Dual Battery Selector Switch – Cup Mount. The Perko Dual Battery Selector Switch allows the addition of an extra battery for accessories. This switch provides the flexibility of selection between batteries or combined operation of both batteries in parallel for starting or charging. It can also serve as an emergency main disconnect switch in the event of an electrical problem. All models are make before break design and are ignition protected.. Features: For two or more batteries. Unique oval shape mounts in a round hole. Removable actuator key.