Lifespan is a key factor affecting the cost of lithium-ion batteries. Therefore, all major manufacturers have adopted various means to improve the service life of their own lithium-ion batteries. The lifespan of lithium-ion battery is closely related to many factors. The battery management strategy has a significant impact on the service life of lithium-ion battery packs. In order to better manage the entire battery pack, the control circuit (BMS) needs to be single in the battery pack. The health status (SOH) of the battery is monitored in real time to adjust the power and charge of the battery pack in time to improve battery life and performance. The traditional SOH monitoring method mainly collects the capacity decline and power decay data of the battery. This method is simple and rough, and cannot distinguish the attenuation caused by different attenuation mechanisms, so the SOH state of the battery cannot be accurately determined, so this A monitoring strategy is a relatively inefficient method.
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Yu Merla of Imperial College of Science and Technology in the UK has developed a temperature and voltage based SOH monitoring method, Temperature and Voltage Differential Method (DTV), based on the outstanding use of entropy and enthalpy to analyze the state of lithium-ion battery electrodes by Maher and Yazami. Working, the method only needs to measure the temperature and voltage changes of the battery during continuous discharge, and the SOH state of the lithium ion electricity can be inferred from these data.
Yu Merla uses four 5Ah square lithium-ion polymer batteries (positive NCM, negative graphite) in parallel to form a battery pack (one of the batteries has been pre-treated to test whether the method is effective or not, and the battery pack uses active air cooling. ), and placed a type K thermocouple in the middle of each battery to measure the surface temperature of the battery.
In the experiment, it was found that at the beginning of the discharge, the three new batteries had a larger current, but in the later stage of the discharge, the current of the battery was higher than that of the three new batteries. Generally speaking, large current discharge will accelerate the decline of the battery. After the load is removed, there is a voltage difference between the batteries with different degrees of attenuation. Therefore, charging and discharging will occur between the parallel batteries to reach the same voltage, which will further The battery is degraded, so it is necessary to identify the battery with excessive attenuation and replace it in time for the battery pack.
The data of the collected battery voltage and temperature are differentially processed by MATLAB software. The position and width of the peak of the dT/dV curve can be used to characterize the SOH state of the battery. From the analysis results, it can be seen that the battery is aged during the discharge process. The dT/dV peak has a lower voltage and a wider peak width. In practical applications, if the voltage of the dT/dV peak of a certain battery is found to be significantly lower than that of other batteries, the peak width is significantly larger than that of other batteries, which indicates that the battery has aged and the current distribution in the parallel battery pack is uneven. The aging battery needs to be replaced in time.
In the charging process, due to less heat generation, heat dissipation is faster, and dT/dt<0, so a negative peak appears. Therefore, two points of dT/dV=0 appear on the curve. Due to the close relationship between the voltage and the phase type of the active material, the phase transition point of the active material can be tracked according to the position of the point. From the statistical data, it can be seen that during the charging process, the voltage of the two dT/dV=0 points and the voltage of the negative peak of the aged battery are significantly higher than the voltages of the other three normal batteries, so the data can also be used for detection. The SOH value of the battery.
Compared with low-frequency AC impedance method, ICA method, DHTV method and other detection methods, DTV method only needs a limited number of thermocouples to collect battery temperature data. By differentially processing with the voltage data collected by BMS in real time, the single battery can be obtained. The SOH state, so the cost of using the DTV detection method is much lower than other methods.
Due to the low cost and simple method, the DTV method is very suitable for the SOH state detection of the single battery in the battery pack, and the aging battery is found in time to remind the user to replace, thereby effectively improving the life and safety of the battery pack. This technology can be used for SOH status monitoring of electric vehicle battery packs, smartphones and laptop batteries to improve battery life and performance.
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