A modular battery pack for mounting to a vehicle frame of a vehicle. The modular battery pack comprising a plurality of modules arranged adjacent to each other, wherein at least one module constitutes an electrical energy storage module. Each module comprises first and second opposite side walls arranged to face respective side walls of adjacent modules, and to provide structural integrity of the modular battery pack. Each side wall comprises a common set of interfaces for interfacing between adjacent modules. The modular battery pack also comprising at least one control unit having a side wall comprising the common set of interfaces, and also a vehicle interface for interfacing with the vehicle.