I once remembered such a data. If there is 5% uncertainty in the SOC calculation, the size of the battery pack must be increased by 5%, resulting in a significant increase in battery cost. Suppose, to increase the capacity of a 16KWh battery pack by 5%, an increase of $300 (data at the time) is required. If estimated at today's price, it is also 800 to 1,000 yuan, which is also a considerable cost investment.
Let's compare the expression forms of battery parameters at home and abroad, and the gap can be seen. In the battery system parameter tables of domestic products, it is often seen that the lower limit of SOC is 15% or 20%, which has almost become the lower limit step common to all lithium-ion batteries, and the unique attributes of the selected battery cell cannot be seen. Let's look back at the expression of early leaf product parameters: the lower limit alarm SOC value is 16.25%. We put aside the testing methods behind the data, and only in terms of the accuracy of expression, domestic products have already lost a lot. This rugged demand pattern for battery systems is also reflected in the design process. As refunds approach, cost contradictions will become increasingly prominent, cost issues will become more important, and new product designs need to be carefully calculated.
Analyze the available range of SOC, which is determined by many factors. On the surface, it is unfair to attribute the responsibility for determining the size of the SOC range to the level of BMS management. Good BMS estimation accuracy also needs more complete battery parameters to support. As the saying goes, a good horse needs a good saddle.
First clarify two concepts: What is the available range of SOC? Depth of discharge?
According to the definition in GBT19596 (T=25℃, first introduced from the state of charge):
State of Charge (SOC): "The percentage of the capacity that can be released in the current battery according to the specified discharge conditions to the available capacity." The percentage of the SOC state range is generally from 0% to 100%. However, considering the reaction characteristics of chemical batteries: threshold boundaries, static and dynamic differences, magnification differences, estimation accuracy differences, etc., SOC estimation needs to leave a buffer space to ensure that the battery is always working in a safe area.
SOC usable range: SOC range minus SOC buffer area, the remaining part is SOC usable range. As shown in Figure 1, c-d interval, 15%~95% usable range.
Discharge depth DOD: "The parameter indicating the discharge state of the battery, which is equal to the actual discharge capacity, and the percentage of the available capacity." Numerical relationship: SOC=1-DOD. DOD is more a reflection of the current battery capacity and a measure of the depth of discharge. For example, when expressing battery life, it is often used as a pre-parameter, 1C/1C DOD 80%, 3000 cycles.
Attached figure 1, borrowing the battery parameters of world-renowned brands to illustrate the inclusion relationship between SOC range, SOC available range, battery threshold range, and safety range.
Figure 1
Relevant factors for the precise range of SOC available: first, the completeness and accuracy of battery parameters
Because lithium-ion batteries are chemical products, their energy form is the mutual conversion of chemical energy and electrical energy, and the charge-discharge curve is non-linear. Among them, capacity, energy, and power are greatly affected by factors such as ambient temperature, temperature rise rate, current rate, and SOC state.
If the accurate battery performance test is completed, the entire test process is very time-consuming and labor-intensive. In order to respond to rapid market demand or subsidy policies, many manufacturers artificially speed up the launch of products and sell products while testing. This practice lays hidden dangers for the engineering application of batteries.
The reason for the long product testing time is reflected in each link. Only the standard cycle or the working condition cycle is 3 to 6 months. This is only a factor of the battery product itself. If it is combined with the equipment status, the time will be longer. At present, there are no less than 12 safety tests for battery cells. There are also more than 16 system function tests, as well as regular function and performance tests. If SOCs at different temperatures are superimposed, the test workload is very huge. . It is conceivable how long the product maturity cycle will take for a finalized active material formula to the launch of a qualified product.
The completeness of battery parameters depends on sufficient and multi-sample testing of individual cells. Under normal circumstances, the battery parameter model proposed based on demand is a multi-dimensional state relationship of battery parameters and a comprehensive battery evaluation. This is also a parameter label that the product should have.
Judging from the drawing test, it is generally carried out in several steps. First, the basic functions are tested, and the data that meets the requirements are first tested, similar to the grid of the map, from large to small. For example, the SOC test step is stepped by 5% or 10%. If faced with an interval with higher test accuracy requirements, it is still far from enough. For key sections, you need to focus on testing, at both ends of the battery charge and discharge curve, low-temperature power state, etc.
The above explanations are more aimed at individual battery cells. If you are more sensitive in terms of heat, consistency, power, and energy from the perspective of the system, the difficulty of the test will increase accordingly.
In addition, the more important point is the stability and accuracy of the test equipment. At present, many manufacturers still choose expensive imported equipment for testing in some key links. This is why, mainly to ensure the accuracy and stability of the test. Fortunately, the status quo has changed in recent years. Domestic test equipment manufacturers have devoted themselves to training and growth. The test equipment launched by a large number of excellent manufacturers such as Nebulas Technology not only compares favorably with similar foreign products, but also has a more down-to-earth price and thoughtful after-sales service, which has achieved a good reputation. And recognition.
Second, the correctness and accuracy of the BMS algorithm
The accuracy of the BMS algorithm we mentioned the most is based on the requirements of the battery system. Regarding the issue of optimizing the usable range of SOC, it is not entirely correct to start unilaterally with BMS. As mentioned above, the integrity of battery parameters is also an important factor. It is difficult for clever women to cook without rice, and BMS is timid in the face of missing data.
When referring to the SOC algorithm, the most frequent words are "estimate" and voltage "Approx". This is not inconsistent with the SOC accuracy requirements. Because of the characteristics of the cell itself, the "current state" does vary with the length of time, temperature, and C value. For example, SOC 5%, ValusStatus Approx. 3200-3400mV. Dynamic voltage and OCV value, static shelving time, there are certain differences. This is precisely the difficulty and charm of algorithm strategy.
Of course, if the user-friendliness is considered for the display of the instrument, by establishing the true SOC corresponding relationship with the background, it can be considered as the SOC value facing the user.
The accuracy of SOC estimation is different under different working conditions. Under normal circumstances, we will put forward requirements for BMS, SOC accuracy of 5% or less, in fact, the understanding of BMS engineering is that this accuracy represents the largest error, not the only one.
The available range of SOC determines the lower limit through precise and meticulous strategy control and precise values.
Comprehensive analysis and optimization of the available SOC range is to determine the lower limit of the battery under different conditions and working conditions. The buffer zone of the battery upper limit is very small, so there is not much room for mining. The cache of the upper limit is mainly for charging safety, ensuring that it is not charged for the purpose. In fast charging, the SOC is 80%; in slow charging, it can reach more than 95% by trickle charging. The lower limit of the battery is mainly to consider the discharge conditions and the ability of the discharge current to change, which will affect the power output or driving experience. At the same time, the width of its cache is still very large.
Give an example to illustrate the relationship between the determination of the lower limit and the working conditions:
VOLT has an optimal lifetime safety window (58~65%), which is a more important part of its strategy. This window has different SOC lower limit values ​​according to different working conditions. In normal working mode, the lower limit is set to SOC=30%; in mountain road working mode, the lower limit is set to SOC=45%. This principle is easy to understand. When in the mountain road mode, the C value of discharging or charging (energy recovery) changes greatly. In order to prevent instantaneous overdischarge (undervoltage), overcharge (overvoltage), set The limit voltage reaches the safe state of the battery.
Differences in the available range of EV and HEV SOC
Because the tasks and roles of battery systems in EVs and HEVs are different, the C value requirements are different. EV emphasizes the large driving range; HEV or PHEV emphasizes the dynamic power mixing capability, including the energy recovery capability of large current. The difference in the use of functions also determines the difference in its limits.
At the same time, the higher HEV or PHEV C value will naturally affect the service life of the battery. Therefore, the window or limit value also needs to consider the life factor. As shown in the table below, the battery's depth of discharge has a very large impact on its life.
summary:
Through the above analysis, the key to the SOC available range is the accuracy of the battery parameters and the accuracy of the BMS algorithm. These two aspects are indispensable. At the same time, under the premise of ensuring battery safety, in the face of various working conditions, BMS strategies and algorithms cannot be one-size-fits-all, and more accurate and multi-level implementation is required. On the premise of battery safety, fully utilize battery capacity and optimize and maximize the usable range of SOC.
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