A battery module (100, 170) adapted for use with a cooling fluid to cool said battery module, said battery module (100, 170) including a plurality of battery cells, each having opposite first and second terminals (102a, 102b), and an outer casing (10, 17) defining an internal space that accommodates said battery cells therein and that is for the cooling fluid to flow therethrough wherein said outer casing (10, 17) has a first opening and a second opening for the cooling fluid to flow in and out of said internal space, a first wall, a second wall, said first and second walls (11, 12) being opposite to each other, said outer casing (10, 17) further having opposite third and fourth walls (13, 14) that interconnect said first and second walls (11, 12), said first and second openings (112, 114) being respectively formed in said third and fourth walls (13, 14); wherein said outer casing (10, 17) has a first engaging portion disposed on said first wall, and a second engaging portion disposed on said second wall; said battery module (100, 170) further includes a first electrode disposed on said first wall and electrically connected to said first terminal of each of said battery cells, and a second electrode disposed on said second wall and electrically connected to said second terminal of each of said battery cells; said battery module (100, 170) is adapted to be coupled to another battery module (100, 170) which is adjacent to said first wall with said first engaging portion of said battery module (100, 170) fittingly engaging said second engaging portion of said another battery module (100, 170), and with said first electrode of said battery module (100, 170) contacting said second electrode of said another battery module (100, 170) for establishing an electrical connection; and said battery module (100, 170) further includes a plurality of spaced-apart straps being electrically conductive and each interconnecting two adjacent ones of said battery cells, each of said first and second electrodes (116, 122) being connected to corresponding ones of said straps so as to establish electrical connection to each of said battery cells. a plurality of battery cells, each having opposite first and second terminals (102a, 102b), and an outer casing (10, 17) defining an internal space that accommodates said battery cells therein and that is for the cooling fluid to flow therethrough wherein said outer casing (10, 17) has a first opening and a second opening for the cooling fluid to flow in and out of said internal space, a first wall, a second wall, said first and second walls (11, 12) being opposite to each other, said outer casing (10, 17) further having opposite third and fourth walls (13, 14) that interconnect said first and second walls (11, 12), said first and second openings (112, 114) being respectively formed in said third and fourth walls (13, 14); wherein said outer casing (10, 17) has a first engaging portion disposed on said first wall, and a second engaging portion disposed on said second wall; said battery module (100, 170) further includes a first electrode disposed on said first wall and electrically connected to said first terminal of each of said battery cells, and a second electrode disposed on said second wall and electrically connected to said second terminal of each of said battery cells; a first electrode disposed on said first wall and electrically connected to said first terminal of each of said battery cells, and a second electrode disposed on said second wall and electrically connected to said second terminal of each of said battery cells; said battery module (100, 170) is adapted to be coupled to another battery module (100, 170) which is adjacent to said first wall with said first engaging portion of said battery module (100, 170) fittingly engaging said second engaging portion of said another battery module (100, 170), and with said first electrode of said battery module (100, 170) contacting said second electrode of said another battery module (100, 170) for establishing an electrical connection; and said battery module (100, 170) further includes a plurality of spaced-apart straps being electrically conductive and each interconnecting two adjacent ones of said battery cells, each of said first and second electrodes (116, 122) being connected to corresponding ones of said straps so as to establish electrical connection to each of said battery cells.