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