This paper introduces Battery-Sim-Agent, a novel framework that leverages Large Language Models (LLMs) to address the challenging problem of battery parameter estimation. The core idea is to frame parameter estimation as a reasoning task, where an LLM agent interacts with a high-fidelity battery simulator in a closed-loop manner. The agent receives multi-modal feedback from the simulator, including quantitative error metrics and visual overlays of voltage curves, allowing it to form physically-grounded hypotheses and propose targeted parameter updates. This approach contrasts with traditional black-box optimization methods, which often lack interpretability and can be sample-inefficient. The authors demonstrate the effectiveness of their approach through extensive experiments on a diverse benchmark suite of simulated battery chemistries and operating conditions, showing significant improvements over traditional Bayesian optimization methods. Furthermore, the framework's practical applicability is validated on real-world battery datasets, highlighting its potential for real-world applications. The paper's main contribution lies in the innovative use of LLMs to mimic a human scientist's workflow for battery parameter estimation, offering a more interpretable and efficient solution compared to existing methods. The experimental results are compelling, demonstrating the framework's ability to handle complex long-horizon degradation fitting tasks and its robustness across different battery chemistries. The authors also introduce a dynamic memory module that allows the agent to learn from past interactions, further enhancing its performance. Overall, this work presents a significant step forward in the field of battery parameter estimation, offering a promising new direction for scientific optimization.