Hi
"If there are two diodes at the panel, these are usually in parallel with 6V sections of the panel"
Nope, the ones in the junction boxes are definitely in series with the whole panel. There are two possible places to connect the outgoing cable in each junction box on my panels. One each side of the diode. Connecting the cable to the other side of the diode removes it from the circuit, so my question remains, is there any good reason to have a diode in series on both the positive and negative side?
"Adding a diode in a minimal system would probably not add any advantage."
My question was: "How much DISadvantage does it add, as I want to use diodes to charge two totally independent battery banks at the same time. I know there are PV regulators available that have outputs to charge more than one bank, but they cost money and I assume they use diodes to split the charge anyway?
I do not want these banks connected in parallel under any circumstances, not even through a relay, but I would like to have some of the energy from my PV panels diverted to the starter battery to make sure that is always topped up, even in long periods of not using the engine. And, I don't want to spend any more money than I have to. Unlike an RV, a yacht can not pull over to the side of the road when something happens at sea, and there is no towtruck to call. The starter battery is used only for cranking the engine and nothing else.
For the engine alternator, I use a charge splitter like the one in the photo, a smart regulator and sense the battery voltage direct from the batteries which compensates for the voltage drop through the diode. However, I try to avoid using infernal combustions to charge my batteries at all cost. My engine alternator may be off grid, but it is not renewable energy, and... it's noisy and smelly and the engine warms the boat up, which I don't need in the tropics.
Regards,
Owen Morgan
Yacht Magic
Falmouth Harbour, Antigua