*Article* **Evaluating and Analyzing the Degradation of a Battery Energy Storage System Based on Frequency Regulation Strategies**

**Chen-Han Wu, Jia-Zhang Jhan, Chih-Han Ko and Cheng-Chien Kuo \***

Department of Electrical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan; d10907002@mail.ntust.edu.tw (C.-H.W.); d11007004@mail.ntust.edu.tw (J.-Z.J.); wing0990242@gmail.com (C.-H.K.)

**\*** Correspondence: cckuo@mail.ntust.edu.tw

**Abstract:** The capacity aging of lithium-ion energy storage systems is inevitable under long-term use. It has been found in the literature that the aging performance is closely related to battery usage and the current aging state. It follows that different frequency regulation services, C-rates, and maintaining levels of SOC during operation will produce different battery aging rates. In the simulations, the researchers used real frequency data to generate SOC curves based on the Taiwan frequency regulation services under different C-rates and different battery SOC target levels. Then, the aging formula of lithium iron batteries (LiFePO4 battery, LFP battery) and the proposed improved rainflow counting algorithm were used. The capacity aging situation and economy under different usage scenarios were analyzed. The simulation results showed that using a high C-rate and a low SOC level had a higher net profit, and the income of dReg was more than that of sReg. The SOC of BESS has an important impact on the life cycle. Keeping the SOC at a lower level will help prolong the life cycle and increase the net income. In dReg0.5, maintaining the SOC at 30% would yield 8.5% more lifetimes than 50%, 20.6% more lifetimes than 70%.

**Keywords:** battery energy storage system (BESS); frequency regulation service; battery degradation; automatic frequency control
