The transition to decarbonized energy systems requires the adoption of new strategies and solutions to guarantee efficient power system management. Battery Energy Storage Systems (BESSs) represent a key component of the energy infrastructure, increasingly used to provide grid support, control and flexibility, making reliable real-time validation of their control systems increasingly relevant. This paper presents the development and Hardware-in-the-Loop (HIL) implementation of a real-time model of a utility-scale BESS. The objective is to provide a reliable replica platform to test the industrial control system under realistic operating conditions, including grid-connected and islanded operation. The plant model is developed in Matlab/Simulink using OPAL-RT ARTEMiS libraries, including step-up transformer, power conversion system, DC bus and battery banks. Each battery bank is represented by an equivalent circuit model parameterized from experimental data, providing an effective compromise between accuracy and real-time execution needs. The model is interfaced with the real BESS control system replica through analog and digital I/O, enabling closed-loop validation in a fixed-step real-time environment. Validation against real plant measurements shows very good agreement for battery voltage and good agreement for DC current. Islanded-operation tests further confirm correct load acceptance and stable stand-alone voltage and frequency regulation.