A battery management system (BMS) described herein determines the internal resistance for a cell that may have an internal short circuit. In one aspect, the BMS monitors the voltage across each of a plurality cells that are coupled in series. If the voltage across one of the cells differs from the voltages across the other cells, the BMS can flag the cell as potentially having an internal short circuit. Once flagged, the BMS can use a simulator that stores a model cell that has similar characteristics as the cells monitored by the BMS to determine the internal resistance of the flagged cell. In one aspect, the simulator changes the value of a surrogate resistor that is parallel with the model cell until the voltage across the model cell matches the voltage of the flagged cell. The value of the surrogate resistor indicates the internal resistance of the flagged cell.