Randomly cuts off on battery!

Hey, could really use some help before I make a very expensive decision -

I’ve had this issue ever since I bought this second hand - it seems that my MBP does NOT like running on battery power! I’ve noticed how long the Mac will last before it completely cuts out without warning will depend on how hard I’m working it, but usually, it’ll only run for about a few seconds before it completely shuts off, and then I have to power it back on again - despite the fact the battery would be on 100% (even confirmed by the battery indicator lights on the side). Sometimes it’ll even cut out when it’s still loading up the OS!

I’ve replaced the battery with a brand new one, reset the SMC countless times, upgraded it to macOS Catalina and still nothing!

Is there anything I can do to solve this?