Direct Comparison of State-of-Charge and State-of-Energy Metrics for Li-Ion Battery Energy Storage

This paper presents a direct experimental evaluation of differences between state-of-charge (SOC) and state-of-energy (SOE) metrics for lithium-ion storage batteries. The SOC-SOE metric differences are first investigated for single constant-current-constant-voltage (CCCV) cycles under room temperature (25°C) conditions to understand the significance of incorporating voltage into the SOE metric. Experimental results show that the SOC-SOE difference values are a function of CRate, where larger C-Rates result in larger metric differences. This investigation has also shown that lower ambient temperatures increase the SOC-SOE difference values, mirroring the effects of temperature on battery voltage. Finally, tests investigating the effects of battery aging on these metrics indicate that the SOC-SOE difference values increase as the batteries age, suggesting that the SOC-SOE difference variable holds potential for use in state-of-health estimation.