# Adaptive Thermal Control of Cell Groups to Extend Cycle Life of Lithium-Ion Battery Packs
## 1 Introduction  
Lithium-ion batteries (LiBs) are at the heart of electric vehicles (EVs). Despite their many advantages such as compact size, high charge and discharge efficiency, high usable capacity, and the ability to tolerate deep cycling, LiBs require careful voltage monitoring for safe operation, and their operating conditions must also be controlled to prevent temperature extremes. Thermal management is an important aspect of EV battery packs, and the heat generated during battery operation needs to be effectively removed from the system so as to maintain the cell temperature within prescribed limits. Hence, LIBs employ a battery management system (BMS) to maintain individual cells within safe operating limits of voltage, current, and temperature. Many BMS aspects have been covered in the literature [1; 2; 3], in which researchers investigated the role of various parameters to improve thermal performance and safety, increase battery life, and reduce maintenance costs. However, one key area which has remained a significant challenge is capacity imbalance within a series/parallel lithium-ion battery pack over its lifetime. This imbalance can result from thermal gradients, current flow discrepancies caused by terminal connections, or inherent differences in individual cell impedance due to cell manufacturing discrepancies [4]. In addition, heat generation rates can vary based on the cell type and even across cells of the same type within a pack [5; 6]. The conventional method to deal with such imbalance is oversizing the passive or active electrical balancing components, which results in a suboptimal system. Compounding the problem is the tendency for cell capacity to fade at a faster rate after a high number of cycles (>600, saturation degradation stage) is reached [7]. Failure to adequately balance cells can lead to poor charge and discharge rate performance, a drastically reduced battery pack lifespan, and even thermal runaway. The two primary strategies for achieving a balanced battery pack are active balancing in which cells may be charged or discharged to obtain the same voltage within a pack,and passive balancing which involves only discharging cells. [PERSON] et al. [8] published a comparison study between active and passive balancing and found that active balancing performed better over 1000 cycles than passive balancing but at the cost of added complexity and the energy used to perform the voltage balancing process. One important practical aspect of battery pack construction is reducing thermal gradients within the groups of cells inside the pack. For economic and safety reasons, often there is a series or parallel cooling loop with a single chiller for the entire pack. [PERSON] et al. [9] studied the thermal gradients within battery packs and determined that cells near the interconnect terminals experience higher currents than cells in the center of a parallel group of cells. This led to conclusions about how the resulting thermal gradients affected cell-to-cell imbalance that are used as the basis for our work. Thermal effects on LiBs [10] have been well studied in the literature. For example, [PERSON] et al. [11] demonstrated that the thermal environment has a huge impact on battery degradation within a commercial battery pack. They used electrochemical impedance spectroscopy at different operating temperatures as a diagnostic tool and their results confirmed an increase in solvent resistance and significant thickening of the solid electrolyte interphase (SEI) at high and low temperatures, respectively. [PERSON] et al. [12] proposed a coupled electrochemical-thermal-mechanical model for LiB aging. Their model predicted that high ambient temperature caused the SEI layer to increase in thickness, low ambient temperature was responsible for anode plating, and high C-rates led to cathode material loss. [PERSON] and [PERSON] [13] linked the variation of battery electric vehicle range and efficiency with ambient temperature which would impact the cooling design. In [14], [PERSON] et al.