| The power battery of electric vehicles is generally composed of hundreds of battery cells connected in series and parallel,but due to differences in capacity or/and impedance between individual cells,the cycle life of the battery pack is often shorter than that of individual cells.Therefore,when grouping,batteries with similar parameters(voltage,capacity,resistance,etc.)should be selected and grouped together to improve the consistency of individual batteries in the battery pack.The assembled individual batteries are then connected in series or parallel through processes such as resistance welding,ultrasonic or laser welding,but the consistency of electrical connections during connection also affects the performance of the battery pack.In this paper,the effect of electrical connection impedance among LiFePO4/C individual batteries on the performance of the battery pack were studied.The electrical connection is controlled by adjusting the tightness of the connection screws between individual batteries(The impedance value of the normal electrical connection point in the battery pack is about 0.2 mΩ,and electrical connection points with higher impedance than the normal are set as faulty electrical contact points(FECP)).The results show that the presence of FECP affects the temperature distribution uniformity of the battery pack.When FECP exists in the series circuit of the battery pack,it can be easily detected by monitoring the temperature changes at the connection points.However,if FECP eexists in a parallel circuit,only after the voltage inflection point appears in the later stage of charging and discharging,the FECP in the parallel circuit will show the characteristics of higher temperature than other normal connection points However,during the charging and discharging process,the temperature of the FECP in the battery pack will ultimately be higher than the temperature of other normal connection points,resulting in higher temperature of the individual batteries adjacent to the FECPand affecting the cycle life or safety performance of the battery pack.In addition,the presence of FECP in parallel battery modulessignificantly reduce the capacity of the entire battery pack.At the same time,during the discharge process,the presence of FECP will also cause some individual batteriesin the parallel module to reach the discharge cutoff voltage earlier,thereby stopping the entire battery pack from discharging.The presence of FECP in a series circuit will significantly reduce the charging and discharging power characteristics of the battery pack,thereby further affecting the cycling life of the battery pack.Further,in the study of cycle lifeof battery pack with FECP,the cycle cutoff condition was that the capacity decayed to 80% or the discharge terminal voltage of the individual battery dropped to 2.0 V.The cycle life of the normal 3P3 S and 1P3 S battery packs was 1210 and 1260 ccycles,respectively,which is less than 1510 cycles of an individual batteryindividual.However,a 3P3 S battery pack with one FECP in the 3P parallel module and a 1P3 S battery pack with one FECP in the series circuit automatically stopped the cycle at the 1097 th and 381 st cycles due to the discharge terminal voltage of an individual battery being lower than2.0 V.At the same time,as the cycle number of increased,the voltage difference between the highest and the lowest voltage of the individual battery at the end of battery pack discharge also increased,which is also a characteristic data of battery’s health status.The discharge voltage difference in the battery pack with a FECP is much higher than that of a normal battery pack,but the discharge voltage difference of battery pack is much higher than the charging voltage difference under the same condition.The higher the connection impedance value of the FECP in the battery module,the greater the voltage difference between the individual batteries in the battery pack and the temperature difference between the connection points,and the higher the charge/discharge rate of the battery module,the more significant the difference between the pressure voltage and the temperature difference.At the same time,it has found that an electrochemical workstation with an auxiliary voltage divider module can be used to quickly and non-destructively detect the presence of FECP in the battery pack.If the AC impedance spectra curves of each individual battery in the parallel battery pack can relatively coincide,it indicates that the consistency of each individual battery and the parallel electrical connection point is good.When the impedance of the positive and negative electrode connection points is close to each other and both are greater than the normal value,the AC impedance spectra curves of the parallel individual battery can overlap.However,at this time,the ohmic impedance of the battery pack tested by the main channel will be higher than that of a normally connected battery pack.If the impedance of one of the positive and negative electrode connection points is higher than the other,the position and shape of the AC impedance spectra curves of the parallel individual battery will deviate.The AC impedance spectrum curve of the smaller current battery will move in the direction of smaller Ohmic impedance,and the charge transfer impedance will also decrease with an increase in the impedance value of one connection point.Moreover,if there are differences among the individual batteries in parallel battery pack,the AC impedance spectra curves of each individual battery in this method will not overlap.The high internal resistance battery will exhibit a characteristic of high Ohmic impedance,but there will be no charge transfer impedance changes inconsistent connection points among the individual batteries.The battery with large internal resistance will exhibit a characteristic of large ohmic impedance,but there will be no change in charge transfer impedance caused by inconsistent electrical connection points between individual batteries. |
PDF Full Text Request