How the system works varies between EVs, but generally speaking the 12V battery is trickled charged by the main traction battery when the vehicle is in use, but is not charged when the vehicle is off ...

[Robert Nixdorf] frequently needs to use this high-end audio recorder, but it sucks dry a set of eight AA batteries in ... power bank to provide 12V output to Quick Charge his audio recorder.

Optionally, you could use a handheld volt meter to determine your battery’s state of charge. Touching the two prongs on the .

For comparison, many USB ports in cars output just 0.5A, which means your phone battery ... larger than the 12V socket itself, yet have one or two USB ports for powering or charging two devices ...