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