If you've got a 6s, you might be eligible for a replacement:
https://www.apple.com/uk/support/iphone6s-unexpectedshutdown...
https://www.apple.com/uk/support/iphone6s-unexpectedshutdown...
To prepare your iPhone 6s for the battery replacement process, please follow the steps below:
Back up your data to iTunes or iCloud.
Turn off Find my iPhone
Erase data and settings in Settings > General > Reset > Erase all Content and Settings
Just to replace a battery??