For automotive circuits, we recommend an auto-ranging meter with at least 20MΩ of internal resistance. Make sure to use the correct multimeter setting for a car battery.

Still, the time it takes to charge a 12V car battery will depend on three variables: the battery capacity, charger output, and the state of discharge.

Using some alligator clamps and a multimeter, Big Mess o’ Wires figured out much the USB charger draws in standby mode, then figured out what effect, if any, it’d have on a car’s battery: Is ...