Alarming development!

[Seabbatical]

[Please note that this post is a revised and updated version of one that was previously posted in the Electronics forum, as this forum may be more appropriate.]

On my 2006 Sea Ray 270 Amberjack equipped with a Mercruiser 350 MAG MPI Horizon and B3, I am experiencing the following issues w/ the Mercury Smartcraft alarm:

• With the ignition switch turned off, both batteries are disconnected from their cables, shore power is turned on, and the battery charger is turned on at the AC/CD distribution panel, the Mercury Smart Craft emits an audio alarm. It is was not a periodic beep, but a continuous ear splitting alarm.
• With the ignition switch turned off, both batteries are disconnected from theur cables, shore power is turned on, the battery charger is turned OFF at the AC/CD distribution panel, the Mercury Smart Craft does NOT emit an audio alarm.
• With the ignition switch turned off, battery #1 is connected to the cables, battery #2 is not connected, shore power is turned off, and the battery selector is turned to off, the Mercury Smart Craft emits an audio alarm.
• With the ignition switch turned off, battery #2 is connected to the cables, battery #1 is not connected, the battery selector is turned to off, and shore power is turned off, there is NO alarm.
• With either battery #1 or battery # 2 connected, shore power turned off, the battery switch set to the corresponding connected battery, and the ignition switch set to on or the boat is actually running, the Mercury Smart Craft gauge does NOT show any error messages. It indicates that everything is OK. The gauges also appear to be indicating normal readings, and the engine seems to run fine.

Please note that since the development of these issues, both batteries have been replaced w/ new ones to eliminate any issues w/ them.

Also, I have located the Mercury Smartcraft alarm. It is right behind the tach gauge and is attached by two short wires to a bundle of wires emanating from the gauge. Neither wire seems to be grounded.

It seems as if there is some thing very specific causing the above observations.

Thanks in advance for any suggestions on where to begin to tackle this issue,

SeaBat