[nibbleshifter]

In many cases a battery replacement = dead.

Im unsure about the 2012 MBP (not having had one), but if its an internal battery, battery death usually means user unfixable, for most users.

[tverbeure]

It’s not supposed to be user replaceable, but there are plenty of kits and YouTube videos available on how to do it.

[nibbleshifter]

Kits and YT videos pretty much always means "beyond the average consumer", meaning either paying a repair shop hundreds of euros in labour/profits, or just replacing the device.