battery dies at 50%

First try a System Management Controller (SMC) reset:

Reset the SMC of your Mac - Apple Support

If that fails to correct the issue, your battery has likely failed and requires replacement.

Batteries are replaceable by Apple. I do not consider the battery to be a user-replaceable part. You cannot buy a legitimate Apple battery as a service part, and the process to remove the old battery, which is glued in place, involves potent, highly flammable solvents. The process of removing the old battery can also cause irreparable damage to delicate (= expensive) internal components.

Start here to get an estimate for the battery service:

Mac Repair & Service - Apple Support

NOTE: in general, Apple stores do not do battery service in-store because the nasty solvents required are strictly governed by OSHA, HAZMAT, and local fire codes and cannot be used in a retail environment. The fastest service the mail-in/ship-in option. If you take it to an Apple retail store, you can add 2-4 days to the time because they have to ship it to the central service facility anyway. I use a local UPS Store for this and am 100% satisfied with the process.

Using a private provider, even some Apple Authorized Service Providers, for battery service costs much more because of the solvent issue. Lacking the ability to use the solvents in their shops, they have to replace the entire upper deck assembly including keyboard, trackpad, battery, and other parts. They will charge you twice what Apple does.