The VSR acts as an isolator, allowing the auxiliary battery (inside the box) to charge directly from the crank battery while the alternator is producing at least 13.2V (i.e. the engine is running). When the ignition is switched off and battery voltage drops below 12.7V, the VSR automatically cuts the link between the auxiliary and crank batteries to prevent the crank battery from being discharged.

This Voltage-Sensitive Relay can also be used in 12v setups without a Hardkorr battery box. 

PLEASE NOTE: this product is not suitable for vehicles with a variable-voltage alternator. Please check with your vehicle manufacturer prior to purchase if you think your vehicle may be equipped with a variable voltage alternator.