NOVELTY - A power storage unit has a groove(s) (20d) formed at a power-generation-element-case of each power storage module (20a) such that the power-generation-element-case breaks at the groove in response to an excessive increase in the pressure in the power-generation-element-case. It is formed in a portion of the power-generation-element-case that does not face any other power storage module.    USE - Power storage unit (i.e. battery pack) for mounting in a vehicle (claimed).    ADVANTAGE - Even if the power-generation-element-case of the power storage module breaks and deforms, it does not contact any other power storage module, and thus the power storage modules can be arranged close to each other to make the power storage unit more compact in size. Because the grooves of each two battery cells facing each other in the direction of gravity are arranged such that their battery cases break in the opposite directions, when gas is released from the lower battery cell, the gas needs to move a longer distance before reaching the upper battery cell, i.e. the distance the released gas moves in the coolant is relatively long, and thus the time the released gas remains in contact with the coolant is relatively long. The longer the released gas remains in contact with the coolant, the more efficiently the gas can be cooled. Thus, even if the released gas contacts the upper battery cell, the upper battery cell is not heated excessively by the released gas, and thus gas production in the upper battery cell due to heating can be prevented.    DETAILED DESCRIPTION - A power storage unit comprises power storage modules (20a), each constituting a power generation element and a power-generation-element-case containing the power generation element and disposed adjacent to each other; and a case containing the power storage modules and an insulative fluid. A groove(s) (20d) is formed at the power-generation-element-case of each power storage module such that the power-generation-element-case breaks at the groove in response to an excessive increase in the pressure in the power-generation-element-case. It is formed in a portion of the power-generation-element-case that does not face any other power storage module.    DESCRIPTION OF DRAWING(S) - The drawing is a perspective view showing the exterior of the battery cell.    Power storage module (20a)    Positive terminal (20b1)    Negative terminal (20b2)    Groove (20d)