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 comprised of one or more battery cells held in place between side rails and secured by end plates and mounting pins. The end plates also include flanges which secure the end plates behind the pins. The pin arrangement provides more secure holding and helps prevent torque loosening during operation. The battery array also includes cell retainers between each cell to reduce thermal transfer between cells, and an insulation liner between the battery cells and the side rails to improve creepage and clearance.