40HR / 6H4 Charging System / Battery sizing

Hello everyone,

I have an Yamaha 40HP 3 cylinder two stroke (6h4) and measured the voltage on the rectifier output with a scope. Without load it looked like a rectified AC voltage up two 50V, with a motorcycle battery connected it was between 13V at idle and 17V at 4000rpm. The voltage is really noisy with a lot of spikes.

Can someone tell if the engine has actually a rectifier / regulator combination or if it is just rectified and needs an external regulator? The part number of the build in rectifier should be 6G1-81970-60 based on Yamaha parts catalogue.

I try to determine if this is common behavior and the noise would be smoothened with a bigger battery or if the regulator is damaged. So far I read that on the old outboards 17V is not a rare thing and that special marine batteries should be used that tolerate these voltages, but I am concerned that the spikes will damage some electronics, so if this behavior seems normal I might addd a voltage limiter.


I have another question about the battery sizing: I read in a manual for the 40/50 HP two strokes that a 70Ah battery is recommended. This seems quite big to me. Do you have any experiences / recommendations? I used to run it with a 44Ah battery.


Best regards,
Jan

Need to know the model of the motor. Some have a rectifier only and some have a rectifier/regulator.

The part number you listed is a rectifier. Not a rectifier/regulator.

A voltage limiter would be a rectifier/regulator.

Output is so small that 17 volts is typically not a problem. Certainly was not a problem back in the day when that motor was built. With today's modern electronics things can be different. If the battery is undersized it may contribute to a high voltage situation.

Output of a rectifier is pulsed DC. Is that what you are referring to as "spikes"?

The engine type is a yamaha 6H4 / 40HR 3 cylinder 2 stroke, no autolube, handstart, remote control, I am not sure about the year it was build, my best guess is 1985 - 1987.

From what I read and based on your comment it seems like there was no voltage regulator in the system just a rectifier and an oversized battery. Is this right or were there any optional regulators that could be connected between the rectifier and the battery?


Here are some measurements:


780 rpm idle, rectifier output voltage without load (open circuit):



780 rpm, rectifier output voltage, motorcycle battery connected:



4000 rpm rectifier output voltage without load (open circuit):



4000 rpm, rectifier output voltage, motorcycle battery connected (17V + noise):



Based on those measurements I would not be comfortable to connect modern electronics without a voltage regulator.

My idea is to replace the original rectifier with a "Linear" rectifier/regulator combination, which opens the charging circuit whenever the cvoltage gets too high instead of shorting the coils (like most motorcycle regulators). This way the coil should be protected from high currents / temperatures and the battery and electronics should be protected from overvoltage.

However, I am open to other ideas / suggestions, so if anyone has another idea, please let me know.

All Yamaha regulators are the shunt type. Current is shunted to ground to keep the voltage within limits.

Early USA issued 40 HP two strokes were delivered with a rectifier only. Later models started using a rectifier/regulator.

6H2-81960-00-00

Thank you for that information and the fast response! Then I might just exchange the rectifier to the newer part number that includes the regulator.