A vehicle includes a traction battery and a battery management system. The battery management system includes a controller programmed to output a drained cell diagnostic indicator. The drained cell diagnostic indicator is based on a magnitude of a difference between an average cell state of charge at a start of a battery rest period and a cell state of charge estimated at predetermined intervals during the battery rest period being greater than a predetermined value. A response to the drained cell diagnostic indicator depends on the magnitude of the cell drain rate and may include operating the traction battery at a reduced power level and notifying an operator of the diagnostic condition.