>I am also working on an aux battery web page.  The scholarly
>dissertation to date is here:
>http://www.salicos.com/SecondBattery.htm

Tom, I've never had any trouble with the Surepower 1315's.

When you hooked it up, did you "tape up and ignore" the blue wire
from the Alternator?

(the 1315 should do it's thing by comparing the voltages of the two
batteries positive + terminals, no input from alternator or ign. key
is required).

Also, you don't need 8 gauge wire between batteries + terminals and
Surepower unless you want to use the "starter assist mode".   If you
don't need that feature you can use much smaller wires.