Study On State Of Health Evaluation For Li-ion Battery Pack

The current environmental and energy issues have become increasinglyprominent, electric vehicles comply with the trend of the times, it can ease the crisisin a certain extent. However, electric vehicles have not yet popular, the main reason isthe power battery performance cannot meet the requirements. Li-ion batteries havebecome the mainstream of the power batteries because of its relatively superiorperformance. Except research on the battery chemical material, the study of batterymanagement system (BMS) is also in full swing, but current research is still onestimating SOC precisely and balance strategy, evaluate the state of health is a weakpoint.The state of health (SOH) describes the actual capacity constantly decreasingwith the increasing use of cycles. Based on the principle of Li-ion battery, the articlediscusses internal factors such as electrode materials, electrolyte solution, battery poleand external factors such as charge and discharge rate and off-voltage, temperature,the connection of packs influence on battery decay.General battery SOH research focused on estimating single battery SOH, a fewresearch on battery pack SOH. This thesis evaluate the pack SOH depend on bothsingle battery SOH and pack consistency. It gets the internal characteristics of thebattery through establishing the single cell model and a series of experiments toidentify the parameters. Comparing different batteries ohmic resistance andpolarization resistance, concluded that ohmic resistance is important to the batteryperformance, so the thesis evaluate pack consistency depend on ohmic resistance andSOC. On the other hand, because the battery discharge capacity is approximately aconstant throughout the whole life, so the thesis estimate single battery SOH throughthis nature. In the end, evaluate pack SOH through single battery SOH and packconsistence.This article improves BMS strategy by analyzing the SOH. The current BMScontrol strategy based on operating voltage, in fact, operating voltage cannot reflectSOC exactly because of ohmic resistance. To eliminate the effects of resistance, this thesis use open circuit voltage as a basis to determine the strategy. On the other hand,traditional balance only focuses on lowest voltage battery, this article proposes tobalance depend on the lowest SOC from the perspective of balance essence andprotecting packs to keep consistence and extend battery life.