This study examines how heat affects lithium-ion batteries during their operation through detailed computer modeling. Our research uses computational fluid dynamics (CFD) to track heat flow and battery behavior in a rectangular pouch battery design. The model focuses on changes across the battery's thickness, measuring key factors: how electrical current moves, how heat dissipates, and how lithium ions travel through the battery. We tested the battery under two main conditions. First, keeping it a steady room temperature of 298 K, and second, letting it generate heat naturally during use. For each scenario, we tried three different charging speeds: a standard rate (2C at 18 A/m2), a medium rate (4C at 53 A/m2), and a fast rate (6C at 88 A/m2). Our results show important patterns in battery behavior. When batteries charge and discharge faster, they lose capacity more quickly. This happens both when the temperature stays constant and when the battery heats up during use. Interestingly, when heat builds up, the battery takes longer to complete its charge discharge cycles. This occurs because higher temperatures change how the battery works in two ways: they alter the electrical properties of the battery's electrodes and make it easier for lithium ions to move around inside. To address these challenges, we investigated various cooling methods. Our findings suggest that controlling battery temperature is crucial for maintaining performance, especially during fast charging. These insights help explain why battery sometimes underperform and point to ways we can make them work better through improved cooling systems. This study advances our understanding of why batteries heat up during use and how this affects their performance, offering practical solutions for better battery management.