kernel_task using 500% of the CPU. computer unusably slow. can i deactivate thermal throttling ?

You said you had applied thermal paste. I imagine that this could either not be adequately doing its job, or not be the proper paste for the device. Or perhaps some sensor was not adequately reconnected after putting in the new battery, and the throttling happens as a result of not having adequate temperature information.

This should be fixed, not by disabling the proper safety mechanisms that prevent the mac from melting down, but by determining what is actually wrong.

You have installed a lot of **** in this system, and odds are that the runaway kernel task is due to one or more of these system modifications.

In particular, you still have remnants of the creepy "cleanmymac" thing.

Any mac that has been touched by this is likely to need a clean install to regain sanity.

You have Chrome, which is notorious for its "keystone" agents to spike kernel task like crazy.

And you have many many old system modifications, over the years.

You should make sure to have a full backup, and then erase and install.

If you are not willing to do that now, consider installing a clean system on a separate drive, and then compare how the

mac works booting from that clean system. With an SSD, it will likely fly, but even if you use a slow external drive for this test, it will take time booting and launching apps but will otherwise probably run faster than what you have now.

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is desshoigned solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Also, unless you're using a true VPN tunnel, such as between you and your employer's or bank's servers, they are useless from a privacy standpoint: Public VPN's are anything but private.  

That being said you've installed what most of the experienced contributors to these forums consider akin to malware, CleanMyMac. Unless you need the VPN to connect to your employer or bank it should also be uninstalled.

I recommend you uninstall it according to the developer's instructions. You can check to see if you've removed all of the supporting files by downloading and running the shareware app Find Any File to search for any files with the application's or the developer's name in the file name.  For XXXX software you'd do the following search(es): 

1 - Name contains cleanmymac

2 - Name contains macaw

3 - Name contains holavpn

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

Lastly, you're at the edge of free space on your boot drive. I would not let it get any less. This will help with performance.

If you think you have a hardware problem, take it to an Apple Store or Authorized Repair Center for evaluation.

Nobody here can fix a hardware problem.

I have no idea how you might "deactivate thermal throttling", but I would not do that, anyway.

The reasons why you are experiencing the current problem are beyond my level of expertise, but one thing I do know: these devices are built to very tight specifications. Management of heat in a sophisticated electronic device like a mac is not something where I'd try to "outsmart" the engineers that made the device.

I once watched my daughter replace the battery on her 2013 15" MBP. It was not an easy task. She is a software engineer (not hardware), and she did it very carefully, but we were not sure if the mac would boot after this "surgical" procedure. There is too much that can break. In her case, it did work, but something went wrong in yours.

I agree that spending the equivalent of a new mac mini on a 2015 mac is not a great idea.

Maybe it is time to write it off and get a new one (if you can afford it, of course).

i'm asking for the process to disable thermal throttling

Is your MacBook Pro getting hot? If it isn't, the "thermal throttling" isn't the cause of kernel_task running away. Throttling the CPU when it gets hot is not the only function of kernel_task.

If it is getting hot, then disabling it will eventually disable your MacBook Pro permanently.

when you boot the laptop,even if it has been switched off for the whole night,the fans come on immediately. if you then go into the terminal and run the "pmset -g thermlog" command, which tells you if the device has already limited the speed, a normal score is 100, but mine is 21 - which means the CPU is being throttled by 80% from the moment i turn it on

If you've searched the internet for how to turn it off, that is where you would find the answer. Most topics indicating how to subvert the operation of the Mac get removed from this forum. If someone does post an answer, you may get a small chance to see it before the post is removed.

Often in the life of features in macOS, early stages have hidden preferences that can enable/disable the feature (probably because Apple uses them in testing). Once the feature is mature, those toggles are removed and there is no method to disable them.

I think the results of your safe mode test pretty much show that it is a hardware problem.