When we talk about lithium batteries, one thing can not be omitted—Lithium Battery SOC.

We usually judge a good lithium battery according to judging its capacity, voltage, cycle life and C-rate and etc. And as we mentioned previously, SOC, State of Charge, represents the short term capacity of the battery.

Left energy of our mobile phone is very important in our daily life, thus we usually give it an highlight place to remind people how much energy there are in this mobile phone battery. According to it, we can properly estimate how long we can use.

Knowing the amount of energy left in a battery compared with the energy it had when it was full gives the user an indication of how much longer a battery will continue to perform before it needs recharging. It is a measure of the short term capability of the battery. Using the analogy of a fuel tank in a car, State of Charge (SOC) estimation is often called the “Gas Gauge” or “Fuel Gauge” function.

There is an oil gauge on a traditional fuel truck, how much oil is left, and how far it can run, you can count it at a glance.If it is an electric car, the driver needs to know how much power is left in the battery pack.The state of charge, also known as the remaining power, SOC, State of Charge, is a parameter that reflects the current power in the battery pack as a percentage of the overall available capacity.Based on the total mileage of the full battery state, the driver can infer the battery life of the current battery, and some models directly display the battery life.

Electric vehicles with inaccurate power estimates often cause some troubles to car owners. The power fluctuated, and when I was about to stop, I glanced at the power, and there was still 50% left, which was estimated to be barely enough for the return trip.When I came back to start the car after a while, I found that the battery indicator was at 40%, and it seemed that I couldn’t go back.……

Suddenly the power was turned off, and when the power was still 30%, he accelerated with one foot, and suddenly the alarm was that the power was too low, and he stopped.……

The accuracy of SOC has always been the focus of criticism by electric vehicle users. On online forums, you can often see complaints from electric vehicle owners that they have been put halfway.For electric vehicles that have been developed for many years, the accurate estimation of SOC is still not particularly good.

What is the use of SOC estimation?

For example, in new energy vehicles, the SOC will tell the driver the remaining mileage.At the same time, the management and control process of electric vehicles and battery packs also requires accurate data from the SOC.

1. Prevent overcharge and over discharge

As an important threshold for charge and discharge, SOC regulates and protects the battery pack.During the charging process, if the battery pack power is too low, such as less than 10%, the charging current cannot be fully turned on, but the current is limited to charging until it reaches the normal power range before the limit can be released.In the discharge stage, when the power is already relatively low, but it is still above the discharge cut-off power, such as as low as 20% SOC, the power output is generally limited to prevent high currents from causing the system to touch the parking voltage, and it is desirable to operate in the most energy-efficient way and run longer distances.

2. As a low battery current limit threshold

Why do I need to set a current limiting strategy when the battery pack is low?When the battery is low, the battery terminal voltage is low. If a high current discharge suddenly occurs, the internal resistance of the battery will increase rapidly, causing the internal resistance to account for the voltage to rise, and the battery potential minus the internal resistance to account for the voltage after the battery terminal voltage will be reduced accordingly.If the current is large enough, the terminal voltage may be pulled down below the stop supply voltage.If the duration of the low voltage exceeds the delay time, the battery management system determines that the battery voltage is too low to continue working and needs to be powered off.So he took the initiative to disconnect the main circuit contactor of the battery pack.A sudden power failure occurred.Of course, the control authority of the main circuit contactor of the power supply of some models is on the vehicle controller.

3. As the threshold value of the vehicle control strategy

There are a variety of electrical appliances, air conditioners, audio, power steering, brake power and so on in electric vehicles.When the SOC is reduced to a certain extent, the permissions of electrical appliances need to be sorted, such as brake power, no matter what the power is, the power supply must be powered, the air conditioner and audio, the power supply must be stopped when the power is less than a certain value, etc.

How to estimate Lithium Battery SOC?

The algorithms that have been well-known to everyone are the open circuit voltage method, the Anshi integral method and the internal resistance method.Recently studied methods include Kalman filtering method, neural network method, etc.New methods are constantly emerging, but most of them are in the research and discussion stage, and the algorithms in practical applications are still dominated by the old methods.

There is a clear and monotonous correspondence between the open circuit voltage of the lithium battery and the charge of the battery. As long as the accurate open circuit voltage is obtained, the battery power can be calculated.First, the open circuit voltage values at different temperatures and SoCs are measured offline to form a table.After the battery system is used, whenever the power supply is stopped, the tabular data can be called to determine the state of charge of the battery based on the measured open circuit voltage.

1.Open circuit voltage method

The open circuit voltage method is accurate in judging the battery power, but there are many conditions and restrictions.The battery must be allowed to stand for a period of time after the loop is disconnected.This requirement makes online measurement impossible.

Some people conducted research on the above problems and found that the battery terminal voltage and the open circuit voltage maintain a stable relationship after the circuit stops supplying power for a certain period of time.Using this method, the process of standing for a long time can be avoided, and the application range of the open circuit voltage method has been expanded.

2.Anshi integral method

The Anshi integration method can measure the main loop current of the battery pack in real time and integrate it into time, charging to negative and discharging to positive.During the discharge process, the initial power is used to subtract the integral result to obtain the current power; during the charging process, the initial power is used to add the integral result to obtain the current power.

However, there is a problem with the Anshi integral method: it is impossible to directly obtain the judgment of the initial power consumption.In addition, due to the high volatility of the system current, current sampling is carried out at intervals of a certain time, so that the sampled value is not necessarily similar to the average value of a period of time, accumulated over a long time, resulting in more obvious errors, and the error is not eliminated by the Anshi integral method itself.Therefore, the practical application of Anshi integration must be combined with other methods to solve the problems of initial value and cumulative error.

3.Internal resistance method

There is a correspondence between the SOC of the cell and the internal resistance.It is theoretically feasible to measure the internal resistance in real time and then obtain the SOC of the battery.However, the trend of following the SOC change from the internal resistance is very gentle, and a small change in the internal resistance or measurement error can cause a large error in the SOC value.Moreover, there are also various possibilities for errors in the internal resistance measurement process.The measurement contact resistance is too large; the battery current is large, and the interference of the internal resistance of the polarization is large; the temperature rise of the battery is inconsistent, which makes the temperature of the temperature monitoring point inconsistent with the temperature of the battery body, causing the temperature compensation to deviate and so on.The internal resistance method estimates the SOC, and there are relatively few practical application cases.

4.Extended Kalman filtering algorithm

The Kalman filtering method, which introduces the concept of time domain, is to treat a process as a collection of countless states that are continuously played on the timeline.The equation of state is used to describe the dynamic process, the measurement equation is used to describe the observation information, and the estimated value of the previous time and the observed value of the current time are used to iterate with each other to update the estimate of the state variable.Some people describe the Kalman filtering algorithm as: using the Anshi integration algorithm to calculate the SOC, and using the open circuit voltage method to check the results of the Anshi integration.The Anshi integral method corresponds to the state, and the open circuit voltage method corresponds to the measurement.If you want to introduce temperature variables, you must choose a circuit model that expresses the relationship between internal resistance and voltage, because in the battery parameters, internal resistance is a relatively easy to measure and closely related to temperature.

The various algorithms extended on the basis of Kalman filtering have been applied in many computing fields, and have become the most mainstream of the new methods for estimating the remaining power of power batteries.

5.Neural network

In the context of the artificial intelligence pandemic, the neural network method is also applied to SOC estimation.The input parameters are loop current and battery terminal voltage, and the output result is SOC.The rest is a general neural network model.As long as a sufficient amount of data corresponding correctly between voltage, current and SOC can be provided, the output accuracy of the model can be continuously improved.

In theory, neural network algorithms are a very ideal estimation method, but the timing of application is still immature. The accumulation of data and the improvement of computing power will promote the practical application of neural network algorithms in the field of power battery SOC computing.

Factors Affecting SOC Accuracy

From the definition point of view, there are two parameters that directly determine the SOC value, the maximum available capacity of the battery and the current power.Judging from the parameters used in various algorithms, the battery terminal voltage and loop current are the most important measurement parameters.

1.Maximum available capacity

As the number of battery cycles increases, the battery gradually ages.The maximum available capacity of the battery becomes smaller as the degree of aging deepens.The maximum available capacity used in the system to calculate the SOC must be adjusted in real time before an approximately accurate SOC can be obtained.

2.Current measurement

Common factors that affect the accuracy of current measurement include: the accuracy of the current sensor, whether the range is appropriate, the presence of electromagnetic interference, and whether the sampling algorithm is reasonable.

The sampling algorithm, if it samples evenly along the time axis, there will be a problem that the current value of the sampling point seriously deviates from the average value of this stage.A more reasonable way is to increase the sampling density during periods of severe current changes, and reduce the sampling density during periods of gentle current, so that the sampling value is closer to the true value.

3.Voltage measurement

Voltage measurement can be divided into two situations: real-time operating terminal voltage measurement and open circuit voltage measurement.

The main influencing factor in the real-time measurement of the operating voltage is the size of the real-time current. On the one hand, the current affects the size of the internal resistance of the battery, and on the other hand, it determines the proportion of the internal resistance to the voltage.Another accidental factor is the size of the resistance between the voltage measurement point and the electrode ear of the battery. It may be poor contact or there may be too long wires. In short, if this resistance is too large, it reaches the order of magnitude of the internal resistance of the battery, the voltage measurement result will be affected.

Open circuit voltage measurement is a relatively simple situation with relatively few influencing factors. It needs to be proposed that the battery needs to stand for a long enough time.

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