Mercury - Voltage Gauge 24V 16-32 Volt Range White Face 2-1/8" (52 mm) 79-8M0069412. Overview. The Mercury Voltage Gauge 24V (79-8M0069412) gives you a reliable view of your boat's electrical system at a glance. Built to monitor voltage between 16 and 32 volts, this gauge helps ensure your 24V system is charging and performing as it should. It features a white face, black numerals, and a glass lens for clear, easy reading and matches other Mercury OEM gauges perfectly. It fits any 2-1/8" (52 mm)