Review on 🚗 MICTUNING LED Digital Double Voltmeter: Round Panel Voltage Monitor Blue for Cars, Pickups, RVs, Trucks & Dual Battery Packs by Michael Norwood

I use it on my ATV to measure voltage on the main and winch/accessory batteries which are separated by a battery splitter. If the main battery is not connected, you cannot see anything from the second battery either. The main battery must be connected to see the secondary battery. At first I thought it sucked, but actually it's the perfect design. If you connect it directly to the battery to measure the voltage, it will be on all the time and drain the battery over time when not in use. So you need to route it through a relay controlled by the power switch. As the connection to the main battery controls the display you only need 1 relay to prevent the batteries from being drained. If both turn signals were completely independent, you would need to connect 2 relays. If you use batteries of different types/sizes/ages, don't connect them without some kind of battery isolator, otherwise if one of them fails, the good etc. output will be discharged without one.