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Fair question and in some of my living hours, I have had the same thought.
Each system has their own regulator, as you mention, and likely they all see a high voltage, and respond accordingly. Current flows will be very low from each source, and the voltage may sit as high as 14.5 v (or higher if the battery temperature is low).
The system delivering the highest current will be the one that the regulator is in a charging phase with the highest voltage. As I understand systems may have 3 or 4 stage charging phases, and some may include a brief equalisation charge to reduce battery sulphation.
If the 12 v system is under low load (< 30 amps), and with these three charging sources, then each will contribute, again depending on their voltage phase. I have some kit to measure this, so next time I am on the boat I will measure. Under a heavy load (say bow thruster, engine starter, or recovering an anchor (while sunny and on shore power)), then battery voltage will drop and all three will contribute their maximum amps (bulk charging phase). In our boat this would be 60 amps from the alternator, 30 amps from the shore charger and 20 amps from solar.
If the system voltage has dropped (<12.8 v) so that the VSR activates then the house and engine batteries will be operating as separate systems. In this case the alternator will charge the engine battery until it rises to above 13.3 v (or so), and the shore charger and solar systems will continue to charge the house batteries. Above 13.3 volts the VSR will recombine the systems, and we are back in the paragraphs above.
Happy to be corrected, but that was my thinking.
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Hanse 345 - Tenacious. Sailing on the Solent and now to the west coast of Scotland.
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