A battery comprising: a plurality of battery cells; a circuit board which is electrically connected to the plurality of battery cells; and a battery case which has a rectangular parallelepiped shape, and is configured to accommodate the plurality of battery cells and the circuit board therein, the battery case comprising: a first cell accommodating portion which has a rectangular parallelepiped shape and is configured to accommodate a first number of the plurality of battery cells therein; a second cell accommodating portion, which has a rectangular parallelepiped shape and is configured to accommodate a second number of the plurality of battery cells therein, the second number being smaller than the first number, the second cell accommodating portion being shorter than the first cell accommodating portion in one of three directions parallel to sides of the battery case; a board accommodating portion which is adjacent to the second cell accommodating portion in the one of the three directions and is configured to accommodate the circuit board therein; a first partition wall which is arranged between the first cell accommodating portion and the board accommodating portion; and a second partition wall which is arranged between the second cell accommodating portion and the board accommodating portion; such that the circuit board is prevented from being soaked in an electrolyte leaking from the battery cells. a plurality of battery cells; a circuit board which is electrically connected to the plurality of battery cells; and a battery case which has a rectangular parallelepiped shape, and is configured to accommodate the plurality of battery cells and the circuit board therein, the battery case comprising: a first cell accommodating portion which has a rectangular parallelepiped shape and is configured to accommodate a first number of the plurality of battery cells therein; a second cell accommodating portion, which has a rectangular parallelepiped shape and is configured to accommodate a second number of the plurality of battery cells therein, the second number being smaller than the first number, the second cell accommodating portion being shorter than the first cell accommodating portion in one of three directions parallel to sides of the battery case; a board accommodating portion which is adjacent to the second cell accommodating portion in the one of the three directions and is configured to accommodate the circuit board therein; a first partition wall which is arranged between the first cell accommodating portion and the board accommodating portion; and a second partition wall which is arranged between the second cell accommodating portion and the board accommodating portion; a first cell accommodating portion which has a rectangular parallelepiped shape and is configured to accommodate a first number of the plurality of battery cells therein; a second cell accommodating portion, which has a rectangular parallelepiped shape and is configured to accommodate a second number of the plurality of battery cells therein, the second number being smaller than the first number, the second cell accommodating portion being shorter than the first cell accommodating portion in one of three directions parallel to sides of the battery case; a board accommodating portion which is adjacent to the second cell accommodating portion in the one of the three directions and is configured to accommodate the circuit board therein; a first partition wall which is arranged between the first cell accommodating portion and the board accommodating portion; and a second partition wall which is arranged between the second cell accommodating portion and the board accommodating portion; such that the circuit board is prevented from being soaked in an electrolyte leaking from the battery cells.