Numerous battery + screen problems

There are a few things you could be facing. This series has a known issue with the HD SATA cable so if you upgraded the drive you might be hitting the inability of the cable to transmit the data between the drive and the systems logic board (either way). Your higher temp levels leads me to believe you are facing this.

So you need to get a new better cable which is this one: MacBook Pro 15" Unibody Mid 2012 Hard Drive/IR Sensor Cable Replacement and yes! Its the 2012 version of the cable which is the better one! Here’s the guide to put it in MacBook Pro 15" Unibody Mid 2012 Hard Drive/IR Sensor Cable Replacement. There are a few details that also need to be done here. The first is placing a strip of electricians tape on the uppercase where the cable crosses over and we also need to make sure the mid plane clip is not broken near the drives. If you have a good one swap it over.

Here’s a good read on the cable issue: Your Hard Drive Cable Is A Ticking Time Bomb

If the clips are both broken you should get some foam tape to add a bit of protection from the bottom cover damaging the cable. Here’s the placement guide:

We still have one more detail! The cable needs to make a few bends an issue we have seen is people make a deep crease which damages the delicate foil wires within it! So don’t fold! Instead find an old BIC pen to take the Ink straw out as we will use it as a forming brake to bend a nice radius bend no tighter than the diameter of the straw.

OK, we got the cable issue resolved but the stuff you have on the drive right now may have some issues so its best to make a clean backup using TimeMachine, reformatting the drive and then installed a fresh copy of MacOS.

But! You really should stick with Sierra as thats the best macOS for your system! Sierra is the last OS which supports HFS+ which is a better file system for older SATA based systems! APFS is great for PCIe/NVMe drives but its just to chatty for SATA drives (any) SATA has fewer buffers than APFS so it gets choked up!

OK so we’ll go back but we need to have a bootable OS installer thumb drive setup! And Apple has messed up the certificates on many of the OS images! Here’s more on that: If you've got an old macOS install image, it will probably stop working today So what to do?? Here’s a good image for Sierra How to upgrade to macOS Sierra jump down to Step 4 for the direct link, double click on the dmg file to get the installer out. Then follow this guide: How to create a bootable macOS Sierra installer drive One last point make sure you format the thumb drive to GUID /Journaled file system first.

With the created installer drive boot up your system then reformat your internal drive back to GUID/Journaled file system (HFS+) then install the OS. At the end of the install you will be asked if you have a system to migrate your data from or a TimeMachine backup this when you restore your user accounts, apps and data - Done!