If you leave your 2013 plugged in it will maintain the 12v battery at 13.0 volts. I don't know for sure where it gets the voltage from as the high-voltage contactors are opened after charging is complete, but I've measured it. I keep an accessory plug voltmeter plugged into the outlet in the top dash cubby. Interesting to see the voltages the car runs at, they change according to OAT, higher in winter, lower in summer. You can monitor battery voltage after you turn the car off during "retained accessory power" and observe battery voltage drop. Turning on the headlights for a few minutes gives a moderate load test.