Research On The Safety Warning Method Of Lithium-ion Battery Based On Dynamic Impedance

Lithium-ion batteries are widely used in electronic products,new energy vehicles and large-scale energy storage.In recent years,fires and explosions caused by lithium-ion battery abuse have occurred frequently,and safety issues have come to the fore,becoming a major bottleneck for large-scale applications.Under the condition that there is no revolutionary change in the current working principle and manufacturing level of lithium-ion batteries,a battery management system(BMS)with excellent performance is the basis for ensuring the safe application of lithium-ion batteries.The sensing inputs of current mainstream battery management systems are the terminal voltage and surface temperature of a single cell,which have many advantages but also have many problems.In order to improve the safe application of lithium-ion batteries,adding sensing parameters in the BMS is a necessary path.Electrochemical impedance can be used to effectively reflect the electrochemical characteristics and operation of the battery,but it is mostly measured statically.In this paper,the online measurement of dynamic impedance of lithium-ion batteries is used as an entry point to study the relationship between the dynamic impedance of Li-ion batteries and the safety-related characterization of each stage of battery operation,and to provide theoretical and technical basis for the construction of a new BMS,with the following main research contents and results.(1)A scheme for online measurement of dynamic impedance of lithium-ion batteries with current source excitation is proposed.The battery safety problem occurs during the charging and discharging process,and the accurate acquisition of the dynamic impedance of the battery under the working condition is the key to the safety warning.The online measurement scheme utilizes the characteristics of large internal resistance of current source,constant current and strong anti-interference ability,and adopts "four-wire system" to inject sinusoidal current of specific frequency into the battery in working condition,and obtains the response voltage of the same frequency.The "strong-DC-weak-AC and no-phase-shift" circuit is designed to amplify the weak voltage response signal at a high magnification to achieve the measurement of the dynamic impedance of the battery.The "strong-DC-weak-AC and no-phase-shift" circuit in the measurement scheme not only solves the problem of distortion of the weak sinusoidal signal caused by the large DC component,but also realizes the amplification of the weak sinusoidal signal,which provides a technical solution for the integration of dynamic impedance parameter measurement in the battery management system.The test results of the prototype designed and developed by this solution show that the impedance measurement range is wide,the anti-interference ability is strong,and the measurement accuracy is high,which can realize the online measurement of dynamic impedance.(2)A method is designed to calibrate the internal temperature of the battery by dynamic impedance,and a dynamic impedance-internal temperature prediction model is established to realize non-intrusive battery internal temperature measurement and increase the safety warning time.Different batteries have different electrochemical impedance spectra,and there are impedance parameter-battery internal temperature sensitive frequency points on the impedance spectrum.In this paper,the frequency points are selected by studying the dynamic impedance spectrum,and the impedance-internal temperature curve of the battery is calibrated by the static method,and the relationship between the impedance parameter and the internal temperature at specific frequency points is modeled,and the internal temperature of the battery is obtained by dynamic impedance measurement,which realizes the non-intrusive sensing of the internal temperature of the battery.Compared with the external temperature measurement on the battery surface,the isothermal time point is greatly advanced,which increases the safety warning time.Taking the lithium iron phosphate battery as an example,the effectiveness,achievability and interference resistance of internal temperature sensing are verified by probe implantation temperature measurement under 1 C and 2 C charging modes.Taking the 50 ℃ co-temperature point as an example,the predicted sensing time is 591 seconds earlier than the surface temperature sensing time,which provides sufficient time for taking measures to prevent the occurrence of danger.(3)By studying the dynamic impedance change law in the process of single cell battery overcharge,the dynamic impedance acceleration decay characteristics of single cell battery micro-overcharge are found,and the proposed "first-order fitting residual" quantifies the impedance acceleration decay characteristics to realize the effective early warning of battery micro-overcharge situation.The modular use of batteries leads to the deterioration of battery inconsistency,resulting in micro-overcharging of a single cell in the module and causing malicious accidents.By analyzing the comprehensive influence of the state of charge(SOC)and internal temperature on the impedance curve during charging,the accelerated decay of impedance under micro-overcharge conditions is found,and the first-order fitting residual is constructed to quantify the accelerated decay of impedance;using the set residual threshold as the criterion,an early warning scheme based on impedance measurement for the occurrence of micro-overcharge of a single cell is established,and the experimental results verify the effectiveness of the warning scheme and provide technical support for the early warning of micro-overcharge of a new BMS.(4)Through the theoretical and experimental analysis of the thermal runaway process and mechanism of the battery,the slope shift characteristic of the dynamic impedance modulus of the lithium-ion battery before the thermal runaway occurs with the progress of the overcharge is found,and the early warning of the thermal runaway of the lithium-ion battery under the overcharge condition is realized with the slope shift characteristic as the symbol.Thermal runaway during battery charging and discharging is the main cause of combustion and explosion accidents in most battery application scenarios.By analyzing the mechanism of thermal runaway,thermal spread law and overcharge gas production principle of Li-ion batteries,the influence law of gas production on dynamic impedance change during battery overcharge is clarified,and the impedance slope shift characteristic during overcharge progression is found to be universal.With the dynamic impedance slope shift characteristic as a marker,the battery can be warned in advance of thermal runaway,providing a time window to avoid the occurrence of thermal runaway.This early warning method can be realized only by the measurement of dynamic impedance and simple calculation,and has a clear electrochemical theory basis and high reliability.