I agree with Mr. Finch. The iPhone will tell you the battery is bad, but rarely does this happen when the battery "capacity" is rated at 98-100%. But this exact thing happened to me. I had been given an iPhone 6 Plus that had been carried around daily, but seldom used, for 5 years. The 5-year-old battery still had 100% listed capacity, but by being repeatedly charged to 100%, but almost never being drained, the battery developed a very high "internal resistance," which made it unable to provide enough power without its voltage dropping too much for the phone to operate. This is considered a normal, though unfortunate, side-effect of charging the battery to 100% every day for five years, but barely ever using the iPhone.
The result was that this phone would lose power and go suddenly black-screen (unexpected cutoff) when stressing the battery (while using Wi Fi with a bright screen and Bluetooth). This unexpected power-off condition would happen anytime the battery dropped below 50% charge. The battery could be drained all the way to 1%, but only when the power draw was exceptionally light, like playing podcasts through headphones for dozens of hours. The phone's battery info reported that the iPhone had experienced repeated unexpected shutoffs and that the battery needed replacing while at the same time it rated the battery capacity at 98%. This was a 5-year-old original battery. A new battery fixed everything.
