It may be the battery is going bad and/or has a defect.
Remove the rear cover and battery, inspect it for damage and dirt or corrosion on the contacts.
If the contacts are dirty, try cleaning them with a pencil eraser.
Observe the battery packaging for swelling, bubbles, breaks, or burns. If any, then replace.
If the packaging or battery cells do have breaks, handle with care as Lithium batteries can self ignite in the presents of moisture and water.
Have the battery tested or replaced at a cell phone or battery service center.