On Thu, 16 Dec 2010 09:05:40 -0700, "Bob La Londe"
wrote:

"Karl Townsend" wrote in message
.. .
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


Its called a battery isolator. They sell them for pickups with campers all
the time.
But they do NOT function the way the OP envisions.

They charge both together, while allowing the starter to only draw
down one battery, and "house" loads to only draw down the other.

Now, if one battery is lower than the other, it WILL take the lion's
share of the charge.