"Nigel" wrote in message
...

I think the system has a positive earth,

That would confuse things! What kind of boat is this on?

What confuses me the most is why both the Positive and the Negative sides of
the battery are switched. In a typical battery installation the negative
side of all battery banks gets tied gets tied to "ground". If it was a
postive ground system then I would expect the same except it would be the
positive side that is connected to ground.


Ignore for the moment the fact that the batteries have a switched negative
side. The fact that the output of the switches have their negative side
tied together suggests that the system is negative ground. That wire that
connects the two negatives together is NOT what you want to replace!

The voltage sensitve relay needs to connect between the positive terminals
of the two batteries, preferably between the switch and the battery. In
order for the two banks to charge together regardless of the switch settings
the negative sides of thebatteries would need to be tied directly together.

So, the easiest way to install this relay is to add a BIG wire between the
two negatives, then wire the relay directly to the two positives

Rod