Design and analysis of sliding mode observers for State-of-Charge estimation in Li-ion cells under realistic conditions

Accurate State-of-Charge (SoC) estimation is crucial for the safe and efficient operation of Li-ion batteries. At the same time, these demanding performance requirements must be met using the low-cost hardware typically available in Battery Management Systems (BMSs). In this context, Sliding Mode Observers (SMOs) offer a promising solution due to their computational efficiency and inherent robustness. This work investigates the design and tuning of first and second order SMOs for SoC estimation, both relying on a first order Equivalent Circuit Model (ECM) with SoC and temperature dependent parameters identified from experimental data. The two observers are evaluated under a highly dynamic automotive profile through a sensitivity analysis based on three complementary performance indexes, highlighting their relative strengths and limitations to support practical design. Finally, the SoC estimation performance is assessed at two different temperature levels, confirming the effectiveness of both approaches under realistic operating conditions.