A technique has been developed to estimate the internal battery temperature (T-bat) of secondary batteries by measuring the series battery resistance (R-B) at cold ambient temperatures. Tests were performed on both lead-acid and nickel metal hydride batteries at different cold temperatures to obtain useful plots of RB versus Tbat. RB was measured by using a pulse discharge circuit to apply a short-duration current pulse (I-B) directly to the battery. The test results indicated that R-B not only varies with temperature but also varies with the amplitude of I-B. The RB versus T-bat plots were later utilized to predict T-bat from R-B during alternating current (AC) battery heating at cold ambient temperatures. (c) 2005 Elsevier B.V. All rights reserved.