Here's some info from the ASE Supply website on the 1315. HTH. Bit spendy,
aren't they?

The BATTERY SEPARATOR is designed for use in multi-battery applications as a
solenoid priority system to protect the chassis charging system from
excessive loading while allowing auxiliary batteries to be charged. The
BATTERY SEPARATOR has two basic operational characteristics: ASSIST IN
ENGINE STARTING When the starter is activated the BATTERY SEPARATOR compares
the voltage of both battery banks. If the chassis� battery is lower than the
auxiliary battery bank, the BATTERY SEPARATOR will engage allowing the
auxiliary battery bank to aid in vehicle starting. The start signal must be
at least three volts for the operation to occur. PROTECT THE CHARGING SYSTEM
The BATTERY SEPARATOR monitors the battery system to determine if the
batteries are being charged. When the engine or auxiliary batteries (if
1315-200 is used), or the engine batteries (if 1314-200 is used) reach 13.2
volts*, indicating charging is taking place, the BATTERY SEPARATOR will
engage, joining the two battery banks. If the drain on the charging system
by the auxiliary or main battery bank reduces the system voltage below 12.8
volts*, the BATTERY SEPARATOR will disconnect the battery banks from each
other, thus protecting the respective battery banks from excessive drain. A
delay function has been incorporated in the control circuit to prevent the
BATTERY SEPARATOR from reacting to momentary voltage fluctuations and
chattering. The priorities are to assist in engine starting, if required,
and to protect the charging system from excessive power drain.