On the other hand, sometimes a battery seems to be running out, but it can continue to be used for a while after being left for a long time. For example, in the past, the mobile phone was turned off without power, and it could be turned on after a while. In addition, when the remote control car can't run when I was a child, I can remove the battery and put it in the remote control for a long time. How much charge does the battery have? Can you figure it out?
Because of the existence of battery polarization, fast charging is necessary, but we have also introduced many fast charging methods to eliminate the influence of polarization. So who should I share the remaining cans with? The power estimation module in the battery management chip has been firmly connected. Who let him rely on "Mongolia"?
Yes, that's right. In fact, the 100% and 50% power displayed on your car or your mobile phone are hidden. After all, the battery won't automatically tell you how much power is left. Even the problem of remaining power has many different indicators to describe.
★SOC, SOE and remaining power are not the same thing.
As we know, a battery is a device for storing electric energy, and its capacity is generally expressed in Ah ampere-hours or mAh milliampere-hours. Everyone who has studied junior high school physics knows that the current multiplied by time is the electricity, and the unit is c coulomb,1(ah) =1(a) * 3600 (s) = 3600 (c). But if this battery still has 10Ah, it will often make people puzzled. Therefore, we generally don't use the remaining power to indicate how much power is left in the battery.
So we convert the remaining power into percentage: SOC, State? Yes? state of charge
For batteries, SOC is defined as the ratio of the remaining battery capacity to the maximum battery capacity. I didn't understand when I said Ann, but obviously there is still 60% power. Therefore, SOC is widely used.
Although SOC is most commonly used to represent battery capacity, its limitation is that it cannot represent energy. Whether we use a mobile phone or an electric car, we actually consume energy, not the charge in the battery, W=UQ. In junior high school, we know that voltage multiplied by charge is energy. When the battery discharges the same amount of electricity, the SOC drops from 80% to 50%. If the discharge voltage is different, the actual energy released is also different. Due to the internal resistance of the battery, the discharge current loss has a large voltage on the internal resistance. The lower the external discharge voltage, the less energy is released. So the state of SOE energy (residual energy of battery) is the most practical, and its calculation is also the most difficult, especially for electric vehicles. After all, the future working conditions are unknown, and the discharge current and voltage are also unknown.
★ How can ★SOC be "blind"
How much electricity is in the battery can't be measured directly, only the voltage and current can be measured and then calculated (estimated). So 100% and 0% SOC are also artificially defined. Just as we use a multimeter to measure the voltage of No.5/No.7 battery to judge whether the battery is charged, car companies will also define the charging cut-off voltage and discharging cut-off voltage for the battery. Use a small current, pay attention to the small current. Charging to the charging cut-off voltage means that the battery is full, SOC= 100%, and discharging to the discharge cut-off voltage means that the battery is dead, SOC=0%.
These two voltages are determined by considering the safety, life and capacity of the battery and the working voltage and current range of the electrical appliance. It does not mean that the battery cannot be charged when SOC= 100%, nor does it mean that the battery is dead when SOC=0%. Just like the battery that the four-wheel drive couldn't use when I was a child, I can put it in the remote control and continue to use it.
Wally and Kongchong are clear, so how to divide the middle part? There are two basic methods: charge accumulation method and open circuit voltage method.
☆ Charge accumulation method (ampere-hour method)
Suppose we know the total capacity of the battery, calculate the current in use and integrate it:
Get the total power in the use process (including charging and discharging), and subtract the used capacity from the initial capacity to get the remaining capacity.
☆ The charge accumulation method has three problems:
1. Dependence on initial value. The charge accumulation method needs to know an initial value, such as the maximum capacity of the battery. But what is the maximum capacity of the battery? Pay attention to the cell phone battery we usually use. Well, I can't see it. Then look at the charging treasure. Generally, the above capacity will be marked with two values: typical value and minimum value. Just like CPU has physical differences, if it is overclocked with good physical fitness, it will be scrapped with poor physical fitness. There are also individual differences in batteries, such as design capacity of 3000mAh and physical fitness of 3 100mAh. Only 2900mAh is in poor health, so manufacturers will mark the minimum value and average value (typical value). If there is no algorithm and sensor to evaluate and optimize the specific battery independently, and the denominator of SOC is a typical value or a minimum value, then the data will be biased, and even a number greater than 100% may be calculated.
2. Cumulative error cannot be ignored. Many error sources, limited sensor accuracy, low sampling frequency and signal interference will distort the measured value of current sensor. Integration is an ideal situation, but in fact, the current sensors collect discrete data, and the error can not be ignored with the passage of time.
There are also methods to eliminate the error, such as fully charging or fully discharging the battery, but in the actual use of electric vehicles, it lacks practicality. The maximum capacity needs to be measured separately, and complete discharge may damage the battery and the vehicle.
3. Can't cope with the self-discharge of the battery. It is also an inevitable problem over time. The self-discharge current is small, so the current sensor can't measure it accurately. After flameout, the battery management system doesn't work and can't be monitored.
☆ Open circuit voltage method
The more the battery charge, the higher the electromotive force. It can be considered that there is a monotonous relationship between battery power and electromotive force. SOC corresponds to battery electromotive force one by one. The electromotive force of the battery cannot be measured, but it is generally believed that the external open circuit of the battery is an open circuit, and the open circuit voltage measured when the current is zero is the electromotive force of the battery, so the SOC of the battery can be determined by the SOC-EMF curve. This curve can be obtained by experimental test.
☆ The open-circuit voltage method also has many problems:
The curve is a sample curve measured in the laboratory, which may not be completely consistent with the characteristics of the battery actually carried on the car, resulting in errors.
2. With the change of ambient temperature, the battery will age periodically, its characteristics will change and its curve will shift. Batteries with different temperatures and aging degrees have different SOC-EMF curves. If this point is not considered, it will also lead to inaccuracy.
3. Dynamic monitoring is impossible. Zero working current means that the vehicle stops completely, which makes the open-circuit voltage method completely unsuitable for dynamic SOC monitoring. In addition, even if the vehicle is not driving and charging, the screen, communication, BMS and other units in the vehicle are still working, so only a small enough current threshold can be determined, and the external voltage measured under this current can be approximately used for SOC-EMF curve.
Moreover, not only the current threshold, but also the time threshold should be considered. Because of the voltage rebound effect and a large number of capacitive components in the vehicle circuit, it takes some time for the working voltage to rise to the open circuit voltage after parking, so a short stop such as waiting for the traffic lights is not enough to complete the calculation of SOC by the open circuit voltage method. Application scenarios are further limited.
4. The SOC-EMF curve of lithium battery has a wider platform area, which is the advantage of lithium battery over lead-acid battery, that is to say, the voltage can be maintained at a higher level in most discharge intervals. However, in the commonly used SOC range 10-90%, the difference of open-circuit voltage measurement of tens, tens and millivolts will cause the SOC to fluctuate greatly, and will also amplify the influence of other errors on the results. Therefore, the reliability of open circuit voltage measurement is extremely high.
5. Voltage has hysteresis effect, that is, the open circuit voltage is different when charging and discharging under the same power. For the application of frequent charging and discharging (energy recovery) of electric vehicles, the error caused by hysteresis can not be ignored.
It can be seen that the two methods have their own shortcomings, but they seem to complement each other. They are United. When the battery is working, the charge accumulation method is used, and when the battery is stopped, the open circuit voltage method is used to eliminate the accumulation error of the charge accumulation method. At the same time, it also solves the initial SOC problem after the battery is placed for a long time without self-discharge. It looks perfect, so it has been widely used in BMS, but it can't solve the shortcomings of open circuit voltage method itself, such as aging, environmental temperature problems, voltage hysteresis and so on.
And eliminating the error will lead to the jump of SOC, such as SOC before correction? 65%, changed to 62% after revision, which is one of the reasons why many early electric vehicle owners found that the power display changed after the flameout was restarted. In real life, this will also affect users' confidence in the battery life display.
Although there are still defects, these two methods are still the optimal solutions at present, at least in hardware, and others, such as internal resistance method and load voltage method, are even less suitable for electric vehicles. Car companies and researchers are also studying ways to improve the accuracy of SOC estimation through optimization algorithms (such as neural network, Kalman filter, fuzzy algorithm, etc.). ) and build more accurate batteries (dynamic response, aging, etc.). ) model.
And most importantly, the greater the discharge current of the battery, the smaller the discharge capacity. As shown in the figure, when 1C discharges, the discharge amount is close to 8Ah, while when 18C discharges, the discharge amount is only 6.5Ah, just like pouring yogurt, which leaves a lot of residue on the cup wall, but it can be emptied slowly.
★ What will happen if ★SOC is not allowed?
The accuracy of "Meng" will seriously affect the actual use of batteries. To give the simplest example, the mobile phone has been used for a long time. After the battery is aging, the display power of 10% often APPears, but opening an app may cause the phone to suddenly lose power. This is that the SOC estimation program of mobile phone is not done well and the battery attenuation cannot be predicted. The SOC calculated by husband formula is not accurate. Putting it in the car will lead to inaccurate estimation of cruising range, which is even more problematic. It seems that there is still a cruising range of 30km, and the vehicle suddenly broke down.
In addition, battery overcharge or overdischarge will lead to the decline of battery performance and life. Therefore, many manufacturers will set redundancy to prevent the battery from over-discharging. For example, a 100kWh battery is only allowed to use 90kWh. But how to know that 10kWh has been used at this time depends on BMS. If the vehicle is allowed to continue driving, the battery will be damaged.
This is a situation where there is more "Meng", and conversely, there is a problem if there is less "Meng". For example, the actual SOC of a vehicle is 98%, but BMS estimates that it is only 93%. At the same time, the vehicle is facing a long downhill, because BMS misjudges the SOC of the battery, which leads to excessive energy recovery and overcharge of the battery, which is also unfavorable to the battery.
In addition, when charging, as mentioned above, the optimal charging curve needs to be adjusted in real time according to the current SOC of the battery. If the SOC is not accurate, there is no control. How many big things can be done? SOC is very aware of the battery's ability.
The accuracy of ★ Meng "is not necessarily the accuracy of display.
The more accurate the measurement and estimation of SOC, the better, but the value displayed to the driver does not necessarily need to be completely true. Based on the problems mentioned above, such as calibration in SOC calculation or voltage rebound after high power discharge, the value calculated by SOC may be 69% one moment ago and 7 1% the next. If the vehicle is actually displayed, the driver will not think that this is an algorithm error, but will distrust the estimation of vehicle power, resulting in a decline in vehicle endurance. This is undoubtedly to be avoided.
Therefore, there is still a layer of processing between SOC estimation and instrument display value, and how to deal with it depends on the car company's own calibration. It is particularly important to note that the output voltage decreases with the decrease of SOC, so if the capacity of a battery is divided in half, the first half can output more energy, which is inherently more durable.
If car companies want to make the whole power consumption process more linear, they can make a change in the display value and display the actual estimated SOC=60% on the instrument as 50%. Of course, other strategies can also be adopted, such as Aion? The practice of S makes the first 20% and the last 20% of the head display particularly durable. This is also possible, because users are often sensitive to the data at both ends of the head and tail, especially in the low power stage, making the display data practical will greatly affect the driver's judgment on whether he can reach his destination before the power runs out.
Of course, for the sake of battery life, you can not only set aside a part of reserved power, but also dig out a full charge. For example, when the SOC is estimated to be 95%, it will be 100%, which reduces the depth of battery charging and discharging and is beneficial to prolong the cycle life of the battery. But at the same time, more redundancy will also sacrifice the actual endurance of vehicles, and different car companies have different trade-off decisions.
★SOC is not equal to cruising range
In addition, it should be noted that electric vehicles will display the cruising range in addition to the percentage of SOC, and the estimation of cruising range will be further based on SOE (note, it is not SOC). We know that SOE is more difficult to estimate than SOC, because the more the vehicle needs high power output, the greater the discharge current, the greater the loss and the less SOE. At the same time, when discharging at high current, the total amount of charge discharged to the discharge cut-off voltage is also less. Therefore, SOE has a strong correlation with driving conditions. The calculation of cruising range needs to consider the driving condition again on the basis of SOE, and the uncertainty is too great, so the accuracy of the estimation of cruising range of electric vehicles is worrying at present.
And it depends largely on the prediction strategy of car companies. Some car companies use fixed mapping, such as Weilai's dashboard battery life, which is based on the current SOC value and converted by full battery life equivalent. Fully charged 400km, half charged 200km, 30% charged 120km. The other is based on the average energy consumption of the previous journey, such as the estimated value of Weilai in the central control screen. Based on SOE calculation (residual energy divided by average energy consumption), the result is conservative and has a strong correlation with driving conditions, but there is the possibility of jumping, such as long uphill first and then long downhill.
In contrast, the SOE of a fuel vehicle can be accurately measured, because how much energy is contained in gasoline is directly proportional to the amount of remaining gasoline, and will not change due to driving conditions. We only need to consider the influence of driving condition variables once. However, electric vehicles need to consider once when calculating SOC (the greater the discharge current, the less power they can discharge), once when calculating SOE (the greater the discharge current, the lower the voltage, and the less energy they can discharge the same amount), and finally once when calculating cruising range. It is not difficult to understand why the estimation of electric vehicle cruising range is a long-standing problem.
References:
1 Tan Xiaojun. Design of battery management system for electric vehicle. Sun Yat-sen University Press. 20 1 1
2 Jiang Jiuchun. Application technology of electric vehicle power battery m? Beijing jiaotong university press, 20 16
, Guo,,. Overview of state of charge estimation of power lithium-ion batteries J. Power technology, 2018.42 (9):1398-1401.
The author of this article is the Kicking Car Gang? Route? 64
This article comes from car home, the author of the car manufacturer, and does not represent car home's position.