Expected voltage at idle and at RPM?

I just replaced the battery in my '07 WR450 (purchased the bike a couple of months ago.) I'd like to know what AC reading I should get when the bike is running, indicating that the battery is getting a charge.

Here's my story, in case any of these facts would help the answer:

I went to charge the battery in the bike a couple of days ago, anticipating a ride over the weekend. It seemed to take a charge (battery tender) but the estart was fairly weak. I took out for a ride anyway, since I figured I could kick the bike if needed. After a couple of hours (and after a trail that was a little too technical for me, rocky and steep) the bike started to run rough at higher RPMs.

On the way down the canyon, I noticed that the turn signals (it's got a bd kit on it) would impact the roughness of the engine running. Finally, the bike quit altogether, feeling like I suddenly lost ignition. I waited for 20 minutes or so, and the bike fired again, but only ran for a couple of minutes before the ignition quit again.

Got it home, put it on a charger overnight, and the battery wouldn't take a charge.

New battery installed, fires right up. However, I don't notice an increase in voltage when the bike is idling, or if I rev the engine (checked both AC and DC.) I get 13.5 DC, but no? AC.

My guess is that I should be getting AC while the bike revs at 2-3K RMS. Can someone provide a bit of guidance on what I should be seeing? The bike is very low mileage for its age, around 750 or so.

Thanks in advance for your help!

Chris Wareham

After trolling through the shop manual, I measured the voltage at the connections recommended by Yamaha, (same result as measuring them at the battery.) I also measured the resistance through the "source coil" which I assume is the charging coil, and got a reading of around .3 ohms (correcting for the resistance in the meter.)

The manual says that if the voltage isn't in spec, and "If no failure is found in checking the source coil resistance, replace the rectifier/regulator."

I'm not sure how the rectifier fails / what it does when it fails. It almost seems like if the stator is kicking out enough voltage, and the R/R quits working, you'd just get an unregulated amount of voltage to the battery, but I'm new :). How does it fail when it fails?

Has anyone had a similar experience and fixed the issue replacing the R/R?



Edited by cwareham

No AC on the battery, only DC; batteries do not provide, nor charge AC power by design. Voltage at 13.5 should be fine, what you are looking to not see, and therefore indicating a bad RR, huge spikes (above 15v or so) nor the charging voltage continue to rise as the RPM's increase.

Sounds like you had a bad battery. FYI, a WR will run with a "dead" battery, just not a bad one. Yours sounds like it had a bad cell.

To answer your second post question look at the name of the part: regulator/rectifier and it works the same on all electrical circuits.

Regulator portion takes the power the stator puts out and regulates it to a constant voltage. By design a coil continues to build more voltage as the RPM's increase, can be 20v or more, which being sent back t your 12v bike battery will destroy it. You are testing for this function in what you did above, usually bikes are regulated to the just under 14v area and no more. If you see voltage rise off idle and keep climbing, its bad.

Rectifier portion can do two functions: rarer seen is smoothing the DC output so you don't get spikes and your electronics have a consistent voltage running them (the regulator handled that usually but I've seen some in newer setups) or the most common of converting AC to DC power. By testing your charging power at the battery like you did and see very low AC levels (you'll always have some AC on a rectified circuit) it shows that portion seems to be working as well as your major output is DC.

They usually combine them into one package since they are quite intertwined with each other.

This help? It seems tricky and I'm always amazed how many people say they don't get electronics when its simple flow just like hydraulics, cooling systems or oil pumps. Your system appears normal.


Edited by miweber929

I have the same bike and I just got finished putting in a BD kit.

Did you send the stator to them for the 'ground float' mod to the stator?

After I put it all together, I measured with a mater.

I have a new battery so it measures 13.5V not running

When it runs, at idle it makes 13.7 - 13.8V

A good battery should measure 12.2-12.8 V (or more if its really new or freshly charged)

When attached to a running vehicle, 13.2 - 13.8 V

Hi Bob-

The BD kit had been installed when the bike was new, several years ago. I assume that if there was a mod required it was done then. I don't know for certain though.


Thanks much for the clarification, that's VERY helpful. The voltage I was seeing on the battery was right after it was charged; I'll keep an eye on it once it settles down to the mid-12's and see if I'm getting voltage higher than that when the bike is running.

All the best!


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