A battery system includes a plurality of battery modules, each of the battery modules including a plurality of stacked battery cells and a battery module monitor (BMM) configured to monitor the respective battery cells; a battery system monitor (BSM) configured to communicate with and control the BMMs; a housing enclosing the battery modules and the BSM, the battery modules being arranged in the housing such that a swelling compensation channel is formed between the battery module and the housing; and an optical communication system (OCS) configured to connect the BSM with the BMMs via optical signals propagating along the swelling compensation channel.