When it comes to Idiot Lights, exactly who’s the Idiot?

 

(WARNING: TMI on boat DC electrical systems ahead.)

Since shortly after we bought the boat we have been frustrated by an apparently false alarm from a battery voltage sensor on the starboard engine.  We say apparently false because the engine runs fine and the starboard starting battery always seems to have adequate voltage.  And the digital voltage gauges on both engines show nearly identical readings.  But, boy that flashing light (and its now-silenced horn) has been annoying.  Last year, we even had a new alternator installed and had the alarm issue checked subsequently by two mechanics, but to no avail.  The little light stayed with us. But things ran fine so we left it for the winter to-do list with our chosen engine specialists here at Herrington Harbour North Marina.

And boy were WE wrong!

The boat electrical expert that (finally)   came to look at it could find no problems with the warning circuit.  He fed it proper voltage and it went out.  But he did deduce that the new starboard alternator we bought last year was producing no power.  In 2020’s COVID-disrupted supply chain, we had settled for a less expensive but available, aftermarket alternator manufactured in an unnamed  Pacific Rim country that doesn’t recognize Taiwan independence.   And we paid the price with a very short lifecycle.

BUT!

It turns out that the boat’s DC electrical system is designed in such a way that it can mask this type of failure from the uninformed (i.e. us and apparently some mechanics).  

To WIT…….

(WARNING TMI gearhead stuff ahead!)

The boat has five separate battery banks.   Port starting, Starboard starting, Generator starting,  Bow thruster, and House (lights, fridge, electronic, etc.)   These are all connected to a beast called a battery isolator, a bit of diode-infested magic that distributes incoming charging current to the various battery banks according to their need.   Charging current can come from onboard battery chargers when we have shore or generator power available  OR from the two (count ‘em, two)  engine alternators when we are underway.  By this configuration, the two alternators provide some redundancy.  One can feed a marginally adequate power level to the isolator to keep all the batteries at a minimal charge level, even when the other alternator is not functioning properly.  (But it takes both to really charge things up.) 

And THEREBY hangs the tale!

While the starboard alternator had failed, the port alternator was shouldering the whole charging load, including keeping the starboard starting battery at an adequate voltage level.  And little did we know that the warning light (Despite displaying a battery symbol. Hello Volvo?) was really warning us about no output from the starboard alternator.    So, we sat fat, dumb, and happy seeing 12.5 volts on the starboard battery gauge, even in the presence of the Idiot Light.  Logical conclusion = false alarm.

WRONG, bucko!

Bench testing of the knock-off starboard alternator verified its failure.  And the installation of an OEM-spec alternator Friday morning (and a corresponding lightening of our wallet) now gives us 13.8 volts on the starboard battery voltage gauge and NO WARNING LIGHT!   

YABBA DABBA DO DO DO!

We would have only noticed the underlying truth if we lost the use of the other alternator and after both engine batteries died.  But by then, of course,  we might have been stranded at sea……. 

One can almost hear the over-worked port alternator sigh with relief.  

So, who’s the Idiot?  Not us! (At least not anymore. 😊 )

 

Thanks for following.