Equalization of valve-regulated lead-acid batteries: issues and life test results

The importance of state-of-charge (SOC) balance, or equalization, is well known. Results of accelerated life testing are presented to evaluate equalization requirements and to compare passive and active equalization approaches for valve-regulated lead-acid (VRLA) batteries. In both heavy cycling duty and high-temperature duty, battery degradation appears very early during expected life in the absence of equalization. The degree of equalization is critical: results show that voltage differences should be held to less than 15 mV/cell to prevent SOC separation in repeated cycling. The tests confirm that conventional overcharge-based passive equalization is effective for VRLA batteries - if there is sufficient time to ensure SOC matching among cells. Most proposed active voltage equalization methods in effect transfer the problem of SOC matching to external voltage matching of sensors and magnetic elements. Matching at the 15 mV/cell level is costly. A switched-capacitor approach has been identified that avoids this limitation. Test results show that switched-capacitor equalization is useful, particularly when the series string is too long to support enough time for passive equalization.