Hey Tom, et al

The Sure Power battery separator you're using is a definite improvement
over the simple battery relays like the Hella or its higher-powered
cousins.  It allows higher power charging of the auxiliary after your
starting battery has been nearly fully charged, which usually happens after
only a mile or so of driving.  This can greatly ease the load on your
alternator.

The battery combiner from West Marine is similar in operation, but appears
to use a solid-state switch instead of a relay.

As Mr. Bill points out, though, there is almost always a down side.  In
this case, the increased complexity of the control circuitry over a simple
relay or diode isolator means more electronic parts to fail.  But the
enhanced performance and elimination of alternator overloading likely more
than compensates for this.

Stanley