A voltage sensitive relay (VSR) is the best way that can always ensure you to get the dual-battery vehicle started. It allows two batteries to be charged from your alternator.  Once your main starter battery is sufficiently charged, it will begin charging the second battery too. Fully automatic with emergency override.   Features: Fully Automatic: Automatically combines two batteries when charging and isolates the two batteries when not charging.  Unique Characteristic: There is no voltage drop compared to the battery isolation diode (0.6V).  Universal Size: Fits heavy duty modular fuse box series (BF43x) and battery switch, matching mainstream products in market.  LED Light: Built-in LED indication light (ON when VSR starts). It also has a remote LED port that indicates the status of the VSR.  Durable & Easy Use: The crust is made of rugged fiberglass reinforced nylon, surface or panel mounting options provide flexibility for installation.  Optional Storage Mode: When you do not need the VSR function, can cut the red line and connect an ON-OFF switch to control the power on and off.   Specifications: Surface Material: Glass Fiber Reinforced Nylon;  Nominal Voltage: DC 12V;  Combine / Open Voltage: DC 13.3V / 12.8V;  Continuous Current Rating: DC 125A;  Intermittent Current Rating: DC 140A;  Power Consumption: 10mA (Standby) / 330mA (Engage) / 0mA (Storage Mode);  Item Size: 7.2 * 7.2 * 6.1cm / 2.8 * 2.8 * 2.4in;  Package Size: 9 * 8.2 * 7.7cm / 3.5 * 3.2 * 3.0in;  Package Weight: 180g / 6.3oz;   Package list: 1 * Voltage Sensitive Relay (VSR);  1 * Set of Screw