This application provides a single-
cell battery, an
energy storage device, an
energy storage system, and a power supply
system. The single-
cell battery includes a casing; a
cell, including a cell body and a tab connected electrically; a restraining member, including a side plate, a bottom plate, and a guide plate, the side plate being disposed around the outer periphery of the bottom plate and the bottom plate and the side plate defining a mounting groove, the bottom plate having a through groove, the guide plate protruding from the surface of the bottom plate facing the mounting groove and disposed around the outer periphery of the through groove; a tab passing through the through groove and partially located within the mounting groove; and an end cap
assembly disposed on the side of the restraining member away from the cell body, the end cap
assembly including an adapter, the adapter being connected to the end of the tab away from the cell body; the dimension of the tab passing through the through groove is w1, the dimension of the guide plate along a first direction is w2, the distance from the connection point of the adapter and the tab closest to the cell body to the end face of the tab away from the cell body is w3, then 2≤(w1-w3) / w2≤8.