As a professional specializing in the new energy industry, I will be sharing fundamental knowledge about electric vehicle technology in the coming days, based on my in-depth understanding of the field of new energy power lithium-ion batteries, to help you truly understand the industry's core technologies. This time, I've chosen to focus on the State of Charge (SOC) of the power lithium-ion battery management system. Firstly, SOC is the core of the bMS, which in turn is the core of the power lithium-ion battery, and the power lithium-ion battery is the core of new energy vehicles; SOC is crucial to new energy vehicles. Secondly, the new energy vehicle sector is too vast to discuss in depth; discussing it in a more accessible way allows for better control and deeper learning.
SOC is short for the current remaining charge/capacity of a power lithium-ion battery. Cars use SOC to understand the current state of charge. Through SOC, we can discuss the comprehensive influencing factors and form a macroscopic system concept.
I. Current Situation Analysis
Without an accurate SOC, the following situations may occur:
1. Overcharging/over-discharging can shorten battery life and cause the battery to fail completely.
2. The uneven consistency is not ideal, resulting in reduced output power and decreased dynamic performance;
3. To prevent breakdowns, excessive redundant power is set, reducing overall energy output;
Therefore, accurate SOC estimation is of great significance. For car owners, SOC directly reflects the current state of the battery and how far it can still travel, ensuring a smooth journey to the destination. For the battery itself, accurate SOC estimation involves nonlinear effects from open-circuit voltage, instantaneous current, charge/discharge rate, ambient temperature, battery temperature, storage time, self-discharge rate, coulombic efficiency, resistance characteristics, initial SOC value, DOD, etc. Moreover, these external characteristics influence each other and are also affected by different materials and processes. Therefore, accurate SOC estimation becomes very important, and its algorithm is one of the core competencies of relevant companies.
Next, we will discuss the current status of the SOC algorithm, analyze its influencing factors, and discuss practical problems.
II: Current Status of Algorithms
Currently, the mainstream methods for estimating SOC include the discharge method, the ampere-hour integration method, the open-circuit voltage method, the neural network method, and the Kalman filter method.
■ The discharge method involves conducting a discharge test on the battery and determining the battery capacity based on the amount of electricity discharged. However, in actual driving situations, the remaining battery power is used for driving, so the discharge result cannot be used as a standard for estimating battery capacity.
■ The ampere-hour integration method calculates the current charge by integrating the current and time under initial and operating conditions. The accuracy of the current state of charge (SOC) depends heavily on the accuracy of the initial and instantaneous current. However, as time goes on, the error accumulates significantly and cannot be corrected independently.
■ The open-circuit voltage method is based on the relationship between the static open-circuit voltage and the state of charge (SOC) of batteries with different material systems and processes.
However, an accurate open-circuit voltage requires a period of rest to recover. This is because the charging and discharging processes allow the internal chemical reactions of the battery to continue for a period of time, prolonging the polarization state and forming a polarization potential. This increases or decreases the instantaneous open-circuit voltage, making the simple open-circuit voltage inaccurate under actual operating conditions due to vehicle interference. Therefore, the open-circuit voltage measured under operating conditions can only be used as a reference and is not the true open-circuit voltage.
■ The neural network method uses various instantaneous data such as local voltage, current, temperature, and internal resistance to form the input layer, automatically summarize rules into the hidden layer, and then converge and optimize through the output layer of the system model to form the instantaneous SOC. The information of each layer does not communicate with each other and has no connection. However, there are currently no commercially viable convergence, optimization, and modeling techniques, which are characterized by high cost and poor stability. The technology is still in the research stage.
■ The Kalman filter, proposed by Rekalman of Hungary in 1960, is a digital filtering algorithm based on the minimum mean square error, used for optimal estimation of the state of dynamic systems. Its advantage is its strong ability to correct initial errors; its disadvantage is that it requires strong data processing capabilities, and its accuracy is determined by the battery model. It is currently a hot research topic.
In summary, neural network methods are too difficult, Kalman filtering has been extensively studied but lacks practical operational data, discharge methods are impractical, and ampere-hour integration and open-circuit voltage methods alone result in significant errors. Currently, the mainstream approach combines ampere-hour integration with open-circuit voltage methods, which is easier to implement. Passenger vehicles from companies like Huizhou Yineng, Kele, and CATL have achieved errors within 5% to date.
The ampere-hour integration method and the open-circuit voltage method are influenced by many factors. Decomposing these factors is essential for us to gain a deeper understanding of battery characteristics and can also help us to continuously improve and refine the development direction of SOC accuracy.
III. Influencing Factors
The accuracy of State of Charge (SOC) is closely related to that of power lithium-ion batteries. Even when using ampere-hour integration and open-circuit voltage for calculation, correction factors for other influencing factors are still required. Factors such as open-circuit voltage, instantaneous current, charge/discharge rate, ambient temperature, battery temperature, storage time, self-discharge rate, coulombic efficiency, resistance characteristics, initial SOC value, DOD, as well as material characteristics and processes are interrelated and jointly determine and influence the SOC state. We will break them down one by one below.
■ Open-circuit voltage refers to the voltage value across the battery terminals when no load is connected. Because there is a curve-like relationship between the stable open-circuit voltage and the State of Charge (SOC), a specific batch of batteries can have its SOC value determined by fitting the relationship between open-circuit voltage and SOC. However, in actual operation:
Higher temperatures result in higher open-circuit voltages. As temperature increases, the electrolyte viscosity decreases, the dielectric constant increases, the ohmic resistance decreases, and the voltage increases. Higher utilization of electrode active materials reduces activation polarization and lithium-ion migration resistance, further increasing voltage, while also improving capacity and discharge power. The opposite occurs when temperature decreases.
(The accompanying images are based on lithium iron phosphate test data.)
The lower the internal resistance, the higher the open-circuit voltage.
Charging increases the open-circuit voltage because, due to electrode polarization, the electrochemical reaction rate cannot keep up with the charge transfer rate, creating a polarization potential. This causes the open-circuit voltage to be higher than the stable value for a period after charging and at the end of the charging process. The higher the charging rate, the greater the polarization, and the larger the error between the instantaneous voltage and the true voltage. (This is also why high-current charging doesn't provide much power; the voltage value in a high-rate charging state is temporarily higher, leading to a higher SOC value. If the high-rate charging error coefficient is not taken into account, the SOC value will be severely distorted.) Discharging behaves in the opposite way.
■ A high instantaneous discharge current causes electrons to migrate out, but positively charged lithium ions do not, increasing the negative electrode potential. Conversely, the positive electrode gains electrons, but positively charged lithium ions are not yet inserted, decreasing the positive electrode potential. Both effects work together to lower the total open-circuit voltage. The higher the discharge rate, the more pronounced the difference; instantaneous discharge is the opposite.
■ Higher temperatures result in lower internal resistance, faster electrolyte ion migration, and increased electrode activity, which can relatively improve battery capacity and output power. Actual state of charge (SOC) increases with increasing temperature and decreases with decreasing temperature.
■ The storage time is due to two reasons: the decay of polarization potential and the reduction of charge caused by self-discharge. When the time is long enough, the product of this and the self-discharge rate is the charge correction deduction.
■ Coulombic efficiency is the ratio of discharged capacity to charged capacity. Better coulombic efficiency results in better battery stability, less capacity loss, and a longer lifespan. Coulombic efficiency is related to temperature, discharge rate, depth of discharge (DOD), and cycle life.
■ The initial SOC value directly affects the instantaneous SOC calculated by the ampere-hour integration method and the OCV method. It is generally accurately calibrated after battery equalization, and its influencing factors are the same as those of SOC.
■ Different depths of discharge (DOD) result in different stable open-circuit voltage values. Overcharging and over-discharging can cause irreversible capacity loss in the battery, and will directly reduce the overall capacity of the battery.
■ Internal resistance is divided into AC internal resistance and DC internal resistance. Power and capacity factors are mainly affected by DC internal resistance. DC internal resistance is further divided into ohmic internal resistance and polarization internal resistance. Ohmic internal resistance is affected by electrode materials, electrolyte, diaphragm, etc.; polarization internal resistance is divided into ohmic polarization, activation polarization, and concentration polarization, and polarization internal resistance is closely related to materials, processes, and operating conditions.
The internal resistance characteristics can be summarized as follows:
■ In terms of material properties, the voltage slope of the positive electrode is large, such as the three-phase change of ternary lithium, and the voltage is easy to calibrate. The slope is small, such as the two-phase change of lithium iron phosphate, and the voltage is difficult to calibrate. The temperature and voltage characteristics of the electrolyte are important. The larger the temperature and voltage window, the more stable the electrolyte, the greater the cycle efficiency, and the smaller the capacity loss. The wettability, porosity, thickness, and resistance of the separator are also important.
■ On the one hand, the heat dissipation process, electrolyte system, and compaction density are crucial factors that directly affect material properties and ambient temperature. On the other hand, the process also directly affects the consistency of the battery. The better the consistency, the more accurate the SOC calibration.
(Some parts are in a stable state, and some are in a working state)
In general, the factors affecting State of Charge (SOC) are as described above. These factors interact non-linearly, making precise SOC calibration extremely difficult. Precisely calibrated SOC can improve battery lifespan, increase output power, enhance economy, and reduce maintenance costs. Furthermore, a foundation in precise SOC calibration can also contribute to battery safety.
IV. Battery Safety
In the development of new energy vehicles, safety is paramount; without safety, environmental protection and economic efficiency are meaningless. The Battery Management System (BMS) is primarily responsible for battery protection, monitoring, and information transmission. Protection is based on monitoring, which includes external battery characteristics such as voltage, current, and temperature. State of Charge (SOC) is the transmitted information calculated based on these monitored external characteristics. SOC informs the driver of the current battery level and also allows the vehicle to understand its own charge level, preventing overcharging and over-discharging, improving battery balance, increasing output power, and reducing unnecessary redundancy. The underlying system employs complex algorithms to ensure the vehicle's continued safe and stable operation, thus enhancing safety.
■ Overcharging and over-discharging
Overcharging refers to continuing to charge a battery after it has reached its full charge state. Determining whether a battery is fully charged is based on battery safety and the maximum charging value needed to maintain its reversible cycle capacity. Continuing to charge after reaching full charge will lead to excessive lithium ion release from the positive electrode, causing crystal structure collapse; temperature rise; irreversible degradation of the positive electrode material; reduced active capacity of the positive electrode material; increased side reactions in the electrolyte; and the release of oxygen and heat.
Lithium dendrites may precipitate on the negative electrode, puncturing the separator and causing an internal short circuit; increased temperature causes the SEI film to dissolve and fall off, reducing cycle life and increasing potential ohmic resistance.
Overcharging and over-discharging will reduce battery life under normal circumstances, cause irreversible capacity loss, reduce output power, and reduce range and climbing performance; in severe cases, it can lead to fire and combustion. Many accidents are caused by overcharging and over-discharging.
■Balance and Consistency
New energy vehicles derive some or all of their energy from electricity, which drives the motor controller, motor operation, air conditioning, instruments, and so on. Batteries are composed of individual cells forming modules that form battery boxes. Since individual cells have low voltage and capacity, they are connected in series and parallel. Series connection increases voltage and output power, while parallel connection increases capacity and extends driving range.
However, inconsistencies in individual battery cells lead to a significant reduction in output power and driving range, which in turn can cause overcharging and over-discharging. At this point, battery balancing is necessary. Although active and passive balancing are currently popular in China, we will not discuss the differences between the two, but rather focus on current balancing metrics.
Currently, the three main balancing indicators are actual battery capacity, battery terminal voltage, and battery state of charge.
▲Battery actual capacity balancing aims to make the actual capacity of batteries more consistent. The method is to continuously charge the battery pack in an overflow state with a small current, that is, to use the overcharge method until bubbles appear on the positive and negative plates, thereby eliminating the impact of small capacity on the overall battery performance. However, overcharging affects battery life and is unsafe.
▲Battery terminal voltage equalization aims to bring the terminal voltages closer to a uniform level. However, in reality, during charging, the terminal with higher internal resistance has a higher voltage, requiring discharge equalization, while the terminal with lower internal resistance has a lower voltage, requiring charging equalization. Conversely, during discharging, the situation is completely reversed: the terminal with higher internal resistance has a lower voltage, requiring charging equalization, while the terminal with lower internal resistance has a higher voltage, requiring discharging equalization. This makes the entire charge-discharge equalization process very chaotic and the effect is not ideal.
▲Battery state-of-charge (SOC) balancing aims to ensure consistent SOC values, thereby improving power output and guaranteeing safety. However, the challenge lies in the fact that SOC is influenced by numerous uncertain factors, making accurate SOC estimation crucial.
■ Improved driving range
Precise SOC allows you to confidently reduce extra redundancy, increase usable battery capacity, and extend driving range.
V. Summary
In today's booming new energy vehicle market, safety is paramount. State of Charge (SOC) is a core component of Battery Management System (bMS), ensuring battery safety, improving power performance and cycle life, and significantly enhancing economic efficiency. Addressing safety issues is crucial for the industry's long-term development; mastering core competencies is essential for companies to potentially become industry unicorns.
