This is very good. The accuracy I determined was within .2 volts on a 12 volt battery. I have another model, which I would strongly recommend, with an on/off switch that I prefer because you don't drain the battery with the LED readout always staying on. I use this device to check battery health on my motorcycle batteries when I store them over the winter. Therefore, having an on/off switch is beneficial so that I don't have to always disconnect this device from the battery when I need to check the voltage. I can just leave it on and take it off when I put on the battery tender If you plan to use this for the intended auxiliary USB ports, then connecting it to the ignition will prevent drain and you can use this without issues.