On Adaptive SOH Equalization for LiFePO4 Battery Packs

Abstract

Equalizing SOH between Li-ion batteries is critical for Battery Energy Storage Systems (BESS). It cannot only decrease the SOH deviation between batteries, but also increase total available charge until the end of life. To address this issue, an adaptive SOH equalization algorithm for LiFePO₄ batteries is proposed in this study. In situation of temperature difference distribution within the range 5℃ from module to module, the SOH equalizer by controlled depth of discharge (DoD) is applied to slow the rate of capacity fading. By realizing adaptive control loop, the exact cycle-life model is not necessary to determine the DoD value. The appropriate proportion control parameter K is determined by the adaptive control algorithm. The battery pack with the adaptive controller can increase 14.0% available charge in simulation cases with different temperature. Moreover, it can also increase 13.3% available charge in simulation case with both different temperature and capacity. The implementation includes the SOH equalizer and the battery management system. The platform provides both complete SOH equalization and the protection for battery energy storage systems. Experimental results that cycled an aged module and three fresh modules indicated that SOH gap between each module didn't reduce. Other controlled experiment cycled 4 fresh modules, and result showed the SOH gap between each module is reduced.