An Overzealous Alternator

Ed Douglass of Sturgeon Bay, Wisconsin, asks:

“My Westerbeke/Universal M-35 diesel engine came equipped with a Mando 51-amp alternator. From its appearance, I would say it is an automotive-type alternator. It puts out a constant 14.6 volts, which not surprisingly causes my wet-cell batteries to boil. It does this even after I have charged the batteries to capacity with my shore-power charger. The regulator is internal to the alternator and is machine-sensed. I have looked extensively for a replacement regulator, but it appears none are available.

I have followed the diagnostic procedures described in the 2nd edition of your Boat Owner’s Mechanical and Electrical Manual. Nothing is wrong with the wiring. I took the alternator to a repair shop, where it was mounted in a test jig. It put out a steady 14.6 volts. The “set point” for this alternator, according to the specifications book, is 14.6 volts. The technician was satisfied that the alternator was working properly.

Your book does not mention “set point.” You do say, repeatedly, that an alternator with an output of more than 14.4 volts is bad for wet-cell batteries and deadly for gel-cells. I am concerned not only about the batteries, but about damage to electronic equipment and LED light bulbs.”

Nigel Calder replies:

I don’t know enough about the guts of a voltage regulator to know if, or how, you could adjust the regulated output. What you could do is induce some voltage drop between the alternator and your batteries, which is the opposite of what we normally try to do! A voltage drop of 3 percent would knock you down to around 14.2 volts at the batteries. One way to do this would be to insert a battery isolation diode in the circuit. These typically introduce a voltage drop from 0.3 to 0.5 volt, but you need to check this, as some of the newer isolation devices have little or no voltage drop

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