Research on SOC of automotive NiMH No.7 battery

　　In order to estimate the state of charge (SOC) of NiMH No.7 battery, based on the analysis of factors affecting the SOC value and traditional SOC estimation methods, a new idea was adopted based on the actual situation, that is, the working status of the battery is divided into static , recovery, charging and discharging three states, and SOC estimation is performed for the three states respectively. In the estimation process, factors that affect the SOC value are dispersed and eliminated. Especially in the charge and discharge state, the dynamic recovery amount of electricity based on the Coulomb efficiency factor is used to improve the ampere-hour metering method, which solves the problem of the ampere-hour metering method. The problem of cumulative error. Experiments show that this method improves the accuracy of battery SOC calculation and meets the application requirements of power vehicles.

　　Lithium batteries have been widely used in industry, daily life and other fields, and the estimation of battery state of charge (SOC) has become an important part of battery management. However, due to the complex structure of the battery, the state of charge of the battery is affected by factors such as discharge current, battery internal temperature, self-discharge, aging, etc., making it difficult to estimate SOC. Current SOC estimation methods include: open circuit voltage method, ampere-hour measurement method, internal resistance method, neural network and Kalman filter method. Foreign V.Pop and others proposed the EMF-SOC model, which is a model of the relationship between battery electromotive force and state of charge to estimate SOC, which is equivalent to the open circuit voltage method. This method is used to estimate the battery after it has been left standing for a long enough time, and cannot be estimated in real time; Some people also use the ampere-hour measurement method or the Kalman filter method to estimate SOC. The ampere-hour measurement method leads to inaccurate estimation due to large current fluctuations or long-term accumulation of measurement errors; the Kalman filter method has many problems in establishing an accurate and practical battery dynamic model. Therefore, this article adopts a new idea to estimate SOC based on the actual situation of lithium batteries in application, that is, dividing the working status of the battery into three states, and estimating the SOC of each state one by one. During the estimation process The factors that affect SOC are eliminated, and the SOC values in the three states are mutually premised, thereby improving the estimation accuracy of SOC.

　　1. Battery working status and SOC estimation

　　The battery status can be divided into three states according to the actual situation, which are defined here as static, recovery, and charge and discharge. Their relationship is shown in Figure 1.

　　1.1 Stationary state

　　The static state of the battery refers to the state in which the battery has completely recovered after the battery stops working. It is converted from the recovery state and can be directly transferred to the charge and discharge state. The calculation amount of SOC in this state is used as the initial value of SOC estimation in the charge and discharge state. Since this state is characterized by zero current and no polarization, its SOC value has a good correspondence with the open circuit voltage. Therefore, the open circuit voltage method can be used to directly estimate the SOC value of the battery. The relationship between the open circuit voltage of the battery and the SOC value The curve is shown in Figure 2.

　　The relationship between the battery's open circuit voltage and SOC value

　　In the static state, the battery capacity is mainly affected by the self-discharge phenomenon, which causes the battery power to decrease with time. Using the corresponding relationship between the open circuit voltage and the SOC value to estimate the SOC can eliminate the power loss caused by self-discharge. influence, so that the SOC value can more accurately reflect the state of the battery.

　　1.2 Restoration status

　　The recovery state refers to the transition stage of the battery from a discharged or charged state to a resting state. Generally, the time spent in this stage is 8 hours (this value is an empirical value). The calculation amount of SOC in this state is used as the initial value of SOC estimation in the charging and discharging state. The SOC estimation at this time mainly considers the change of battery power after discharge or charging. quantity. After entering the recovery state from the discharge or charging state, the battery power will increase with time. The reason for the change is that the polarization phenomenon occurs inside the battery during the discharging or charging process, and part of the power is not used for actual charging and discharging. Slowly accumulating, when the battery stops working, the polarization phenomenon will slowly disappear and the accumulated power will be restored.

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