If you have a power port on a newish car:
1. check if the port is powered when the car is parked, or not. Some are, others are only powered when the key is in the "accessory" or "run" position. The voltage at the port if the car is running will be >14V if the battery was not badly discharged just before the engine was started.
2. if the port is powered while parked, then it will measure <13.2V depending on how long the car has been sitting, and the state-of-charge of the battery. If you see <12.0V, the battery is low enough that the car might not start...
3. some power ports are live with the car parked, and with the engine running, but are interrupted while the key is in the "start" (crank) position. Some get powered as the key is turned to "run", and then unpowered while starting, and then power comes back as the key springs back to the "run" position. Some are also powered if the key is turned to the "accessory" position (i.e. the position you use to listen to the car radio while parked).
4. If you measure the battery voltage directly, you will see <13.2V if the car is parked. You will see >14V within a few minutes after start with the engine running. You will see the battery voltage sag below ~10.5V during cranking. The voltage drops between the battery terminals and a power port will be minimal, a few tens of mV at most.