Karl Townsend pretended :
I'm looking for a device and don't know what its called...

We've got two batteries on a diesel salt water boat. Looking for
something to charge the second battery only after the primary unit is
fully charged.

Right now, there is an isolator (which is just a couple large diodes)
that appears broken from years of sal****er corrosion. We're looking
to replace this. The idea here is to run the starter off one battery
and make sure its charged before going to the user battery.

Karl

Common stuff in recreatioal vehicle world.
See the local supplier.

--
John G