I did a battery replacement on my 2015 Macbook Pro last night using an iFixit kit. My laptop is old, but besides needing a new battery, nothing else was wrong with it. It worked fine when hooked up to a charger, but it would shut off in a short period of time when off the charger, and I could feel that the batteries were swollen too. But otherwise, it was a great laptop in complete working order. I would have gone through Apple to get the battery replaced, but Apple doesn't do battery replacements for a computer this old. So I felt my only option was to buy the parts and replace it myself.
I thought everything went well during the removal/installation, I thought I was extremely careful. After hooking up the new battery, I plugged in my laptop and turned it on. The login screen came up, but there is now an X over my battery icon. Every time I try to log in, the computer shuts off. I have tried a SMC reset and a NVRAM reset. I have tried disconnecting and re-connecting the battery connector. I didn't think I did anything wrong during the installation; the trackpad works, the speakers work, the keyboard works. But the laptop keeps shutting off, even when plugged in. I can't get past the login screen without it shutting off. I've never had an issue with my charger before. And the new battery won't charge because my computer won't recognize it. I'm hoping the whole thing isn't ruined, I thought I would save some money/reduce waste by repairing my laptop rather than buying a new one, but now it looks like I will have to buy a new one anyway.
If anyone has any suggestions, I would really appreciate it. I'm kind of at a loss of what else to try here. My closest Apple store is over an hour away, and I'm not sure what they would really be able to do for me, with the laptop being 10 years old and having attempted the replacement myself, I doubt they will feel very inclined to help. I'm open to any suggestions whatsoever.