VSR automatically combines two batteries when charging and isolates two batteries when not charging. Built-in LED indication when VSR engages. Voltage Sensitive Relay (VSR) / Automatic Charging Relay FEATURES Automatically combines two batteries when charging and isolates the two batteries when not charging. No voltage drops comparing to battery isolator diodes (0.6V). Dimensions fits Heavy Duty Modular Fuse Box Series (BF43x) and Battery Switch, matching mainstream products in market (BEP style) Robust Glass Fiber reinforced Nylon enclosure Built-in LED indication (ON when VSR engages) Port for remote LED indicating VSR status (LED not included) Surface or panel mount options provides flexibility when installing Optional Storage Mode allows zero power consumption when engine is off. Wiring Example for BF451 VSR  SPECIFICATION Nominal Voltage | DC 12V Combine / Open Voltage | DC 13.3V / 12.8V Continuous Current Rating | 125A Intermittent Current Rating | 140A Power Consumption | 10mA (Standby) / 330mA (Engage) / 0mA (Storage Mode) CERTIFICATION