Area-51m R?, battery

Maybe you're installing the back panel wrong? it could be shorting something inside the motherboard when its on.

Have you ever played a video on the laptop? like those 10 hour videos until the laptop dies, then you hold the power button. and then plug in the AC adapter?

So, since you got it. It's never worked? Diagnostically, it makes a difference.
You said it would charge both batteries (100%?) if the back-panel was removed.
After you leave if off overnight.....you turn it on in the morning, how long does it take to get the "overheated" error? Latest Bios and all firmware/drivers? This doesn't sound like a hardware issue.

Yeah I tried that and it took me 2 days to get it to charge past 0% and got it to 40%

I did nothing is. Also I’ve tried with the back off and same thing.

It will also turn on without charger and say battery temporarily disabled will turn on when back to operational temperature, and then still turn on! Like? I can use it after it says that and stay on the laptop. But then when I plug it in, it won’t charge. 

Maybe I didn’t word my issue good enough lol sorry. So it’s been perfect for a year now, I love it. No errors nothing. Then one morning it didn’t charge and I was like hmm weird. And while using it it updated and I reset it. This was the first time I seen the battery overheated and temporarily disabled message on startup. So I bought a replacement battery. Same issue. So I assumed when it charged once with the back off that was it, no. Tried it again and said overheated battery I’ll just keep plugging and unplugging the charger and battery cable that goes to the battery / motherboard until it says charging and hope it keeps charging. I have updated everything possible. I went to dell website and did auto it says no updates, I did manual to download things to get them up to date. Still same issue. It just weird that it charges sometimes and sometimes says overheated battery and the battery is cold to the touch. So that’s why I believe it’s the battery to motherboard cable, otherwise it’s the motherboard but I’m not certain. I had a tech friend look at it and he said it makes no sense either. So possible defect? Connection error?

To verify, you are talking about the external cables, yes, not the internal cables?
Simply put, it will either be that or the updates. The fact that a hardware issue happened at the same time as an update, is ridiculously low. The issue began before your updates, yes?
My machine rarely leaves the desk...ever. Do you unplug/plugin every day, meaning, you suspect a worn-out connector, internally or externally. Yes? 

Wait...what if this IS caused by a software/firmware update. The charging works, the battery, works, the system works...it just gets an error code so it refuses to charge. It could be the hardware is fine and the system keeps getting the wrong "battery status". Does this sound correct?

I’m talking about internal cable, from the battery to motherboard. And I only use one of the two charging ports and mine does not leave the spot it’s in either. So that’s why I’m lost. It can’t all of a sudden be both charging ports when I only use one. And sometimes charges and every other time say battery temporarily disabled. It’s ridiculous. And no I don’t suspect that, I did order new laptop jacks to make sure but no, I don’t use 1 of them and don’t take it out and in all the time. That’s why I assumed the connector cable from battery to motherboard, the internal cord.

Sort of yes, if I’m in bios and it’s plugged in it says temporarily disabled and if I unplug it changes to battery healthy. I’ve updated firmware and Reset it to basic. With no added software added. I read somewhere a mosfet could be bad? Like regulating voltage but I’m not that tech savvy. Lol

Good news is, you probably have two perfectly good batteries.
When you first got the battery error, was the battery actually hot? Meaning, not a false error? It may have damaged something. Maybe...
If the error never occurred before the update, then I would be pointing the gun at the firmware. (don't shoot though, it's not covered by your warranty.)

If it is firmware, I’ve updated it with 0 postive results how would I test that.

Neither batteries were ever hot when I checked and it said that, I figured a chip was malfunctioning after all the tests came back negative like it’s perfect lol. So what do I try..?