Karl Townsend wrote:

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



Battery isolators are readily available from normal marine component
sources. It is also common to use the 1, 2, 1+2 high current battery
selector switches to select the battery or batteries you want to run off
of manually.

If you use a diode type isolator, be sure to wire the sense lead from
the alternator directly to one of the batteries (doesn't matter which
one) *after* the isolator so that the charge voltage is regulated
correctly taking into account the diode voltage drop.