Chosen Solution
As the title says, I had a customer bring in an iPad Pro 12.9 that will not stay on unless plugged into a charger. When it is disconected, it will power down within seconds, and all it will display is a dead battery and lightning cable. As soon as the iPad is plugged into a power source, it powers right up and displays the battery is at 100%. I have ordered a new battery for the iPad, but I have a couple questions. Is it possible to test the new battery in the iPad without tearing the entire thing apart to see if a new battery fixes it, and if not, what are the chances this is a bad battery or a bad board? Is it worth the trouble of tearing the entire device apart and put it all back together just to find out the new battery does the same thing?
Use a battery utility, such as coconutBattery (for Mac) or 3uTools (for Windows). Anything less than 70% of design capacity will require replacement. You can also use the Battery Health Diagnostic (Settings\Battery\Battery Health (Beta)) in iOS 11.3 and above. It will tell you what the health of the battery is. If you have access to a USB Ammeter, you could use that to determine if the phone is really drawing current when it says it’s charging.
