A critical aspect of Li-ion batteries is their inherent non-linearity, which significantly impacts the performance of EVs and requires a comprehensive understanding to maximize the battery's operational life. Non-linearity refers to the battery's behavior not being directly proportional to its charge or discharge rates, temperatures, and state of charge (SoC) levels. This poses challenges for the Battery Management System (BMS) that monitors & controls battery operations.
Gandoman et al. (2022) conducted an important study aiming to better comprehend the voltage behavior of Li-ion batteries under various conditions, particularly focusing on 'Open Circuit Voltage (OCV)'.
✔️Key Findings:
- The study showed that the Open Circuit Voltage (OCV) behavior of Li-ion batteries was more stable at 25 °C as compared to other temperatures when they conducted an extensive experiment on NMC batteries, spanning over 300 cycles, to analyze the voltage behavior under various conditions.
- However, at a higher temperature of 45 °C & lower SoC levels (20%), the OCV fluctuations were more noticeable.
- When comparing OCV distribution at different temperatures, the battery's voltage behavior was found to be most variable at 45 °C for 20% & 50% SoC, while the most stable conditions were recorded at 10 °C & 25 °C for an 80% SoC.
- High environmental temperatures resulted in more non-uniform movement of lithium ions through the electrolyte, leading to fluctuations in OCV over the battery's lifetime. This effect was more pronounced at high kinetic energy.
- They also found that the OCV tends to decrease when the discharge rate (C-rate) is ≥2C during the battery's lifetime, most apparent at medium & low SoC levels.
- The study revealed that environmental temperature changes did not significantly impact the OCV at an 80% SoC. However, increasing the C-rate at 10 °C & 45 °C slightly increased the OCV rate over time.
The key factors influencing OCV behavior were found to be internal heat generation, environmental temperature, SoC, & internal resistance.
- At 45 °C & 10 °C, due to temperature & internal resistance factors, OCV behavior remained smooth. In contrast, at 25 °C, neither environmental temperature nor internal resistance alone could maintain smooth OCV behavior.
- The OCV increased for the first 50~100 cycles & then stabilized, highlighting the essential role of increasing internal resistance in stabilizing the OCV rate.
The findings from this study will be pivotal in developing more robust and efficient battery management systems, ultimately leading to the enhanced performance of EVs.