The performance of large-scale stationary energy storage systems such as vanadium flow batteries (VRFB) can be severely reduced by ambient temperature fluctuations due to changing thermodynamic properties. In this work, a new non-isothermal VRFB model is developed and its thermal behavior at room (25 °C) and low (5 °C) ambient temperatures is investigated. The model allows to predict the dynamic behavior of temperature, voltage, capacity and power of the VRFB taking into account the variation of the electrolyte viscosity. Validation of this model is carried out based on available numerical and experimental data. The model is used to simulate the operation of a 5 kW VRFB in constant electrolyte flow rate and constant pump power modes. The results show that at low temperatures in constant electrolyte flow mode, there is a power drop due to intensive pump operation. In constant pump power mode, a battery capacity drop of up to 12 % is observed due to increased electrolyte viscosity. The obtained results emphasize the significant change in battery dynamic performance at low ambient temperatures, indicating the importance of developing optimal operating strategies for specific climatic conditions. The developed modeling principles can be extended to advanced real-time VRFB simulations.