J & J Bavaria Battery selector switch

[ICENI]

I am considering adding a third service battery to my battery bank.

My boat (purchased in July) has 2 existing service batteries installed beneath the saloon settee berths.   I will be fitting another battery selector switch and need to order it.   Before I do, I need to ascertain the type of selector switch Bavaria fitted to the boat.   It is coloured yellow but I feel that it only selects battery 1 or both batteries?   (Not 1, 2, or both as is usual)

The boat is laid up miles away from my home so I am hoping someone can advise me regarding this please?

Before retiring I was an electronic engineer so I know what I am doing.   I do not, however remember the DC wiring in respect of this issue.

[Symphony]

There is no selector switch - just an on/off switch in the negative. One battery is just for start and the other for house and they are charged through a split charger, probably a diode. If you just want to increase your house capacity then wire your new battery in parallel with the existing house battery, although adding a new battery to one of unknown age is probably not a good idea. The OE set up would have been a start battery of around 80AH capacity and a house of 110AH, although of course they may have been changed over the years. You may wish to consider changing the switching and charging arrangements to individual isolators for each bank plus a parallel switch and a VSR to manage the charge. Something like the BEP Marine switch cluster is a neat way of doing it