Exemplary embodiments of the present invention provide flexible, multi-voltage battery modules having multiple cells that are nested together. The cells can be, for example, cylindrical lithium ion cells. To increase cell package density, the cells can be disposed in a nested configuration so that adjacent cell centers form equilateral triangles. The cells can be placed in a housing or case with interlocking tabs that allow multiple modules to be connected together. Within a module, the cells can be connected in different configurations by buss bars at the top and the bottom of the battery cells. The different configurations may provide different voltages for the module.