The hidden missing black cable, 25 May 2021

I started recently having a new electrical problem when starting the engine. An alarm would be triggered for the engine alternator producing more than 15 volts of power. A red scary beeping alarm triggered the minute you start the engines. And by the way, I had to start the engine in the middle of the night while sailing through the Raz-de-Sein because of a general battery failure...

The Mastervolt alarm message is a bit cryptic. The only understandable part is "hypertension". A as common problem for boats as for humans?

To make it more dramatic, a second alarm on the electrical panel is triggered: 15 volts. This is clearly too much, but what can one do while sailing? First, switch the alarm off. Then try to figure what may have become wrong.  
 

As usually, I started with a visual inspection of the battery bank for corrosion, loose connexion or anything else. But all looked Ok!

The battery bank and switches

I spotted the connexion of the minus cables together using a very funny (unprofessional?) approach. On the left plot there are three cables attached, and on the right one, four cables, including the black cable bridging the two plots. Thinking about potential corrosion, I started unscrewing the wires, just to notice that one of the bottom black cables on the right had slipped away from the plot and was no longer connected to the others! I tried to fit it back, but clearly there was no way to fit four cables in this plot. This is why I used a serflex hose clamp to clamp the missing black cable with the other outside of the plot, as you can see on the figure below.  

The minus (unprofessional) connexion fixed with a serflex hose clamp

Today, with the electrician onboard, we tested the alternator, which was providing constant 14 volt power while operating at any rotation speed. And just to check, we removed the faulty black cable, which immediately reproduced the problem of generating higher unstable voltage. The explanation seems to be that with the black unplugged, the controller of the alternator becoming blind is producing as much power as it can, without further regulation.

The fitting of the black cables on a correct bus, as it should have been done from the beginning.

These kind of issues should have been fixed free of charge by the company building the boat, as it was an obvious poor connexion with one missing cable. But it was not the case...