T&T: (Potentially) Catastrophic Electrical Shut-down

[Robin]

When you returned to your boat and found the acrid smell, likely your
batteries were low on electrolyte and probably quite hot...it's quite possible
some plates warped causing internal shorts and final death and some addtional
heat. At this point they were effectively "gone" or close to dead.

Under such circumstances, even with plain old worn out batteries, monitors
will often show full charge... incorrectly. All they know is that at some
voltage little current flows. It looks just like a fully charged battery. But
when disconnected from the charging source, battery voltage drops rapidly and
little power is available.

I'm assuming your boat is gas powered  or electronically controlled diesel as
a mechanicallly controlled diesel will run with no battery power. An advantage
of old diesel twechnology!!

Apparently, as you noted, power was lost when you reduced speed...likely
because alternator output was close to zero and dead batteries could not keep
the ignition hot. Even if you were charging multiple banks via a combiner, it
would have disconnected when voltage dropped and your dead start batteries
would still have killed your engine(s).

Usually, nearly dead batteries look more like an open than a short so charging
virtually stops.,.few ions for anything ....current drops to a low
level...hence I'm not sure your inverter/charger would have been putting out
high current...only the  modest level  equalization current....

Likely with new batteries everything will be ok.

Your experience is one situation which points out the potential value of
separate battery banks for twin screw boats....hopefully banks purchased at
different times don't die simultaneously...

Glad you made it thru this without serious consequences. It's another reminder
how careful we all need to be when boating.

Rob Brueckner
Hatteras YF