Mercury - Voltage Gauge Gray Face 2-1/8" (52 mm) 79-8M0065963. Overview. The Mercury Voltage Gauge (79-8M0065963) lets you monitor your boat's charging system and battery health at a glance. It's designed with a clean gray face, bold white markings, and a glass lens for easy reading in any light. Built to fit a standard 2-1/8" (52 mm) dash opening, this gauge ties seamlessly into your SmartCraft-capable system when the proper voltage sensors are available. It's reliable, easy to install, and des