Equalization requirements for series VRLA batteries

State-of-charge (SOC) balance is a necessary task in any high-rate series battery system. This equalization task is evaluated here through cycling and hybrid vehicle platform tests of valve-regulated lead-acid (VRLA) batteries. It is shown that when voltage matching is used as the basis for SOC equalization, voltage differences must be kept small. An upper limit of 15 mV/cell was found from the tests, although a limit of 10 mV/cell is more realistic for a practical system. A diffusion model is developed for voltage-based equalization processes. A switched-capacitor equalization approach introduced previously is shown to be a low-cost way to meet the accuracy requirements while allowing the charge diffusion process to be accelerated. The tests show that switched-capacitor equalization supports lifetime extension in series strings.