Different power supplies change analog pin readings

raschemmel,
That makes sense. I will do that in the future. Thank you for the suggestion!

Keep in mind that when you screw up, you only have to answer to you.
As a professional Electronics Engineering Technician, working for Apple or Google, while they have plenty of money to buy stuff, if they order some expensive microprocessor board and I or any other tech blows it up
doing what you did, it will not bode well for our career with that company, if you know what I mean....
(so yes, we always check it BEFORE we throw the switch. Ideally, it is a two man operation where one person is operating the hardware and he requests a check by a second person to produce a witness that the voltage was in fact correct. When the person doing the QC says "yeah, it's good, your clear to turn on the power. " then he becomes the witness so if the boss says "Did anyone else check the power before you
turned it on ? " they are "off the hook" and the primary suspect becomes the board. (manufacturing defect)
Make sense ? Otherwise, it's just the one tech saying "But I checked the power before I connected the board !" and nobody to back him up. In my line of work, if some tech says "Can I get a witness ?" to you,
that's what he means. (he's not referring to a Southern Bible thumper)

“Bible thumper” describes the actions of a Christian fundamentalist, evangelist, or preacher who raps the bible with his knuckles to emphasize his sincerity while haranguing his audience, the congregation, or the crowd of curious bystanders.

Bible Thumper: "Can I get a witness ?"

Wawa! It works! I remembered that we had a spare Arduino at work so I borrowed it. It gives accurate and stable readings now. I attached my schematic. I found a program called Fritzing that I used to make this.

When it's convenient for you, I have a couple questions.

For powering the Nextion display and Arduino I'd like something hard wired instead of using a USB adapter. I have an extra one of these. What do you think of it? If this isn't good, do you have a suggestion? I looked and there are so many dc-dc converters available.

Also, I was reading that there can be spikes in car electrical systems. Do I need to worry about the connection between the VW battery and the Arduino?

Thanks!

raschemmel:
HINT: NEVER use variable resistors for a voltage divider, (especially cheap ones like those) !
(they will NEVER be consistent !)

That's equivalent to saying never use variable resistors!

Cermet presets are absolutely fine for this, and they are rock-solid. Witness all the cheap LM2596
buck converter boards on eBay that use 10-turn cermet presets for exactly this purpose.

You're right about not using cheap components for precision applications - but that ought to be
self-evident!