Yes, I really do "need" a 12V USB charging adaptor with a built-in voltmeter.
Driving my VW camper van around, I frequently swap the USB charger with a voltmeter. I'm always curious about how much current my alternator is putting out, and if my house batteries are charging up. I also worry that my kid may plug into the wrong 12V outlets and drain the starting battery. This charger lets me know what is going on as I charge. I NEED TO KNOW!
There are a few chargers with voltmeters, I picked this one as it has both 2.4A and 1A charging, the swivel lets me angle it so I can read it while driving, and the display is super easy to read/understand. When the engine is running the voltmeter tells you what current it sees at your 12V port, with the engine off it's reading the charge from the battery.
Thanks to help from our readers, I successfully put together and sized my house battery, solar panel and portable fridge to allow me uninterrupted cold storage, regardless how long I stay out. This voltmeter helps me manage the system. I'll write up a post what I ended up going with soon.