The VCU communicates with the BMS via the Master Module and all battery packs need at least one Master unit. The Satellites expand the capability of the Master by an additional 18 cells and 3 thermistors per additional Satellite connected. The Satellite Modules are connected to their Master via a high - speed serial interface (isoSPI) that allows additional units to be daisy chained together, adding as many Satellites as necessary for the battery pack configuration.