The 1315-200 (bidirectional) are 200A battery separator modules with an integrated relay for 12V systems. The units monitor both the main and auxiliary battery banks. If the main battery (for the 1314-200) or either battery bank (for the 1315-200) is above the connect threshold, the relay connects the two banks together. If the main battery  (for the 1314-200) or either batteries (for the 1315-200) are below the disconnect threshold the unit will open the relay. The connect threshold is set to a nominal voltage of 13.2V, which would only be reached when the charging system is operating. This will cause the relay to close and the charging system can charge both banks of batteries. The disconnect voltage is set to a nominal 12.8V, which is near the full charge resting voltage of the batteries. This will cause the relay to be opened shortly after the engine is stopped, attempting to preserve 100% of the starting battery capacity for engine cranking.  When the engine is started, the start signal causes the controller to check if the auxiliary battery is within about 0.85V of the main battery; if so, the relay contact is closed.  Thus providing the starter system with maximum battery voltage available.