The SOC (state of charge) algorithm has always been one of the key technologies for the development and application of battery management systems (BMS). Therefore, technical articles discussing SOC algorithms are very common, and the high accuracy of SOC estimation by companies is often a highlight of publicity. However, detailed explanations and definitions of SOC are not often considered, which results in the reference value of the SOC algorithm results being greatly reduced. Obviously, if the concept of SOC is vague, how can we get a precise SOC? Therefore, the author hopes to use this article to analyze SOC values in several dimensions and the role of these SOC values.

　　Roughly speaking, SOC = remaining capacity/rated capacity. To accurately express the meaning of SOC, the denominator of the calculation - rated capacity (TotalCapacity) and the numerator - remaining capacity (ResidualCapacity) must be more rigorously defined. The following is the definition of SOC by some companies and organizations:

　　(1) The United States Advanced Battery Council (USABC) defines SOC in its "Electric Vehicle Battery Experiment Manual" as: the ratio of the remaining power of the battery under a certain discharge rate to the rated power (Ah) under the same conditions.

　　(2) South Korea's Kia Motors defines SOC as: SOC = remaining available energy/total available energy (Wh).

　　(3) Japan's Honda Electric Vehicle EVPlus defines SOC as: SOC = remaining power / (rated power - power attenuation); remaining power (Ah) = rated power - net discharge - self-discharge - temperature compensation power.

　　The first difficulty of the SOC algorithm is to define the rated capacity and remaining capacity for different "functional requirements". At the same time, once these two parameters are observed from different property dimensions, temperature dimensions, and battery life cycle dimensions, it is possible to calculate different SOC value. First explain what "functional requirements" are. After calculating the SOC value of the battery pack system, multiple functional modules will call the SOC value as their input. At the same time, different functional modules have different requirements for calling the SOC value. "Functional requirements" can be roughly divided into three categories:

　　1. User reference requirements:

　　The first category is the most common requirement, that is, users need to evaluate the remaining available energy of the battery system to decide how to use the product. Therefore, users are more concerned about the SOC relationship corresponding to the running distance or usage time.

　　2. Reference requirements for vehicle control strategy:

　　The second category is the SOC value that the vehicle control strategy needs to refer to to manage the driving strategy. In particular, hybrid vehicles need to always control the SOC value within a suitable area to achieve energy conservation and emission reduction (the SOC cannot be too high to ensure that braking energy can be recovered as much as possible) and improve performance (the SOC cannot be too low to ensure that the acceleration process high power output), improve energy efficiency (maintain operation in the low internal resistance SOC range), and extend battery life (maintain long-term operation with shallow charging and shallow discharge). Therefore, the vehicle controller is more concerned about the SOC relationship between power characteristics and life attenuation.

　　3.Battery management algorithm reference requirements:

　　The third category is the SOC value that needs to be referenced in the battery management algorithm. Since the battery system will transition from the BOL state to the EOL state with use and storage, the BMS needs to manage the entire life cycle of the battery system. Therefore, the battery management algorithm is more concerned about having an internal benchmark so that the algorithm can find an equivalent SOC relationship in any state between BOL and EOL. Similar to the time value model used in engineering economics, funds at different stages are calculated through the discount rate algorithm to convert or compare.

　　It can be seen that in order to meet the reference requirements of different "functional modules" for SOC values, the meaning of SOC values needs to be more diverse, and the SOC values output by different functions must be more accurate. Next we need to discuss which dimensions should be used to define the SOC value:

　　1. Dimensions of capacity properties

　　When performing the capacity integral operation, we can choose ampere-hour (Ah) as the unit according to the law of conservation of charge, or we can choose watt-hour (Wh) as the unit according to the law of energy conservation. As shown in the figure below, the capacity C is the X-axis and the voltage V is the Y-axis. The point on the X-axis where the 1C discharge ends at different temperatures is the battery's power (mAh) at the current temperature, and the area formed by each discharge curve and the X and Y axes is the energy of the battery (wh) at the current temperature. It can be seen from the figure that the battery voltage platform drops significantly in low temperature environments. Therefore, even if the total power loss is not obvious at low temperatures, the total energy will be greatly reduced. Therefore, when the SOC value is used to measure battery life, it is obviously more suitable to use the energy (Wh) dimension to characterize it. For example: If calculated using the dimension of electricity (Ah), there will be a situation where the energy (wh) released during the 100% to 50% process is more than that released from 50% to 0%. Therefore, the user may make excessive adjustments to the battery life. Optimistic judgment leads to breakdown halfway. This is the first dimension to consider that defines the nature of capacity.

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