# Adaptive Thermal Control of Cell Groups to Extend Cycle Life of Lithium-Ion Battery Packs
## 1 Introduction
modified an air-cooled thermal management system by incorporating evaporative cooling to increase heat dissipation and extend the use of LiBs in harsher environments. [PERSON] et al. [15] designed and fabricated an air-cooled LiB thermal management system with metal-foam-based heat exchanger plates. Their experiments showed that an airflow of 1100 mls\({}^{-1}\) per cell ensured that the temperature rise of the coolant air was less than 10 \({}^{\circ}\)C over ambient even for 200 A charge/discharge cycles and the power required to drive the air through the heat exchanger was less than a conventional liquid-cooled thermal management system. However, current LiB thermal management research has focused more on liquid cooling. [PERSON] et al. [16] focused on designs to enhance temperature uniformity and proposed a methodology to minimize temperature differences across an automotive battery pack by optimizing a serpentine channel design for liquid cooling. [PERSON] et al. [17] conducted an optimization study for cooling a LiB module composed of 90 cylindrical cells. They concluded that, of the three main types of battery thermal management systems (air cooling, liquid cooling, and phase change material cooling), liquid cooling systems are the most popular and practical in EVs because of their high cooling efficiency. They numerically studied the effect of many parameters such as channel geometry, water inlet velocity and temperature, contact angle, width of the plates, and flow direction. In a recent review, [PERSON] et al. [18] summarized the role of thermal design parameters such as the maximum temperature of the cell and/or battery module, uniformity of cell temperature, the magnitude of temperature differences, coolant inlet temperature, flow velocity, and pressure drop for LiB thermal management systems. They also examined various studies that conducted structural design optimization in which the type of flow channel, channel number, channel configuration and geometry, and cell-to-cell spacing were the design parameters. The studies were conducted for various thermal management systems such as air cooling, water cooling, and the use of phase change materials and heat pipes. One important note in this review paper is that the optimal operating temperature range for LIBs was determined to be 25-40 \({}^{\circ}\)C and that the temperature difference between cells should be less than 5 \({}^{\circ}\)C. In practice, such a small margin may be difficult to achieve in long series-cooling loops. Several optimization methods to keep the operating temperature within the ideal range are discussed. Genetic algorithms use the Darwinian theory of evolution to create responses to specific thermal problems [18]. Another unique and fairly recent optimization method is particle swarm optimization which is inspired by the cooperative behavior exhibited by animals to find an optimal solution [18]. There have also been attempts to use hybrid approaches to cooling systems, such as combining liquid plate cooling with phase change materials. One main conclusion of this review paper was that liquid cooling is much more effective than air cooling due to the former's high convective heat transfer coefficient. Therefore, liquid cooling is a better option to improve the LiB's thermal efficiency and overall battery performance. In our study, we examine a new concept of actively controlling the temperature of each individual cell group connected electrically in series during charge and discharge cycles. This approach differs from the current practice in which a chiller unit drives the coolant through the entire battery pack using a series/parallel cooling circuit without any thermal control at the individual cell group level. The primary benefit of this novel approach is the ability to maintain a capacity balance across the series cell groups purely by thermal control to increase the useful life of the pack.