Test And Analysis The Performance Of LiFePO₄ Power Batteries

LiFeP04 battery is attracted great attention and used in many fields for their performances characters, and is the hotspot in the field of battery. The characters of batteries in series is different from the isolated ones, so it is necessary to have an insight view of the battery group and obtain the clues to manage the batteries in order to solve the problem of the safety and prolong the longevity of the batteries. In this dissertation, many attentions have been paid to the research about the battery group, such as the signal acquisition, the property of the electro-discharge, consistency and evaluation of the statement.In order to measure the voltage of each single-battery, and the whole current and temperature of the battery group, the first kind of multi-section monitoring chip LTC6802 will be used to sampling a small part of differential voltage from the high common-mode voltage, and a passive balanced charging circuit will be designed by the MOSFET switch related to the chip.To analyze the process of electro-discharge and the traits of battery group, a group with ten 40Ah batteries will be tested under different SOC with different consistent electro-discharge, and effects caused by the change of the discharging current for the characters of the batteries will be investigated, as well as the effects caused by the SOC will be studied. There exists a steady discharging platform for the discharging process of LiFePO4 batteries, for there will cause great change only at the start state and end state, moreover this system will have the capacity to have high-rate of electro-discharge under low SOC state. And the energy effectiveness of electro-discharge will increase by the argument of the SOC, on the other hand, it will decrease by the argument of the ratio of electro-discharge. If the capacity of this group suits the Peukert model, then the loss will become small, when the rate of electro-discharge is high, and the performance of electro-discharge will be better than other batteries. Each battery in the LiFePO4 batteries is almost normal distribution whether with or without imposed current, so the consistency of the batteries can be determined by the distribution of open circuit voltage and polarizing voltage.The statement of the battery group will be estimated by the Dual extended Kalman filter. First, the SOC of the group will be analyzed on the basis of the definition of single battery's SOC. Then the models of the open circuit voltage, working voltage, efficient of electro-discharge ratio, of the battery group, will be set up by the experimentations. In order to isolate the effects of the quantity electricity loss caused by the high current, the model of electro-discharge efficient will be built up by two-stage electro-discharge experiments. For the battery group in certain situations can be equal to a single battery, so the battery's SOC will be calculated by ampere-hour measurement and Kalman filter algorithm, and the quality of SOC of the battery group will be analyzed on the basis of the correlation function of the consistent error constant.