A method for measuring the isolation resistance between a chassis and a battery terminal in an electric vehicle. The method can include measuring an open voltage (VP(open)) of the positive terminal; measuring an open voltage (VN(open)) of the negative terminal; measuring a voltage (VN(S1 closed)) between the negative terminal and the chassis with switch S1 closed; measuring a voltage (VP(S1 closed)) between the positive terminal and the chassis with switch S1 closed; calculating the ratio (VP(open)/VN(closed)) of the voltages VP(open) and VN(closed); calculating the open ratio of the open voltages (VP(open)/VN(open)); calculating the difference between the ratio (VP(S1 closed)/VN(S1 closed)) and the open ratio (VP(open)/VN(open)); and multiplying the value of the test resistor (R0) by the difference between the ratio (VP(S1 closed)/(VN(S1 closed)) and the open ratio (VP(S2 open)/VN(S1 open)) to obtain the isolation resistance.