A battery pack, comprising: a case, comprising an accommodating cavity and a drain hole in communication with the accommodating cavity; and a battery module mounted inside the accommodating cavity; wherein a reservoir cavity is provided below the battery module to store a liquid, and in communication with the drain hole; wherein the case comprises: a first case; and a second case enclosing the accommodating cavity with the first case and located below the first case; wherein the drain hole is provided in the second case; wherein the battery module comprises: a battery cell assembly comprising a plurality of battery cells arranged side by side; a cooling system abutting against the plurality of battery cells and configured to cool each of the battery cells; and wherein the drain hole is configured to discharge a fluid leaked by the cooling system; the volume of the reservoir cavity is greater than or equal to the capacity of the coolant in the cooling system; wherein the size of an opening of the drain hole on one end in communication with the accommodating cavity is greater than the size of an opening of the drain hole on the other end in communication with the reservoir portion, or, a film is provided at an end of the drain hole to prevent the backflow. a case, comprising an accommodating cavity and a drain hole in communication with the accommodating cavity; and a battery module mounted inside the accommodating cavity; wherein a reservoir cavity is provided below the battery module to store a liquid, and in communication with the drain hole; wherein the case comprises: a first case; and a second case enclosing the accommodating cavity with the first case and located below the first case; wherein the drain hole is provided in the second case; wherein the battery module comprises: a battery cell assembly comprising a plurality of battery cells arranged side by side; a cooling system abutting against the plurality of battery cells and configured to cool each of the battery cells; and wherein the drain hole is configured to discharge a fluid leaked by the cooling system; the volume of the reservoir cavity is greater than or equal to the capacity of the coolant in the cooling system; wherein the size of an opening of the drain hole on one end in communication with the accommodating cavity is greater than the size of an opening of the drain hole on the other end in communication with the reservoir portion, or, a film is provided at an end of the drain hole to prevent the backflow. a first case; and a second case enclosing the accommodating cavity with the first case and located below the first case; wherein the drain hole is provided in the second case; wherein the battery module comprises: a battery cell assembly comprising a plurality of battery cells arranged side by side; a cooling system abutting against the plurality of battery cells and configured to cool each of the battery cells; and wherein the drain hole is configured to discharge a fluid leaked by the cooling system; the volume of the reservoir cavity is greater than or equal to the capacity of the coolant in the cooling system; wherein the size of an opening of the drain hole on one end in communication with the accommodating cavity is greater than the size of an opening of the drain hole on the other end in communication with the reservoir portion, or, a film is provided at an end of the drain hole to prevent the backflow. a battery cell assembly comprising a plurality of battery cells arranged side by side; a cooling system abutting against the plurality of battery cells and configured to cool each of the battery cells; and wherein the drain hole is configured to discharge a fluid leaked by the cooling system; the volume of the reservoir cavity is greater than or equal to the capacity of the coolant in the cooling system; wherein the size of an opening of the drain hole on one end in communication with the accommodating cavity is greater than the size of an opening of the drain hole on the other end in communication with the reservoir portion, or, a film is provided at an end of the drain hole to prevent the backflow.