Battery pack

1. A battery pack comprising a plurality of prismatic secondary batteries with spacers each interposed therebetween, wherein

the prismatic secondary batteries each have

a flat wound electrode body that is formed by winding a positive electrode plate having a positive electrode active material mixture layer and a negative electrode plate having a negative electrode active material mixture layer with a separator interposed therebetween,

a prismatic outer body that has an opening, a bottom, a pair of first side walls, and a pair of second side walls and houses the wound electrode body, and

a sealing plate that seals the opening,

the first side walls are larger in area than the second side walls,

the wound electrode body has a wound positive electrode core-exposed portion on a side of one of the pair of second side walls and a wound negative electrode core-exposed portion on a side of the other of the pair of second side walls,

a positive electrode current collector is joined to the positive electrode core-exposed portion to form a positive electrode-joined portion,

a negative electrode current collector is joined to the negative electrode core-exposed portion to form a negative electrode-joined portion,

the wound electrode body has a power generation part in which the positive electrode active material mixture layer and the negative electrode active material mixture layer are layered with the separator interposed therebetween,

the power generation part has a flat portion having a flat outer surface, a first curved portion having a curved outer surface and located nearer to the sealing plate than the flat portion, and a second curved portion having a curved outer surface and located nearer to the bottom than the flat portion,

a boundary between the flat portion and the first curved portion is defined as a first boundary, and a boundary between the flat portion and the second curved portion is defined as a second boundary,

in a direction perpendicular to the bottom, a distance between the first boundary and a sealing plate-side end of the positive electrode-joined portion is denoted by L 1 and, in the direction perpendicular to the bottom, a region of the power generation part, the region extending toward the first boundary from a position 0.25×L 1 to a position 0.75×L 1 distant from the sealing plate-side end of the positive electrode-joined portion, is defined as a first region,

in the direction perpendicular to the bottom, a distance between the first boundary and a sealing plate-side end of the negative electrode-joined portion is denoted by L 2 and, in the direction perpendicular to the bottom, a region of the power generation part, the region extending toward the first boundary from a position 0.25×L 2 to a position 0.75×L 2 distant from the sealing plate-side end of the negative electrode-joined portion, is defined as a second region,

in the direction perpendicular to the bottom, a distance between the second boundary and a bottom-side end of the positive electrode-joined portion is denoted by L 3 and, in the direction perpendicular to the bottom, a region of the power generation part, the region extending toward the second boundary from a position 0.25×L 3 to a position 0.75×L 3 distant from the bottom-side end of the positive electrode-joined portion, is defined as a third region,

in the direction perpendicular to the bottom, a distance between the second boundary and a bottom-side end of the negative electrode-joined portion is denoted by L 4 and, in the direction perpendicular to the bottom, a region of the power generation part, the region extending toward the second boundary from a position 0.25×L 4 to a position 0.75×L 4 distant from the bottom-side end of the negative electrode-joined portion, is defined as a forth region,

when viewed in a direction perpendicular to one of the pair of first side walls, the area of a region pressed by each spacer, the region being in a region that lies in the one of the pair of first side walls and overlaps at least one of the first region and the second region, is 20% or less of the area of the region that lies in the one of the pair of first side walls and overlaps at least one of the first region and the second region, and

when viewed in the direction perpendicular to one of the pair of first side walls, the area of a region pressed by each spacer, the region being in a region that lies in the one of the pair of first side walls and overlaps at least one of the third region and the fourth region, is 20% or less of the area of the region that lies in the one of the pair of first side walls and overlaps at least one of the third region and the fourth region.

2. The battery pack according to claim 1 , wherein,

when viewed in a direction perpendicular to the other of the pair of first side walls, the area of a region pressed by each spacer, the region being in a region that lies in the other of the pair of first side walls and overlaps at least one of the first region and the second region, is preferably 20% or less of the area of the region that lies in the other of the pair of first side walls and overlaps at least one of the first region and the second region, and

when viewed in the direction perpendicular to the other of the pair of first side walls, the area of a region pressed by each spacer, the region being in a region that lies in the other of the pair of first side walls and overlaps at least one of the third region and the fourth region, is 20% or less of the area of the region that lies in the other of the pair of first side walls and overlaps at least one of the third region and the fourth region.

3. The battery pack according to claim 1 , wherein,

when viewed in the direction perpendicular to one of the pair of first side walls, a region of the one of the pair of first side walls, the region overlapping the flat portion and a region located between the first region and the third region, is pressed by each spacer, and

when viewed in the direction perpendicular to the other of the pair of first side walls, a region of the other of the pair of first side walls, the region overlapping the flat portion and a region located between the first region and the third region, is pressed by each spacer.

4. The battery pack according to claim 1 , wherein,

when viewed in the direction perpendicular to one of the pair of first side walls, the area of a region pressed by each spacer, the region being in a region that lies in the one of the pair of first side walls and overlaps the flat portion but does not overlap the first region, the second region, the third region, or the fourth region, is 70% or more of the area of the region that lies in the one of the pair of first side walls and overlaps the flat portion but does not overlap the first region, the second region, the third region, or the fourth region.

5. The battery pack according to claim 4 , wherein,

when viewed in the direction perpendicular to the other of the pair of first side walls, the area of a region pressed by each spacer, the region being in a region that lies in the other of the pair of first side walls and overlaps the flat portion but does not overlap the first region, the second region, the third region, or the fourth region, is 70% or more of the area of the region that lies in the other of the pair of first side walls and overlaps the flat portion but does not overlap the first region, the second region, the third region, or the fourth region.

6. The battery pack according to claim 1 , wherein,

when viewed in the direction perpendicular to one of the pair of first side walls, a first recess is provided in a portion of each spacer, the portion opposing a region that lies in the one of the pair of first side walls and overlaps at least one of the first region and the second region, and

when viewed in the direction perpendicular to one of the pair of first side walls, a second recess is provided in a portion of each spacer, the portion opposing a region that lies in the one of the pair of first side walls and overlaps at least one of the third region and the fourth region.