Battery Issues After SMC Reset

Greetings. I recently bought a used Macbook Pro 2011 era model (i7, 8mb RAM, 500gb drive, running High Sierra 13.2.6). It worked fine but the battery life drained pretty quickly. Seemed like it was something normal for a computer of its age.

I bought a replacement online (same exact model of the original Apple battery). After installation everything seemed to run smoothly. It had more battery life. This went on for about 2 weeks of normal usage. Normal meaning me primarily using it to run Ableton Live 10 with a lot of audio processing.

Then I noticed that once the battery percentage hit about 20%, the machine would suddenly shut down. I would plug it in and boot it back up, the percentage would be at 0% as though it was totally depleted. This happened several times after I had charged it to full earlier.

I read that it possibly needed an SMC reset. I followed these directions pertaining to laptops with removable batteries: https://support.apple.com/en-us/HT201295

Once I restarted the machine, the performance capacity seemed very hindered. Normal operation of the OS seemed sluggish, trying to play a session in Ableton Live maxed out its CPU usage meter way beyond 100% (it's resting non-playing usage percentage was on 30-40% with a session loaded, when before it was only about 5-6%).

I had attempted to reset the PRAM/NVRAM as well but nothing changed. I looked in the activity monitor and did not see anything that was taking up a lot of CPU, memory, or energy.

At that point I took out the new battery and put the old one back in (luckily I held on to it). Lo and behold, everything worked flawlessly as before. Except of course it was the old battery with the not-so-long life to it.

I know I can contact the seller of the new battery to get a replacement. But before I do so, is there anything I may be overlooking? It's not really possible for me to take it to Apple because I live about 3 hours from one of the stores.