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What Does a Rectifier Do On a Motorcycle? AC to DC Current

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If you’ve ever been on a motorcycle, you know that it’s not always easy to tell when your bike needs a little help. Many things can go wrong, including a dead battery, a bad alternator, a weak starter, or a blown headlight. You bring it to a mechanic, and you’re told that your rectifier is busted. You scratched your head, asking, “What is a rectifier and what does it do on a motorcycle?” That is what this article will go over.

What is a rectifier?

A rectifier is an electronic device that converts alternating current (AC) to direct current (DC). Alternating current is the opposite of direct current, and it changes direction regularly, while direct current flows in one direction.

Now, what does this rectifier do on a motorcycle? It performs the same function as it does in any other device.

In a motorcycle, when the engine is running, there will be an alternator that generates alternating current (AC) in the system. So, when you talk about a rectifier, you’re also referring to the alternator and vice versa. 

After the rectifier converts the direct current, it is passed on to the battery for storage. This stored electricity in the battery provides power to the motorcycle accessories such as the headlight, starter, indicators, brake lights, horn, etc. 

Long story short, an alternator generates AC generated by the engine’s motion. The rectifier turns AC into DC. Then the battery stores the charge from the DC. 

Does it have a role in starting a motorcycle?

So, what exactly does a rectifier do when starting a motorcycle? By function, a rectifier and a motorcycle starter are not related. However, modern motorcycles require an electric charge to start them up. Therefore, in general, the rectifier is just one of the components that help charge the battery and store power. 

If there is an issue with the rectifier, the battery will not charge, which means the motorcycle won’t start. 

When your battery doesn’t charge, check the alternator/rectifier

Charging a motorcycle battery requires a running engine, a working alternator/rectifier/regulator, and a battery. 

If the alternator is broken, you have zero power, and the battery will not be charged. 

If the rectifier is not working, the generated AC power cannot be converted into DC; the battery requires DC power, so charging becomes a problem. 

Whether these two are not functioning, the result will be the same. There will be no power stored in the battery. If the problem persists, the battery will die out eventually. 

Can I start my motorcycle if my rectifier is broken?

Absolutely! You can even ride it. However, please note that your battery will eventually die out since it is not charging. If your battery still has juice, you will still be able to start your motorcycle, but it’s only good until the last drop. After that, you will have problems starting your bike. 

How do you fix a broken rectifier?

There are two ways to get it fixed. One is to replace it with a brand new unit. The second one is labor-intensive and costly: you have to open the aluminum case, find the defective diodes or MOSFETS (metal–oxide–semiconductor field-effect transistor), remove them, and solder a replacement, then seal it back with epoxy glue for metal (if you have no experience with this, it’s better to consult an expert mechanic). 

Can a broken alternator damage the rectifier?

It’s possible. If the wire around the alternator is damaged, this is an isolated problem, so the rectifier/regulator will not be affected. If there is a vast electrical surge (AC) in the alternator, this will eventually damage the R/R. 

Testing your motorcycle’s charging system

A motorcycle’s charging systems are composed of the following:

  • alternator (or stator coil)
  • regulator/rectifier
  • battery
  • the cables in between
  • fuses and switch

If your charging system isn’t working, it’s one of these components.

There are several symptoms when a charging system is defective. 

  • Your bike won’t start
  • Lights are working, but they dim as you accelerate. 
  • You have a battery warning light. 

To test your charging system, you will need:

  • a multimeter
  • a battery charger 
  • regular tools such as needle-nose pliers, fallen key for the bolts, working gloves, and a trickle charger. 

Note that your battery must be fully charged for this simple test to work. 

The first test is to check the motorcycle’s battery voltage, off, and then on. If you use a battery charger, you will get an indicator light that means the battery is charged. You can start the test once it is completed. 

Test the battery while the motorcycle is off. 

You’re good if the voltage is 12.4V. 

If it’s below 12.4V, you have a battery problem and will need a new one.

If the battery is getting old, it’s probably time to replace it. However, if it’s still new, you have to assess the situation and find out why a relatively new battery is dead or dying. This assessment will make a huge difference. For instance, if the problem comes from a bad regular/rectifier, you will fry the new battery. 

Test the motorcycle while it’s on.

It’s best to disconnect the multimeter first. Then turn on the motorcycle, let it warm up, and check the battery voltage. 

  • Idle reading should be 12 to 13 volts. 
  • Around 3000 rpm, the Reading should be no more than 15 volts.  

The produced current isn’t enough if the voltage is less than 12 volts while idling. A short or something somewhere drawing too much power probably causes it, or you might have a broken alternator/stator coil. Worse, your reg/rec might be completely broken. 

If the voltage exceeds 15 volts at 3000 rpm, the reg/rec may be fried. 

This simple test is just one of several ways to check your motorcycle’s charging system for possible issues. If the symptoms continue, it’s highly recommended that you check with your mechanic and have your motorcycle checked for other related damage. 


So, if you own a motorcycle, you should be aware that one of the most critical components of your bike is the charging system. You must have a good charging system to avoid unexpected breakdowns and better performance. 

I hope this helps you learn about charging systems and the different components of your motorcycle’s charging system. Please let me know if you have any questions in the comments section below.

Until next time, take care and ride safe.