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With growing popularity of grid-connected battery energy storage systems (BESSs), operators require electrical models for optimal utilisation. These models should be provided by suppliers or manufacturers based on testing methods applied to individual cells or modules in specialised laboratories. However, operators are also interested in developing electrical models on their own. This paper explores the feasibility of modelling a grid-connected BESS without dismantling it, using only the data from its energy management system (EMS) and battery management system (BMS). The goal is to characterise a BESS directly on-site, controlling it through the available commands of its power converter system (PCS). The aim is to represent the electrical dynamics of the BESS with an equivalent Thevenin electric circuit composed of open circuit voltage, resistances, and capacitances. The overall usable capacity of the BESS and the efficiency of the PCS are also estimated. The subject of the investigation is a 300 kW/652 kWh Nickel-Manganese-Cobalt (NMC) Li-ion BESS composed of ten racks, each equipped with a PCS and ten battery modules. The analysis proves the feasibility of modelling the grid-connected BESS via data from EMS and BMS. An equivalent cell model is derived, with the open circuit voltage and internal impedance expressed for the entire state-of-charge range. The total resistance assumes values in the range of 1.580–2.424 m Ω , whereas the total capacitance is 609.5–1,580 kF. Consequently, the normalised total resistance is 3%–4%, aligned with the expectations from other NMC Li-ion cells. Finally, the energy capacity and PCS efficiency are reported as a function of the power of the PCS. The usable energy capacity per rack is approx. 59.7 kWh, which is 91.5% of the rated DC energy capacity, and it is independent of the power at which the BESS is discharged. The PCS efficiency is above 94% when operating at 15% of the PCS rated power or higher, during both rectifier and inverter mode. The obtained efficiency curves differ by approx. 2% from the ones reported in the converters’ data sheet. • Adapting existing modelling methods for Li-ion cells to stationary grid-connected BESS. • Proving feasibility of characterising BESS using only data from EMS and BMS. • Modelling BESS by controlling its power converter system, avoiding dismantling. • The internal impedance and open circuit voltage are reported as a function of the SOC. • The obtained results are aligned with laboratory testing campaigns of NMC cells.