Iphone 6s

Try recalibrating your battery gauge:

• Charge to 100%
• Run the battery down until the phone shuts off
• If it shuts off above 1% charge it for a minute only, then continue running the battery down. Repeat until it makes it to 1% before shutting off
• Charge it to 100% again

If the battery is not defective this should fix it.

IF it doesn't charge to 100% after running it down until it shuts off using the Apple USB adapter as a power source you've got a hardware problem or a bad battery. Take it to an Apple store; it's still in warranty.