Commercial-scale, grid-connected battery energy storage system (BESS) typically operates on price-driven or peak shaving charging cycles. However, when installed in a distribution grid next to medium or large-scale photovoltaic (PV) generation, BESS can also play an important role in voltage regulation processes. Therefore, this paper proposes a new, model-driven controller which incorporates BESS into a voltage regulation scheme in order to counteract voltage variation caused by PV power fluctuations. In addition, the method optimizes control efforts and coordinates regulation actions of BESS and remaining grid devices. The algorithm was tested on a model implemented in the real-time digital simulator of a real grid equipped with a medium-scale BESS (0.6 MW/0.76 MWh) and 3.15 MWp of PV generation. As a result, the developed controller managed to regulate grid voltages quickly and without a significant influence on BESS charging cycle. The algorithm can be applied to distribution grids which already suffer from voltage fluctuations or to enhance PV generation in weak feeders by providing much faster and precise voltage control.