Re: High Voltage from Merc 40
You may not have a problem at all.
I am not familiar with your engine, but I know that an unregulated power supply can be all over the place. I have a regulated system and the manual talks current mostly. However in one paragraph, it says that if the battery has charged over 14.5v to replace the regulator/rectifier.
Well, to get that voltage, the regulator must put out somewhere around 16.5 volts at low currents, like are present when full charge is reached; less as the battery is charging; the more charge required, the lower the voltage.
A regulated supply regulates voltage and current. Just having a rectifier (as you have), neither is regulated and for an engine of your size, one would expect significant loading of the source during charging so the 21 volts wouldn't be there all the time.
I'd be more inclined to monitor the battery voltage, measured the day after you shut the engine down (gives the battery time to stabilize). I would not be concerned if it is as high as 13.5 volts. I usually use the number 12.75 as my "charged" number, but it varies with temperature.
The problem with overcharging a lead acid battery is that as you try to increase the cell voltage over the design maximum, the temperature of the battery increases rapidly and the water in the electrolyte/water solution boils out (you can see the bubbles in it) which eventually exposes the plates and once exposed they get too hot and burn up. You can monitor the solution level on most batteries and on sealed batteries (popular today) they resist water evaporation even with moderate amounts of overcharging. You can also put your hand on the side of the battery and if it is comfortable to the touch, you don't have a problem. If you have to take your hand off then yes you are overcharging.
As far as a stator malfunction, putting out too much voltage is not what I would expect as a failure mechanism. Just the opposite is what one would suspect.....nature of the way the beast is constructed.
The rectifier is just several diodes and all they do is convert alternating current from the stator to direct current (that the battery can use) and have no regulating ability, nor anything to do (really) with the maximum voltage available from the stator other than their forward voltage drop which remains pretty constant across the charging current range.
HTH
Mark
