The battery is merely an energy storage and the key for all-electric vehicles is understanding how to use the battery in the most optimal way in order to secure vehicle performance over a long period of time. The operating and controlling strategies of a battery rely on the understanding of the fundamental cell constraints, which are turned into battery and vehicle control strategies, and implemented as algorithms in the battery management system (BMS): the control unit of the battery. The BMS will control and monitor the performance and status of the battery and communicate the operational constraints currently available to the control system of the vehicle. There are many cross-dependent parameters to be understood and to be incorporated in a robust and reliable control system. Input data for the BMS are the state functions, e.g. state of charge and state of health, battery temperature, and usage history, required to secure optimal performance in a durable and safe manner. How this control and communication is handled depends on the battery and vehicle manufacturers, and is not covered in this book. Instead, the underlying fundamentals will be discussed in terms of electrochemical and material constraints. In the following sections, battery control and management will be described: charge control and methods, thermal and safety management, as well as the state functions, i.e. state of charge (SOC), state of health (SOH), and state of function (SOF).
Battery management system
The battery management system (BMS) utilises a number of parameters that are linked to each other and most of the key parameters are path dependent, and the usage and environmental history affects future operational possibilities. Each of these parameters affects the battery control and management system: temperature, voltage range, current, and energy throughput. Temperature is one of the most important parameters for the BMS and the corresponding control strategies. The battery should be used within a specific temperature range, a range defined by the chemistry inside the cell. At temperatures outside this predefined range, higher as well as lower, side reactions may take place, side reactions limiting battery life and possibly causing abuse situations.