Lithium battery SOC calculation method

There is a fuel gauge on a traditional fuel vehicle, and you can tell how much fuel you have left and how far you can run. In an electric car, the driver needs to know how much power is left in the battery pack. The state of charge is also called the remaining capacity, SOC, State of Charge, which is a parameter that reflects the percentage of the current power in the battery pack to the overall available capacity. Informing the driver of the remaining mileage is the value of SOC to the driver. At the same time, the management and control process of electric vehicles and battery packs also requires accurate data of SOC. Today we will take a look at what the battery SOC means, and the calculation formula and method of the SOC of the remaining power of the lithium battery.

what does battery soc mean

SOC (State of charge), that is, the state of charge, is used to reflect the remaining capacity of the battery. It is numerically defined as the ratio of the remaining capacity to the battery capacity, and is often expressed as a percentage. Its value ranges from 0 to 1. When SOC=0, it means that the battery is completely discharged. When SOC=1, it means that the battery is fully charged. The battery SOC cannot be measured directly, and its size can only be estimated by parameters such as the battery terminal voltage, charge and discharge current, and internal resistance. These parameters will also be affected by various uncertain factors such as battery aging, ambient temperature changes, and vehicle driving conditions. Therefore, accurate SOC estimation has become an urgent problem to be solved in the development of electric vehicles.

Lithium battery remaining power SOC calculation formula method

1. Calculation formula and method of ampere-hour integral SOC

Measure the main circuit current of the battery pack in real time, and integrate it with respect to time, the charge is negative and the discharge is positive. In the discharge process, subtract the integral result from the initial electric quantity to obtain the current electric quantity; in the charging process, add the integral result to the initial electric quantity to obtain the current electric quantity. One problem with the ampere-hour integration method is that the judgment of the initial power cannot be obtained directly. In addition, because the system current fluctuates greatly, and the current sampling is carried out once at a certain interval, so that the sampling value is consistent with a period of time

The average value is not necessarily approximate, and accumulated over a long period of time will cause relatively obvious errors, and the errors cannot be eliminated by the ampere-hour integration method itself. Therefore, the practical application of ampere-hour integration must be combined with other methods to solve the problem of initial value and cumulative error.

2. Internal resistance SOC calculation formula method

The internal resistance measurement method uses alternating currents of different frequencies to stimulate the battery, measures the internal AC resistance of the battery, and obtains an estimated SOC value through the established calculation model. The state of charge of the battery measured by this method reflects the SOC value of the battery under a certain constant current discharge condition.

3. Fuzzy logic method

The basic idea of the fuzzy logic method is to use fuzzy logic to simulate people's fuzzy thinking based on a large number of test curves, experience and reliable fuzzy logic theory basis, and finally realize SOC prediction, but the algorithm first needs to have enough understanding of the battery itself, and the amount of calculation Also larger.

4. Fusion algorithm

Current fusion algorithms include simple correction, weighting, Kalman filter or extended Kalman filter (EKF), sliding mode variable structure, etc. The fusion algorithm of simple correction mainly includes the open-circuit voltage correction, the ampere-hour integral method of full power correction, etc.

5. SOC estimation method based on battery performance

SOC estimation methods based on battery performance include AC impedance method, DC internal resistance method and discharge test method. The AC impedance method estimates the SOC through the relationship between the AC impedance spectrum and the SOC. The DC internal resistance method estimates the relationship between DC internal resistance and battery SOC