A modular multi-cell rechargeable energy storage system (RESS) includes a RESS enclosure surrounded by an external environment and having an enclosure tray and an enclosure cover. The RESS also includes a plurality of battery cell blocks arranged in the RESS enclosure. Each battery cell block includes a cell case including a cell vent and at least one electrically insulated structural connector configured to link the cell case with an adjacent battery cell block. Each battery cell block also includes a thermal mitigation barrier (TMB) segment fixed to the cell case and configured to contact the adjacent battery cell block. Each battery cell block additionally includes a thermal insulator configured to line the cell vent. Furthermore, each battery cell block includes a bus bar segment configured to interconnect with a bus bar segment of the adjacent battery cell block.