Definitions and reference values for battery systems in electrical power grids

•Performance values of battery systems for a better understanding between battery manufacturers and power system integrators.•Presentation of a suitable definition for battery energy storage capacity and designation of state of energy (SOE).•Definition of an appropriate reference (test) power value and explanation of the term 'CP-rate'.•Usable energy storage capacity value to describe limited usable energy content of a battery due to operational restrictions.•Clarification of time values regarding constant power battery charging or discharging.

Since more and more large battery based energy storage systems get integrated in electrical power grids, it is necessary to harmonize the wording of the battery world and of the power system world, in order to reach a common understanding. In this regard this article presents different battery content values and their relation to each other. Battery operations typically lead to a change of battery's electric charge or energy content. Based on a simplified battery model the basic values necessary to describe battery operations are clarified. Then the reference values and some acceptance criteria for batteries and secondary cells are defined. Also values describing limited usable energy content caused by operational restrictions are provided. In order to be as close as possible to existing definitions and practical applications, care is taken to be conform to current standards wherever possible in this article. The given set of consistent battery definitions can be used for an agreed design of battery storage systems and provides options for battery performance criteria.