## 1 Introduction
[PERSON] and [PERSON] [41] immersed a prismatic battery into AmpCool AC-100 dielectric liquid and showed that the average battery temperature is reduced by approximately 6 \({}^{\circ}\)C with the immersion method, and the temperature fluctuations of the battery cooled with dielectric liquid are less than those cooled with air. [PERSON] et al. [42] presented that the average and maximum temperatures can be decreased to 6.5 \({}^{\circ}\)C and 9.1 \({}^{\circ}\)C with the use of direct and indirect cooling techniques, respectively. [PERSON] et al. [43] experimentally and numerically investigated the cooling of pouch-type Li-ion batteries and revealed that the highest temperature at the positive tab could be decreased by 46% with the application of immersion cooling instead of natural convection. [PERSON] et al. [44] constructed a battery pack and showed that the temperature of the batteries could be kept at approximately 47 \({}^{\circ}\)C as the maximum value in the considered operating ranges. [PERSON] et al. [45] reported that the maximum temperature and temperature difference of batteries could be decreased by 13.4 \({}^{\circ}\)C and 3.54 \({}^{\circ}\)C, respectively, with the use of transformer oil instead of air. Moreover, the experiments showed that coolant flow rate does not have a significant impact on battery temperature difference under the considered operating conditions. [PERSON] et al. [46] showed that the battery temperatures are approximately 35 \({}^{\circ}\)C and 31 \({}^{\circ}\)C for air and liquid velocities of 10 m/s and 0.01 m/s, respectively, for a discharge rate of 2C. In addition, these results were compared with immersion results, and it was concluded that the immersion method performed the best cooling since the battery temperature is approximately 28 \({}^{\circ}\)C. [PERSON] et al. [47] researched the temperature distribution of pouch-type Li-ion batteries with the immersion cooling method by using dimethyl silicone oil and compared the results with natural air cooling. They stated that the specific heat and flow rate have a significant effect on battery temperature compared to the thermal conductivity of the fluid. [PERSON] et al. [48] clarified that the battery temperature could not be kept in desired value range with the application of indirect and air cooling methods at high discharge rates such as 10C, so it is necessary to use a direct cooling method. As a result of the study, they stated that the immersion of batteries into FC-72 and HFE-7100 fluids could be a solution for battery thermal management systems. Recent growth in the use of electric vehicles has brought up an issue with battery temperature management since the temperature of batteries affects electric vehicles' range and lifespan of batteries. Although indirect liquid cooling methods have been generally used for solving problems about this issue in applications, it is well known that the direct liquid cooling (immersion) method presents higher heat transfer performance compared to the indirect method. It can be understood from the literature survey that the immersion method can be a good option for battery thermal management systems. In the literature, there is a limited number of research on immersion cooling of prismatic Li-ion batteries and heat transfer oils and hydrofluoroethers are selected as working fluids. In this study, an experimental investigation has been conducted to present the effect of immersion cooling on the temperature distribution of the LiFePO\({}_{4}\) (LFP) pouch-type battery. Firstly, natural air convection tests were performed, and then immersion cooling experiments were conductedby using distilled water as the fluid because of its higher specific capacity. The experiments were conducted for various discharge rates (1-4C), immersion ratios (50-100%) and coolant fluid inlet temperatures (15-25 \({}^{\circ}\)C). The variation of average and maximum surface temperatures with a depth of discharge/time was researched for considered operating conditions.