Review on Quick Charge 3.0 12V USB Outlet: Dual USB Car Charger with Power Switch, LED Voltmeter, Waterproof - Ideal for Car Boat Motorcycle RV ATV by Tony Wylie

When connected directly to the battery (via the supplied fuse) you can turn the USB charger off with the power switch so it doesn't drain the battery. When connected to an auxiliary circuit that operates only when the ignition switch is in the ON or ACCESSORY position, the power switch can still be used to turn the USB charger on and off, but the USB Smart Charger will remember its previous state, when the power is on applied once to the next. This is a big advantage. I wanted the USB charger to turn on with the ignition and turn off with the ignition and it does just that. I turn it on once and it remembers when the power goes out. I didn't want to be forced to turn on the USB charger every time I turn the key in the ignition. Despite being an electrical engineer, I somehow managed to connect my USB charger with the opposite polarity during the pre-installation check. derp It didn't work, but my temporary stupidity survived. There should be a diode to protect the USB charger from clumsy mechanics. It should be, but sometimes manufacturers cut costs and do without protective devices. This appears to be a reliable and durable design. Voltage readings are reported to within 0.2V of actual voltage, and my USB charger is matched with another meter to be at least 0.1V accurate. Only minor complaint would be slow. Refresh rate with multiplexed LED voltage display. It looks fine when stationary, but the numbers are fuzzy on a motorcycle or other vehicle with significant vibration.