This Voltage Sensitive Switch (VSS) automatically senses the voltage of two connected batteries or battery banks and provides optimal, efficient charging of the second bank. The VSS allows for two independent battery banks to be combined or isolated based on the presence of a charging source (such as an engine alternator or battery charger). Once the charging source is activated, the VSS lets the first battery of the main bank (engine battery) recharge; when this latter reaches 13.2V, the switch opens the circuit and lets the second battery (service battery) recharge. The emergency parallel function can be operated manually by means of a knob or through an On/Off button on the dashboard. Recess or surface mounting.