Mariner 60 efi charging gremlins

[boyscott]

I have a 2-battery system controlled by a Make-before break 4-way switch (1,2,Both, Off)
Both batteries in good health – 12.5v resting and plenty of electrolyte.
I have a voltage display on the fishfinder in case I get low and normally when the engine is running it reads about 14.3v but when I was out last time it started fluctuating and tripping the alarm below 12.3v.
An engineer checked the engine charging was ok but said I need an intelligent switch to control which battery is getting the charge. While this may be ideal when running with the switch set to “Both” I generally have it on one or t’other so don’t see that it makes much difference. I checked all the connections and tried again but the same problem occurred after about 5 mins running.
Would be grateful for any thoughts or how I can diagnose the fault.
Thank you

[BrianR]

As I suggested on the PBSBAC site, if the same fault is there with either battery connected, I would suspect a faulty switch. Try connecting one battery direct, bypassing the switch.

The suggestion about an "Intelligent switch" is a load of rubbish, as the fault occurs with only one battery conected.

[boyscott]

Thanks - you confirmed my thoughts about the "intelligent" switch. I'll try the tests recommended on the batteries and on the existing switch.