Modular battery system

A battery module 100 comprises a housing, a first pair of power terminals 102 located at a first surface of the housing, a second pair of power terminals 104 located at a second surface of the housing opposed to the first surface, and first and second battery stacks 111a, 111b located within the housing. Each stack comprises a plurality of battery cells connected in series between a terminal of the first pair of power terminals and a terminal of the second pair of power terminals such that the negative terminal of each battery stack is located on the same surface as the positive terminal of the other battery stack. The power terminals preferably comprise a plug or socket to facilitate direct electrical connection of the battery module to one or more identical battery modules. A PCB is preferably located within the housing, the PCB comprising the power terminals, a plurality of battery cell connectors with one or more electrical contacts connected to the plurality of battery cells, and conductive traces between the electrical contacts of the battery cell connectors and the power terminals to facilitate the series connection. The battery stacks within the housing may be connected in parallel.