A novel battery condition monitoring (BCM) technology for lead-acid batteries has been developed. We have developed a highly reliable SOC monitor that improves the estimated precision of the stored capacity to +/- 5% for both the flooded type and VRLA. A novel SOC estimation algorithm was also developed. The SOC value was obtained by the weighting combination of the values from the SOC-DCR table and the current integration. Kalman Filtering Theory contributed to developing this system. In order to improve the accuracy of the SOC-DCR table, we have derived a theoretical equation for the SOC-DCR relationship. This equation helps to interpolate the data and improve the accuracy of the SOC-DCR table. (c) 2005 Elsevier B.V. All rights reserved.