This technical specification provides principles and technical requirements for interconnection of distributed Battery Energy Storage System (BESS) to the distribution network. It applies to the design, operation and testing of BESS interconnected to distribution networks. It includes the additional requirements for BESS, such as connection scheme, choice of switchgear, normal operating range, immunity to disturbance, active power response to frequency deviation, reactive power response to voltage variations and voltage changes, EMC and power quality, interface protection, connection and start to generate electric power, active power management, monitoring, control and communication, and grid-connected tests. The stationary BESSs considered within the scope of this technical specification include electrical forms such as lead-acid, lithium-ion, liquid flow and sodium-sulfur batteries, interconnected to medium voltage (MV) or low voltage (LV) distribution networks via bidirectional DC to AC power converters. This technical specification will specify active and reactive power response and grid-connected testing for distributed BESS, as a supplement for IEC TS 62786-1 ED1. This document specifies interface requirements for connection of distributed BESS with the distribution network operating at a nominal frequency of 50 Hz or 60 Hz. NOTE – Mobile electrical energy storage devices (e.g., electrical vehicles) are under consideratio n for future editions. The requirements of this document can be superseded by laws and regulations where applicable.