I would echo what others have contributed about not routinely drawing
from the house battery for starting if you set up as you describe. What
surprises me is that almost no one selects a setup I prefer: a
large-ish bank of house/starting batteries, without switches, relays, or
problems. "What if you drain the batteries down?" For this purpose,
and for this purpose _only_, I bought a cheap starting battery.
Unconnected to anything. Once a month, I would affix heavy gauge jumper
cables from it to the main bank during a 6-hour motor to re-charge the
self-dischaged capacity. Left the cover off the battery compartment
while under way so there was no way to forget to remove the cables
later. ("Gee, why are the batteries visible from the saloon?") Cables
served for the emergency start, if needed, connected directly to the
starter solenoid.

HTH,
Rich Mechaber

bruce wrote:

I am redoing the wiring on a small powerboat (6.2 meters). As part of
this I intend to use a dual battery system, I have seen a number of
different circuits that would work. The circuit I prefer users a
latching relay, which is activated by the ignition switch, this places
the batteries in parallel when starting and allows the charging of
both batteries at the same time. When the ignition is off the
batteries are separated allowing one be the house battery without
flattening the other.
At long last comes the question; if the house battery is flat what
effect will this have on the other battery during starting, charging
and the circuit in general.

Thanks in advance
Bruce