Chosen Solution
I bought a battery from iFixit and followed the (excellent) replacement instructions on the site, replacing the keyboard while I was at it. I reassembled, and my Mac booted just fine off the battery. I followed the instructions to calibrate the battery: Calibrate your battery before using it: allow it to drain overnight, then charge it to 100% and drain it again until your MacBook Pro shuts down automatically. Charge it again and use it normally. The battery charged up to 100% (over the course of hours, not instantly) and then I drained it again. I had to travel for work the next day, so I left it plugged in for a day or so, and when I came back I found that the battery condition is “Replace Now”, and removing power causes the laptop to power down instantly. I tried resetting the SMC and PRAM, but no luck. (Though, I’m not sure how to tell if either reset actually took place?) I opened it up again and reseated the power connection, but again no luck. I looked around the forums here, but this seems to be a different situation from the others, since the battery did work normally for a while post-repair. Has anyone else seen this?
You’ll need to speak with the IFIXIT store to get a replacement as it sounds like you got a bad battery. The contact info is on the shipping paperwork.
You must have gotten a bad battery, you most likely cant find an apple OEM battery, so I recommend using ifixit’s battery.
