A battery monitor determines an internal resistance of a battery cell of a battery having parallel connected battery strings which accounts for errors introduced by the parallel connected battery strings. When determining the internal resistance of a battery cell, the battery monitor determines a baseline intercell resistance of an intercell connecting the negative terminal of the battery cell to the positive terminal of an adjacent battery. The battery monitor then applies a momentary load across the battery cell and immediately prior to releasing the load, measures the voltage across the battery cell (loaded voltage) and the voltage across the intercell. The battery monitor then calculates the current flowing through the intercell (intercell current) by dividing the measured voltage across the intercell by the determined intercell resistance. Immediately after releasing the load, the battery monitor measures the voltage across the battery cell (recovered voltage) and determines the battery cell internal resistance by dividing the difference between the recovered voltage and the loaded voltage by the intercell current.