A solenoid which is wired first in the 12v system that, when activated, opens or closes and turns 12v power on or off to that system. Found mostly on Motorhomes which will incorporate 2 disconnect systems, 1 for the House batteries and 1 for the Chassis Batteries. usually controlled by either, manually opening or closing the solenoid by turning it or moving a lever, or electronically operated via a remote switch mounted inside the recreational vehicle. with a disconnect turned off, that battery circuit is ‘dead’ and no power will be available from the batteries. Generators and starter motors usually bypass these systems due to high power demands.