Working gloves (optional) because my hands always get filthy when I do this stuff!

Before doing any tests on your motorcycle charging system… you need to use that trickle charger you bought from Amazon or eBay. Leave it on overnight.

You have to charge your battery before you do any other tests, or your results may not be meaningful.

To charge your battery you usually have to remove the seat, and the fairings if you have them. Sometimes you have to remove the tank — I pray this isn’t you!

Mental note: I’m so thankful I have a motorcycle that has a fairing (to keep me warm) but that it’s a bikini fairing so I can access everything.

Test 1: Battery voltage a with motorcycle off and then on

If you are using a charger, the light will indicate the battery is charged (hopefully).

Once your battery is charged, you’re now ready to start testing your motorcycle charging system! Test the voltage of the battery using your multimeter.

• If your voltage is at or above 12.4V — you’re good.
• If your voltage is below 12.4V after a charge — you need a new battery.

Before you buy a new battery — if your motorcycle was acting normally and suddenly you need a new battery… you have to ask yourself: “Why is my battery dead?”

If it’s just old — like, the battery has literally never been changed, or it’s more than 5 years old — then you might feel comfortable changing the battery.

But if a bad regulator/rectifier has fried it, then you will just fry a new one again, wasting time and money. Test the reg/rec — we’ll do that next.

Now do a similar set of tests — with the motorbike on.

Disconnect your multimeter for a sec (so voltage spikes don’t nuke it).

Now turn the motorcycle on, let it warm up for a bit, and check the voltage at the battery again.

• At idle, the voltage should be around 12-13V.
• At 3,000rpm, the voltage should be no more than 15V.

Is the voltage less than 12V at idle? Then something isn’t producing enough current. You might have a short somewhere in the system drawing too much current, you might have a faulty alternator/stator coil, or your reg/rec may have totally failed.

Is the voltage more than 15V at 3,000 rpm? If so, then your regulator/rectifier might be partially fried. You can do some more tests on it (that’s a story for another day). But you can either take it in, or take a pot-shot and buy one and replace it. Worst case, you’re going to need that spare later (they all die eventually).

To dive deeper into what part of your motorcycle charging system isn’t working, look at whether your alternator/stator coil are working, or if your reg/rec is functioning as intended.

Test 2: Alternator/Stator coil

You need to figure out if your alternator/stator coil is producing enough power. You can usually see if it is by testing the output voltage.

The stator coil is a coil (or a series of coils) that sits around the rotor. The rotor is just a magnet connected to the crankshaft of the motorcycle.

When the motorcycle’s engine is turning — i.e. when it’s on (or if you push start it!) — the rotor magnets turn. The magnets turn inside the coils of the stator. The changing magnetic field caused by the turning magnets induces current in the coils. It works the opposite way an electric motor does.

To test your stator coil, you need to test if it’s producing enough voltage. If it is, it’s usually a sign it can carry the load. (Not necessarily, but almost always.)

First, find the output of your stator coil. There’s usually a plug coming out of the whole area.

Don’t know where the output of the stator coil is? On longitudinally mounted engines, like V-twins, you can usually find the clutch on one side of the engine and the stator on the other side. You know the clutch side because the clutch cable and actuator goes to it. You know the stator side because it’s the other side! On that side, you might find the plug.

On transversely mounted engines, like my BMW or many inline-four motorcycles, it’s not so obvious. There, it’s easier to find the regulator/rectifier (under the seat, quite often) and check the voltage there.

Once you’ve found the output of the stator coil, do the following tests:

Tests on the alternator/stator coil with the engine off:

• Check the resistance between each of the pins (if there are just two, then the resistance between the two). Use your multimeter on a low resistance scale. The resistance should be low (0.2-0.5 ohms roughly). If it’s open circuit: that coil has opened up. If it’s a short circuit: bad news, both coils are shorted to ground.
• Check the resistance between the pins and earth (the chassis, or the negative terminal of the battery). This should be open circuit. If it’s anything but, then there’s a malfunction.

If you still haven’t found any problems, you can do some tests with the engine on. You’ve charged the battery, so it should start up.

Tests on the alternator/stator coil with the engine on:

Put your multimeter in AC mode.

You now can test the voltage between the terminals of the stator coil at 3,000 rpm. Depending on your motorcycle, you should get a reading of anywhere between 20 and 50 volts.

What voltage you get isn’t important (unless it’s less than 15 — at 3,000 rpm, you should definitely have enough voltage to charge the battery, i.e. more than 15). The more important thing is that a) there is a voltage reading on all terminals and b) they’re all pretty similar (if you have a multi-phase alternator).

Some older alternators are just two-phase and have only two output wires. With those, that’s the only voltage you measure.

Most modern alternators have three phases and have three output wires. So you have to measure A-B, B-C, and A-C. In this case, your voltages have to be similar.

Test 3: Regulator/Rectifier

The final test is of your regulator/rectifier.

These fail often on older motorcycles because they live a very hard life. For millions of engine revolutions, they have to get a big voltage, invert it to DC, then trim off the excess, bleeding it to heat.

This is what regulator/rectifiers do (they’re actually two things, but they’re usually in the one unit because they’re easy to build that way — they’re made out of high current diodes).

The rectifier converts AC voltage to DC. This is the same as in any power adaptor in your house, like for your phone charger. The AC voltage that comes out of the wall socket needs to be converted to DC to charge your devices. AC can only be used for things that just provided heat or light (where the direction of current doesn’t matter), like ovens, toaster, kettles, and lamps.

The regulator brings the voltage down from high levels to levels where it won’t fry your battery. The alternator produces massive voltages — higher as your engine spins higher. The regulator trims off the excess and only gives your battery what it needs. Sometimes, this is trimming off over 70% of the available voltage!

The sad part about that is that it just discards that excess power as heat. That’s why regulators fry. They get overwhelmed.

Sidebar: I’ve seen it suggested by mechanically knowledgeable people that disconnecting power-drawing things (like lights) can overload a regulator rectifier as the power has nowhere to go. This isn’t how power circuits work. The regulator doesn’t regulate any power not drawn from the alternator.

Otherwise, as a parallel, our laptop’s power supply would overload when the laptop isn’t plugged in. Or batteries would overheat when not plugged in.

So don’t worry about unplugging accessories — regulators overload from too much power been drawn through them, and/or poor ventilation. Focus on good ventilation and not overloading them and you’ll be right.

When a regulator fries, you either get a) no voltage (flat battery) or b) excessive voltage (fried battery, which also means flat battery).

That’s why if you have a dead battery and you replace it without checking your reg/rec, you may just fry the battery again.

To test the regulator/rectifier: Well, if you understand how diodes work, you can set your multimeter to “diode” mode to test the polarity of the terminals of your reg/rec module. The configuration of diodes and the orientation of the bridge depend on your specific motorcycle, so you need to consult the wiring diagram of your motorcycle manual.

If that was gibberish, then here’s a good testing algorithm: If your stator coil was producing the right voltages in the tests above, but the voltages at your battery seem off (e.g. when revving it goes above 15V), then your reg/rec module is dead.

If your reg/rec module is dead, you can buy the same part — or, if you’re game, you can buy a similar part from any other motorcycle, cut the wires, and splice it in.