That's a great question--you can rule out software with a restore, and set up as new, leaving off apps and see if it holds charge. Or you can do the opposite first and just overlay a new battery and see if holds charge with a new battery. I wouldn't full install a new battery since that can affect your out of warranty discounted replacement options at Apple. If the phone is in warranty then do the software restore first and then go to Apple Store or AASP.
DMMonk wrote:
Have a 6s. It charges but quickly drains when not left plugged-in. How can I determine if it’s software or if I need a new battery?
Make an appointment at the Genius Bar at your local Apple Store. They will be able to test the battery for you.
Idris was absolutely correct. All the standard self diagnostics did not work. I did a factory reset and it still wouldn’t hold a charge. I took it to the Genius Bar. The tech didn’t even need to test it. He could see the problem. The battery was “swollen” it was so swollen the glass was separating. I couldn’t notice it until the tech put it next to another unit. Clear as day it was very swollen. So much that they gave me a whole new phone for free. The battery was defective. They really owed me a new battery. But it was so swollen that it was unsafe to open the phone. So they just replaced the phone. Very pleased with Apple right now.
6s won’t hold a charge