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What is a related battery model study?

What is a related battery model study?

As shown in Figure 1, a battery pack consists of some interfaces (such as electrodes) and several battery modules consisting of several battery cells. In a battery module, all battery cells are all connected in parallel, thereby reducing the possibility of battery failure caused by the failure of a single battery cell. The battery modules of the same group are usually connected in series to provide high voltage and energy to the battery pack. The BMS is responsible for protecting hundreds of battery cells from damage and keeping the batteries working properly while driving a pure electric vehicle.

What is a related battery model study?
Figure 1 battery and battery pack model

1. Battery efficient management and scheduling
Rate capacity and recovery effects are the most important physical performance parameters for efficient battery management. The larger the battery discharge current, the smaller the effective capacity of the battery, this phenomenon is called the rate capacity effect; the output voltage of the battery does not decrease with the battery discharge, but rises, this phenomenon is called the recovery effect. We can increase battery capacity by minimizing the discharge rate of each cell and hibernating the battery cells.

The state of charge (SOC) of the battery is used to characterize the remaining power of the battery, and the value ranges from 0 to 1. When SOC=0, it does not discharge externally, and when SOC=1, it means it is fully charged. Because large battery pack performance depends on the electrical state of the aging cells within the pack, balanced state of charge is the most critical factor affecting the performance of large battery systems. In order to obtain better battery performance, a series of battery scheduling and discharge rate reduction measures based on SOC balance are taken to control the battery cells to release energy at a suitable discharge rate. For example, when the motor requires high power, the battery management system connects all battery cells to discharge to increase battery power; on the contrary, when the motor requires low power, the battery management system cuts off some cells with low remaining power to achieve output voltage recovery and Balance of state of charge.

2. Battery thermal characteristics
In addition to discharge behavior and SOC balance, battery thermal characteristics are also important for battery efficiency, operation, and safety. First, cell efficiency is temporarily improved at “immediate high temperature” due to increased chemical reaction rates and ion mobility, but cumulative exposure to high temperatures accelerates irreversible side reactions leading to a decrease in the permanent battery life. Therefore, most BMSs will require limiting each cell to a well-defined temperature range to achieve the desired performance. Every electric vehicle must therefore be equipped with a thermal management system that includes cooling and heating to keep the temperature of each battery cell within a reasonable operating range. When the temperature of the battery pack deviates from the operating temperature range, the thermal management system is activated to ensure the thermal stability of the battery.

In a high temperature environment, the radiator takes away the heat of the battery through the coolant and exchanges heat with the outside air to cool the battery; in a low temperature environment, the battery needs to be heated. For example, the GM Chevrolet Volt uses 144 fins to actively cool or heat 288 battery cells, and its radiator uses a coolant flow valve to control the flow of cooled or heated coolant. The Ford Focus also features an active liquid cooling and heating system for thermal management of its lithium-ion battery pack.

The simple method currently employed in battery cells can operate normally and efficiently during the shelf life of the car, but such a passive, extensive thermal control cannot take full advantage of the thermal management system. By understanding battery thermal characteristics, thermal management systems can be used to improve battery performance without sacrificing battery life: heating the battery cells at high power to improve instantaneous performance of the battery, and heating the battery cells when low power is required Cool down to delay battery life decay.

3. Relevant problem statement
The DC power generated by the battery pack of the electric vehicle is converted in the inverter to drive the electric motor of the electric vehicle. During the operation of the electric vehicle, the power inverter needs a suitable input voltage Vapp to drive the motor. After the battery pack is fully charged, the accumulated time to provide the required power Prep(t) within the output voltage range is defined as the running time top. Therefore, the BMS should ensure that its battery pack provides the required power to drive the motor, while keeping the output voltage not lower than the input voltage threshold during long-term operation, which is longer than the battery’s lifespan. Otherwise, EVs require a larger battery pack or frequent replacements.

By controlling the temperature of the battery cells to achieve a long enough operating time in the life, select the type of cooling liquid for each cooling fin to achieve the purpose of controlling the temperature of the battery each time it is cooled or heated, that is, the type of cooling liquid Used as a control knob for the BMS. Determine the type of coolant for each instant Cfin(t) to maximize the battery runtime top.

4. Overview of battery physical dynamic changes
The dynamic changes of the battery under stress conditions can affect the performance and safety of the battery system. For example, uncontrolled high temperatures can cause batteries to explode; extremely low temperatures can degrade battery performance and even make it impossible to drive electric vehicles. Therefore, the influence and interrelationship of control nodes and external conditions on battery dynamics should be analyzed to improve the safety and performance of battery systems. To this end, the factors affecting battery performance are first determined by bridging different abstract models of physical dynamic changes, and based on this unified abstract model, the dynamic changes of batteries under the influence of different temperatures are discussed.

What is a battery thermal management system?

What is a battery thermal management system?

As the main energy storage form of electric vehicles, the performance of power battery directly restricts the power, economy and safety of electric vehicles. Compared with other types of batteries, lithium-ion power batteries have great advantages in terms of energy density, power density and service life, making them the mainstream of current vehicle power batteries. But its performance, life and safety are closely related to temperature. As many studies have pointed out, temperature is one of the most important factors in battery design and operation. If the temperature is too high, the side reactions of the battery will be accelerated and the performance of the battery will be attenuated, and even lead to safety accidents. Therefore, it is very necessary to study the thermal management system (BTMS) of the battery.

The battery thermal management system obtains the temperature of battery cells at different locations through temperature measuring elements. Accordingly, the control circuit of the thermal management system needs to make the action decisions of the cooling actuators such as fans and water/oil pumps. At present, the temperature sensors of common power battery packs are mostly attached to the inner surface of the battery box or the outer surface of the battery cell. For example, in the third-generation Prius battery pack in 2010, part of the temperature sensor is arranged in the flow channel inside the battery pack; the other part is directly attached to the middle of the upper surface of the cells in some typical positions, and these cells are located in the front of the battery pack. top, middle and rear. The battery thermal management system usually performs hierarchical management according to the temperature region where the battery is located. Volt plug-in hybrid battery thermal management is divided into active (active), passive (passive) and non-cooling (bypass) three modes: when the power battery temperature exceeds a preset passive cooling target temperature, passive cooling mode starts ; and when the temperature continues to rise above the active cooling target temperature, the active cooling mode is activated. However, this is still an extensive control strategy, which leads to a larger safety margin of the battery and reduces the efficiency of the battery.

Next, an efficient and sophisticated battery thermal management system is investigated. The maximum operating time or accumulated time that the battery management system can provide the required power after being fully charged is used as the evaluation index of efficiency. as an indicator of reliability. The hardware and software relationship for integrating and coordinating battery temperature management is shown in Figure 1. For the hardware part, on the basis of understanding the influence of battery thermal properties and external temperature and pressure conditions on battery performance, by calculating these nonlinear physical properties and abstracting these features in cyberspace, an ideal solution to reduce the safety margin is developed accordingly. temperature management system, thereby improving the efficiency of the entire battery system of pure electric vehicles.

What is a battery thermal management system?
Figure 1 BTMS software and hardware relationship

Compared with the common temperature control system, the temperature is selected as the control node to realize dynamic control, and the influence of the thermal properties of the battery and the electrical state of the battery on the efficiency and reliability is analyzed. The cell-level thermal control is adopted as the control strategy of the battery thermal management system. Temporarily boost the performance of the cell layer when high power is required, while sleeping other layers to reduce stress, and the associated model is validated.

The main research contents include the following three points:
(1) Using cyberspace to summarize thermal properties to solve problems related to the efficiency and reliability of temperature management systems.
(2) Design the battery temperature management system, and systematically study the use of temperature as the control core of the temperature management system.
(3) The temperature management system used in the in-depth evaluation proves that it can effectively improve efficiency without sacrificing reliability.

What is lithium-ion power battery charging optimization control?

What is lithium-ion power battery charging optimization control?

Lithium-ion power battery cells generally adopt a constant current-constant voltage charging method, that is, first use a fixed rate of current (0.3C or 1C, etc.) to charge, reach the set charging cut-off voltage (3.6V, 4.0V, etc.) For constant voltage charging, the charging is completed after the charging current is lower than a certain value (such as 0.03C). Relevant studies have shown that the charging current and charging cut-off voltage not only have a significant impact on the charging time and charging energy of lithium-ion power batteries, but also have an important impact on their service life. Lithium-ion power battery systems are generally used in groups of multiple cells in series. If the constant current-constant voltage charging method is still used, it may cause overcharging of some battery cells. On the basis of studying the influence of charging current, charging voltage, overcharge and other charging factors on the service life, with the purpose of prolonging the service life of the battery, an optimized charging method for lithium-ion power battery cells and battery packs is proposed.

1. Influence of charging factors on the life of lithium-ion power batteries

1) Influence of overcharging on the life of lithium-ion power batteries
When the lithium-ion power battery is overcharged, a lot of heat is generated inside the battery, and at the same time, a lot of bubbles are generated in the electrolyte, which causes the active material on the positive and negative plates of the lithium-ion power battery to peel off, which seriously affects the activity of the battery and increases the internal resistance. Capacity has also dropped. At the same time, overcharging may also cause the battery to expand and deform, and even cause serious consequences such as fire and explosion. Research by Wang Hongwei et al. shows that at an ambient temperature of 20°C to 40°C, overcharging will cause the lithium-ion power battery to expand and deform, and the higher the temperature, the faster the temperature rises when the lithium-ion power battery is overcharged, and the higher the maximum temperature. more likely to be dangerous. Therefore, during the use of the lithium-ion power battery, it is necessary to ensure the normal operation of the charger and the protection circuit to avoid overcharging.

2) Influence of charging current and charging voltage on battery life
The charging voltage and charging current directly affect the charging energy and charging speed of the lithium-ion battery. Taking a certain lithium-ion power battery as an example, as shown in Figure 1, as the charging current increases, the charging capacity in the constant current stage becomes smaller, and the constant current charging capacity at 100A charging current is reduced by 8.36% compared with 20A charging current. The total charging time of the battery decreases with the increase of charging current, and the total charging time of 100A constant current charging is reduced by 76.1% compared with the total charging time of 20A charging current. This shows that increasing the charging current has little effect on the total energy charged, but can significantly improve the charging speed. However, high-rate charge-discharge current will cause the battery system to deviate from the equilibrium state, and accelerate the aging of positive and negative materials, thereby shortening the battery life. Therefore, power battery manufacturers need to comprehensively consider charging time and battery life when designing charging strategies. Two charging modes can be set: under normal circumstances, low-current charging should be selected as far as possible when charging lithium-ion power batteries, so as to prolong the battery life; in urgent cases, high-current charging can be used to shorten the charging time, although this will damage battery life.

What is lithium-ion power battery charging optimization control?
Figure 1 The relationship between constant current charging capacity, total charging time and charging current of a lithium-ion power battery

In general, when the charging current is the same, the higher the charging cut-off voltage, the greater the total energy charged by the lithium-ion power battery. However, the higher charge cut-off voltage will cause partial decomposition of the battery cathode material, the performance of the electrolyte will also decline, and the separator will also be oxidized due to contact with the high-potential cathode material. Taking a lithium-ion power battery as an example, as shown in Figure 2, when the charging voltage is reduced from 4.2V to 4.1V, the capacity retention of the lithium-ion battery is better as the number of charging and discharging increases, that is, the battery Longer cycle life. Relevant studies have shown that reducing the charge cut-off voltage by 0.1~0.3V can prolong the battery cycle life by 2~5 times.

What is lithium-ion power battery charging optimization control?
Figure 2 The relationship between the usable capacity and the number of cycles at different charge cut-off voltages

2. Charging strategy based on lifetime optimization
By studying the influence of charging current, charging voltage and overcharge on the life of lithium-ion power battery, in order to prolong the service life of lithium-ion power battery cells and battery packs, this paper proposes the optimization of lithium-ion power battery cells and battery packs. charging strategy.

1) Charging strategy of lithium-ion power battery cells
In order to prolong the service life of the battery, the charger and the charging protection circuit should be safe and reliable. The thermistor can be used to detect the temperature of the battery, and stop charging when the battery temperature exceeds the high temperature threshold to prevent overcharging. After the battery is fully charged, disconnect the voltage in time, otherwise, metal lithium will be generated inside the battery, resulting in permanent capacity loss, and may cause a short circuit inside the battery.

According to different positive and negative materials and battery structure, the charging parameters of lithium-ion power batteries will be different. When determining the charging method of a single battery, factors such as the composition material and structure of the battery, charging time, charging capacity and battery life should be comprehensively considered, and parameters such as charging current, charging cut-off voltage and charging termination current should be optimized and designed. The charging methods of lithium-ion batteries can be divided into ordinary charging and fast charging: ordinary charging is suitable for general household charging or long-term parking charging, using small rate charging current and charging cut-off voltage, charging voltage can be 3.8~4.0V, And use a small charging termination current (such as C/10 or less) to strengthen the protection of the battery; fast charging is suitable for charging in emergency situations, which will greatly damage the battery life, use a large rate current for a short time (1h) Charge more than 90% of the battery inside. The number of fast charging times should be minimized during use.

Appropriately reducing the charge cut-off voltage can significantly improve the service life of the battery. In order to take into account the charging time, the cut-off voltage can be set according to the depth of discharge (DOD): when the depth of discharge is 100%, in order to shorten the charging time, the cut-off voltage can be increased, such as 3.8V; when the depth of discharge is 0, set the cut-off voltage The voltage is 3.5V. In this way, when the depth of discharge is between 0 and 100%, the charge cut-off voltage can be set between 3.5 and 3.8V according to a linear relationship. This approach helps extend battery life and can reduce the overall charging time for Li-ion power batteries.

2) Charging strategy of lithium-ion power battery pack
When charging a lithium-ion power battery pack, if a single constant current-constant voltage charging is used, it is likely to cause some cells to be overcharged. Therefore, the charging of the battery pack needs to be controlled according to the state of the lithium-ion power battery cells to prevent overcharging of the cells, protect the cycle life of the single cells, and help prolong the cycle life of the battery pack. Taking the charging process of a lithium-ion power battery system as an example, the standard charging steps are: charging with a constant current of 1C, when the voltage of a single battery reaches 3.5V or more, the charging current is reduced to C/2. When the voltage of the single battery reaches 3.55V or more, reduce the charging current to C/4, when the voltage of a single battery reaches 3.6V, reduce the charging current to C/8, when the highest voltage of the single battery reaches the cut-off voltage (3.65V) or the battery pack The charging process is completed after the total voltage reaches a certain cut-off voltage.

When the vehicle is parked for a long time, the power supply should be cut off, and the vehicle should be parked in a ventilated, rain-proof, moisture-proof, sun-proof, and fire-fighting place, and should be kept away from flammable and corrosive items. When the vehicle is parked for more than a month, the battery pack must be kept at a state of charge of about 50%, the connecting wire of the battery pack must be unplugged, and the power battery system must be charged and discharged with a small current every three months for maintenance.