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.
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.