Problem with refurbished 6s

[newbie6sphone]

recently purchased a refurbished iPhone 6s, and for the first week it worked perfectly. However, just yesterday it turned itself off out of the blue, to the point where it would respond to anything (the charger icon doesn't even come on when holding down on button)

This has worked drastically throughout the last 24 hours to the point now where it doesn't stay on for longer than about 15 minutes while being used, unless it is plugged in and charging.

Is this a faulty battery or is there anything else it could be? Is it worth buying a new battery kit to try and replace the battery? Is this difficult to do?

Thanks

 

[Audit13]

Did the phone come from Apple?

 

[KGB7]

Refurbished phones have 90 day warranty. Use the warranty to fix or resolve the issue.

 

[Audit13]

Refurbished phones purchased from apple.com carry a 1 year warranty.

Phones replaced under warranty carry a 90 day warranty or until the expiry of the original phone's warranty, whichever is longer.

OP did say who refurbished or sold the phone.