A unique method has been developed for internally heating hybrid electric vehicle (HEV) batteries at cold temperatures using alternating current (AC). The poor performance of these batteries in cold climates is of major concern because they suffer a huge loss in capacity. Another symptom of this low performance is a dramatic increase in the series resistance of the battery, R-B, as the temperature drops. Experiments were performed with both low and high frequency heaters, and several tests were conducted on both lead acid and nickel metal hydride (NiMH) batteries at different AC amplitudes, states of charge (SOCs) and cold temperatures. Low frequency 60 Hz heating was first tested on several different 38 Ah lead acid batteries. The feasibility of using high frequency heating was then tested using a 10-20 kHz inverter on a pack of 6.5 Ah nickel metal hydride (NiMH) batteries. A technique also was developed to estimate the internal battery temperature, T-bat, by measuring the battery source resistance, R-B. (C) 2003 Elsevier B.V. All rights reserved.