While halfway through a 400-nm passage in the South China Sea, the wind died, so I decided to motorsail and punch through the sloppy, rolling seas. It was just before sunrise, and I hoped for the return of the wind with the coming day.
After 20 minutes, I noticed the engine tachometer dropping to zero and the engine panel lights dimming at the same time. This was happening about every 15 seconds. At the same time the alternator would cut out and begin its restart cycle. My diagnosis began with considering a tachometer failure. But I ruled that out since it was relatively new (1,100 hours), and its failure should not cause the panel lights to dim. I concluded that the dimming of the panel lights and the alternator’s cutting out actually were related, because the same circuit that supplies the engine panel lights also provides the field voltage to the alternator.
As an aside, this arrangement is convenient for energizing the alternator from the cockpit without adding a switch (and failure point) in the system. Prior to the last repowering, I was plagued with an engine that was on its last legs and had little power. So little power, in fact, that if the batteries were low and the engine started, then the 100-amp alternator consumed all of the engine power and would not drive the vessel forward at the same time. My nightmare was waking in the night and dragging anchor with insufficient engine power to get underway. Being able to turn off the alternator quickly and devote all engine power to the prop was the motive for this wiring arrangement. When I repowered, I continued the same setup.
Back to the scenario. The engine panel voltage, which comes from the engine battery, made me suspect the engine battery. Activating the digital voltmeter to the battery (normally switched off to eliminate the RF noise it produces), I noticed a fluctuation between 8 and 12 volts when the boat rolled with the waves. I ruled out an open or a short in the wiring, since that should cause the voltmeter to read zero. I finally concluded that the engine battery had defective cells, probably due to deposits in the battery case, which were shorting out two plates each time the rolling motion of the boat caused the liquid electrolyte to surge inside the cells.
I switched the engine battery out of the circuit and switched the engine to the house batteries. The problem was solved. With a sigh of relief, I continued on my passage across the South China Sea. After reaching port, I got to work replacing the engine start battery.