Compact integrated battery packs for hybrid vehicles are fully integrated with an on-board cooling system and include both the battery cells for supplying a primary voltage, such as powering a belt-driven starter generator (BSG) unit for engine start/stop, as well as a direct current (DC) to DC converter for stepping down the primary voltage to a secondary voltage for powering low voltage components and/or recharging a lead-acid battery. The on-board DC to DC converter decreases packaging size and eliminates the need for an alternator, which further decreases packaging size. Primary and secondary voltage output ports can also be implemented for quick and easy connection to other components. Configurations include single-layer and dual-layer arrangements with integrated air or liquid cooling for placement in or beneath a trunk of a vehicle, behind or beneath seats in a cab of a pickup truck, and/or beneath a vehicle.