Abstract: This application relates to the field of battery technology, and in particular, to a box assembly, a battery module, a battery, and an electrical device. The box assembly may include at least a partition piece and two end plates. In the box assembly disclosed herein, a stress relief piece may be disposed at two ends of a main body of the partition piece. The main body may be connected to the end plate by use of the stress relief piece. A corner may be formed between the stress relief piece and the main body. In this way, when the battery module is subjected to stress, at least a part of the stress can be relieved through the corner formed between the stress relief piece and the main body.