I agree it is an interesting one. Although the problem hasn’t recurred (I was back at Rhu last weekend, and connected up to the shore power with no problems) it would be nice to understand the mechanism by which it happened.
The charging system is a split system. I’m not sure of it is split through diodes or a VSR. It could be the latter, as the engine battery always takes priority.
I have just been through the unhappy determination of unserviceable batteries.
The first thing to understand is that the modern charging systems will keep batteries in reasonable trim so long as they are tied to an almost constant mains supply or charge regime. However the voltage (pd., or potential difference) will drop off quite quickly when charging by any mode ceases. Once mine reached 10 volts unassisted (ie., when sailing) my radar and instruments began to malfunction. I am using 2 Silver-Alloy, sealed batteries of 170ah’s, these have a window showing the state of charge that under certain circumstances can mislead however if a disconnection of charging is introduced then the window colour will be found to change quite quickly from green (good charge) to black (requires charge) and when out of service to white at which point they are ready for the knacker’s yard almost without hesitation.
I have now adopted (due to no further options in Greece) to using Bosch Tecmaxx Silver-Alloy batteries of 170ah’s, the deep cycling or not of these batteries is a unknown or at least that is the conclusion my research has reached.
Not too sure if this helps but basically unless the battery holds 12.5 or more volts for more than say 48 hours unassisted and without load being applied it is likely on it’s way to the NY.
SV., Pickled Tink.