Reliability-Oriented Fast Charging Strategies for Lithium-Ion Batteries and EV Chargers

Electric vehicles (EVs) offer a sustainable and eco-friendly transportation solution, reducing greenhouse gas emissions and dependence on fossil fuels. As EV adoption increases, expanding charging infrastructure and improving lithium-ion battery (LIB) technology is crucial. However, key challenges such as fast charging accessibility, extended LIB lifetime, and high EV charger reliability must be addressed. High charging currents accelerate LIB degradation mechanisms, including lithium plating, electrolyte decomposition, and increased internal resistance, leading to capacity fade. While various advanced charging strategies aim to balance charging speed and LIB lifetime, their impact on power electronics converters, particularly dual-active bridge (DAB) converters used in fast chargers, remains a critical consideration. Failures in these converters due to thermal and electrical stress can lead to costly maintenance, charger downtime, and reduced system efficiency. Optimizing battery charging profiles and ensuring the reliability of power electronics converters is essential to developing reliable and efficient charging solutions.
This Ph.D. project investigates the multi-stage constant current (MSCC) charging strategies for LIB performance and lifetime and their influence on DAB converter reliability. Five-stage constant current (5SCC) charging with state-of-charge (SOC)-based stage transitions is selected among various MSCC charging strategies. Taguchi optimization is applied to determine the optimal charging currents for each stage to enhance its performance. These optimized 5SCC profiles are experimentally validated against the standard constant current-constant voltage (CCCV) charging method, demonstrating marginal improvements in LIB performance, such as a reduction in charging time. However, accelerated aging experiments are conducted to investigate its impact and thoroughly quantify LIB lifetime. On average, the optimized 5SCC profiles extended LIB cycle life by 7% to 40% compared to equivalent CCCV charging.
Beyond LIB aging, the impact of these optimized charging profiles on DAB converter reliability is examined by considering charging profiles as mission profiles. The results showed that the higher initial charging current in 5SCC profiles led to increased junction temperatures in power semiconductor switches, accelerating degradation and reducing converter lifetime. In contrast, constant current (CC) charging exhibited higher converter reliability due to lower thermal cycling stress on power components. This trade-off suggests that while 5SCC charging enhances LIB lifetime, it compromises power converter reliability due to increased thermal stress during the initial high-power charging stage.
In addition, a co-optimization framework considering the LIB and the DAB converter lifetime using weighted sum optimization is applied. The weight factors are assigned based on the cost impact of LIB packs and fast EV chargers. The results showed that CC charging outperformed 5SCC in terms of overall system reliability but required approximately 10% longer charging time. However, from a fast-charging perspective, the Tavg-based optimized 5SCC profile appeared as the most balanced solution, offering enhanced LIB lifetime, better converter reliability relative to other 5SCC profiles, and reduced charging duration.