An energy storage system comprising at least one energy storage module adapted to supply electrical energy to a hybrid vehicle. The energy storage module comprises an enclosure having at least one battery array located within the enclosure. The energy storage module includes a thermal pad disposed between the battery array and an interior surface of the primary enclosure. A heat sink is disposed on an exterior surface of the primary enclosure. The heat sink comprises a plurality of fins which may be disposed angularly with respect to a longitudinal axis of the primary enclosure. The height or length of the fins may be varied relative to a fan location to provide uniform cooling across the battery cells in the battery array. An enclosing plate is mounted exterior to the heat sink and defining an airflow cavity, wherein the enclosing plate directs air from the fan across the heat sink.