Enhancing Battery Available Operating Time (BAOT) via a Passive Series-PCM and Optimized Finned Structures

Keeping the battery temperature below a reasonable limit of 50 °C is a primary objective of battery thermal management systems (BTMSs). Accordingly, the battery available operating time (BAOT) can be defined as the time required for the battery maximum temperature to reach 50 °C, which could be adopted as a key indicator for safe and efficient operation. BAOT can be improved through different BTMS configurations. This work focuses on passive solutions, aiming to increase BAOT without requiring pumping power. The study numerically investigates the combined use of phase change materials (PCMs) and fins to evaluate their effectiveness in terms of time percentage improvement (TPI). A preliminary analysis is conducted to assess the need for PCMs and fins at three discharge rates, namely 1C, 3C, and 5C. The results indicate that PCMs are required under all operating conditions, while the use of fins is not always advantageous; in particular, at 1C, fins lead to a reduction in BAOT. The analysis then focuses on the 3C and 5C cases, where topology-optimized fins are employed to dump temperatures under these stress conditions. Three fin arc lengths (ψfin) and eight diffusion coefficients (Rf) are examined. The optimized fin configurations increase BAOT, achieving maximum TPIs of 10.61% and 7.69% for the 3C and 5C cases, respectively, both corresponding to ψfin = 2.75 mm and Rf = 0.10 mm. At 5C, BAOT is limited to only a few seconds; therefore, configurations with PCMs arranged in series are also analyzed using different combinations of four selected PCMs. When coupled with optimized fins, the PCM-in-series solutions yield further improvements, with maximum TPIs of 22.92% for 3C and 62.50% for 5C compared to the single-PCM configuration coupled with optimized fins. The results also show that the optimal diffusion coefficient and PCM arrangement strongly depend on the discharge rate.