When the engine is started, starting and auxiliary batteries are separated allowing only the starting battery to be charged. When the charge voltage reaches 13.3 volts the voltage sensitive relay engages allowing both starting and auxiliary batteries to be charged. This is indicated by the red light in the top LH corner being illuminated.
When the engine is stopped and the battery voltage falls below 12.8 volts the voltage sensitive relay or “VSR” disengages separating the start and auxiliary batteries. The auxiliary battery continues to power any accessories without risk of draining the starting battery.
The Enerdrive VSR “Voltage Sensitive Relay” also has “Dual Sensing” which senses the voltage of both batteries. If either battery is receiving a charge, e.g. a battery charger is connected to the auxiliary battery; the VSR will engage and charge both batteries.