For peace of mind, this kit is a must-have when adding additional electronics to your boat (stereo upgrades, light bars, etc.). This kit comes complete with a BEP Marine® Battery 701S™ Battery Switch, a BEP Marine® Voltage Sensitive Relay (VSR), and a GIL Marine® Composite Battery Box plus all necessary wiring and hardware. This kit will allow you to keep one battery isolated at all times for reserve purposes. The VSR will allow the engines to charge battery #1 until it is topped off at 13.7 VDC, then automatically switch over to battery #2 to charge it. Turn the knob on the battery switch to select using battery #1 by itself, battery #2 by itself, or both batteries together in parallel. You can also switch off battery current altogether. Professional installation recommended. Marine batteries not included. Fits any single battery boat with room for another group 24 battery.