Buy good thermal paste, artic silver 5 has always been my favorit but artic silver MX will also do, DO NOT buy liquid metal, one mistake and it will short out your laptop and is really hard to clean up.
Then open your laptop, take out the heatsink and fan, clean them(do not use compressor on the fan).
Remove old thermal paste with a soft cloth like pillow casing or Carbon fiber cloth and isopropyl alcohol.
Blow out dust, air from lungs, in the rest if the laptop.
Clean CPU with isopropyl alcohol, if dedicated GPU then clean that also.
Apply a pea size thermal paste on CPU/GPU and reattach heatsink and fan. Assemble the laptop and try now.
If the same, try booting a Live Linux distro and see how long time it last, to rule out hardware. Because if it is running good in Live Linux, something in your OS is causing the problem.
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As others have said, use original charger, never mix ram(other laptops might allow it but ThinkPads will not), because it can cause all sorts of problems. Also usually try to keep it the same brand/model and size, some laptops allow the mixed sizes(mostly newer models), some don't.
Buy a battery from a shop so you have warranty if it break something or just stop working, do not buy from Aliexpress. You can buy online but choose a store that has been in business for many years and check battery and store reviews.
I went through a lot before I found one that didn't drain quickly after one week. Ended up with a no name that turned out to be Sanyo 1769 100Wh in new enclosure, it setled in 93.7Wh after calibration.