@Tony Brooks Thank you for this information, it’s super helpful.
The inverter is 400W, so I trust not likely to overheat the switch.
The boat originally came with two solar panels, each of which was charging one of the batteries only. But then the seller put the 4-way switch in and wired them both via a MPPT controller to the switch output, so I could select which battery to charge. This was intended as an improvement.
I hadn’t registered that if the switch is off no battery will be charged! Though I will be living aboard, so it is unlikely I shut down the batteries for an extended period.
By by the way, is the output of the 4-way switch what you are referring to as the alternator terminal? Sorry I’m not completely up to speed with the wiring. I presume that it connects to the alternator, engine and the main switch panel?
it is a single alternator boat, so your wiring suggestion would mean I lose some voltage through the VSR, but I wouldn’t have to think about which battery to charge if moored or cruising etc, right? So without the vsr, the engine battery wouldn’t get a solar top up when moored, unless I selected both. But then it would also be drained by domestic use. Also if I’m cruising and have the switch set to 1, the domestic battery won’t get a top up from the engine. Is this correct?
I haven’t really got going yet, so haven’t figured the pattern of my needs. I think I will delay this decision until I know better how I use the boat.
Please can you explain why the inverter fuse must be outside the battery box in order to satisfy BSS? How is that safer?
thanks again for these comments.