MiniBMS Head End Board is required to process signals from cell modules and control the charger and motor controller as well as buzzer alerts. Only one Head End Board is required for each EV. The Relay is optional for external control of the charging source and we highly recommend also getting the heatsink with the relay.
For a basic system, customers should select a Head End Board and one Cell Module for each Lithium cell in your battery pack. See Product Support page for detailed documentation.