It is probably not related to the battery assuming that the iPhone is otherwise functioning normally. ***Make sure that your iPhone is setup to meet the following requirements before you attempt to update the iOS version over a Wi-Fi Network. Open*** Settings ➡️ General ➡️ Software Update: ***If there is an update available for your iPhone then it will be listed here. Follow the on-screen instructions to download and update your iPhone.***
* ***Your iPhone must have a fast Wi-Fi connection*** . You cannot update over a cellular hotspot or a cellular network.
* The iPhone cannot be in Low Power mode
* The battery state of charge must be above 50%
* Your iPhone must have at least 12GB or more free storage space
* Your iPhone cannot be connected through VPN
* You should leave the phone plugged in until it finishes updating, both to assure that it remains above 50% and to keep Wi-Fi enabled (Wi-Fi turns off when the phone sleeps if not connected to power).
Still not resolved ? Time to make an appointment at the Apple Store or simply take the phone to an authorized Apple service shop for help.