Review on Premium Waterproof 12V DC Voltmeter Digital Display Battery Tester for Marine Car Truck Boat RV - Accurate Voltage Gauge! by Glenn Cho

If you have a single battery device this is a great product for basic information. However, I installed three blocks on the battery to start the boat and trolling motor. The first problem is that there is no fuse, so you have to buy and install a fuse. The second problem is that there is no switch. So if you connect them directly to the battery, they'll be on all the time. If you connect them to the boat power supply, you don't control the "offline" battery and at the end of days. Fishing the last thing you want to find is your starter battery is dead! My solution was to connect them via relays which are powered by the mains after the battery is disconnected, so whenever you have the battery "on line" the relays will turn on and the indicators will turn on when you disconnect the batteries isolate to return home. Then the relays drop out and the indicators go out.