​RE: 1314 or 1315
1314 is one way, engine battery to aux battery only.  1315 is two way.  If
charging the aux battery (solar?) then when aux battery reaches full charge
(13.3 v?) the charging source will also top off the engine battery.
But some have argued that this takes away from getting full charge going to
the aux battery.
I have been running with the 1315 for several years now (and with 120 watts
of solar) and have wondered about this issue.  But with the 1315 and solar
(plus a battery charger wired in for when I have shore power) I have the
assurance that my engine battery will start the engine when I need it.
YMMV