Embodiments of this application provide a battery cell, a battery, and an apparatus using a battery, where the battery cell includes: a housing with an opening; and an end cover covering the opening. The housing and the end cover are welded together to form a fusion zone. The end cover is provided with a first stress-relieving groove on an inner surface. The first stress-relieving groove is configured to recess from the inner surface of the end cover in a direction leaving the inside of the battery cell, so as to relieve stress produced by welding of the housing and the end cover. The technical solutions provided in the embodiments of this application can enhance safety performance of batteries.