Battery energy storage systems (BESSs) are commonly used in smart grids. Voltage deviation or imbalance among cells generally exists in multi-cell battery packs. This work presents a study of the voltage deviation-related phenomena observed during the operation of a grid-tied BESS, Willenhall Energy Storage System (WESS), including the voltage deviation changes during full range cycle and the cut-off mechanism activated by it. Electroimpedance spectroscopy measurements and equivalent circuit modelling were conducted on the same type of cell as that used in WESS to obtain cell-equivalent circuit parameter distributions (the standard deviation and mean). Cell voltage deviation in a WESS-sized battery pack (> 21k cells) was studied using Monte Carlo simulation through a proposed cell level battery simulator. Both experiments and simulations reveal that high cell voltage deviation emerges at the low and high state-of-charge zones where the cell internal resistance has a large value and large extent of deviation.