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, at least one battery array located within the enclosure, and an energy storage controller module located within the enclosure and electrically connected to the battery array. A high voltage junction box is attached to a first end of the enclosure and having a plurality of high voltage connection terminals connected by a plug-in bussed electrical center configured to reduce the possibility of unintentional grounding or shorting, and decreases the safety risk to a technician during servicing. At least one of the high voltage connection terminals is configured to receive a high voltage conductor connected between the energy storage module and an inverter of the hybrid vehicle.