Why is my battery not working?

@chuck37957 the only way to diagnose a battery would be by doing a load test. For that you would need a know load and and ammeter to determine the rate of discharge etc. Measuring the voltage on a battery does not test the battery capacity. In your case where the computer does not work unless it is plugged in, I would most certainly replace the battery without testing the battery.

Since iFixit does not yet have a guide for this, use something like this video Batteries are readily available just stick with places that have a good service record. Stay away from ebay or others like that. There are some shady thing being done with replacement batteries.

While you work on your laptop, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!