One of the key components of electric vehicles is the battery management system (BMS). In order to meet the ever-increasing power and voltage requirements, the battery pack used in electric vehicles has hundreds of battery cells connected in series or in parallel-which forms a complex battery system.

Electric vehicles (EV) have many advantages over internal combustion engine vehicles, including superior performance, high energy density, less pollution, and good acceleration. But electric cars are not perfect. One of the main disadvantages is the need for an expensive battery system with specific maintenance requirements and a long charging time.

One of the key components of electric vehicles is the battery management system (BMS). In order to meet the ever-increasing power and voltage requirements, the battery pack used in electric vehicles has hundreds of battery cells connected in series or in parallel-which forms a complex battery system.

Any lower than ideal battery conditions (for example, over current, over voltage, over charge or over discharge) will cause damage and aging of the BMS. In the worst case, there is a risk of fire and explosion. For these reasons, BMS is required to provide "safety protection" to ensure proper battery performance.

However, BMS functions (such as current and voltage protection during charging and discharging) depend on the working conditions of the battery (load, life, temperature, etc.). This part is completed through battery modeling, which provides a mathematical model of virtual battery, which can verify whether the BMS can work normally for the corresponding battery pack.

1. Status monitoring

Battery status monitoring is essential for optimizing battery safety and performance. Life prediction and aging diagnosis are essential. Among them, battery design, battery performance and environmental conditions are one of many factors that affect battery life.

The State of Charge (SoC) battery evaluation provides information about the remaining capacity of the battery (as a percentage of its total capacity). There are two common methods for SoC evaluation: direct evaluation and model-based evaluation.

Direct estimation is based on preliminary measurements of battery parameters (voltage and current). The two calculation methods used are systems based on ampere-hours (Ah) and open circuit voltage (OCV). However, when adjusting the Ah method for the SOC estimation algorithm, planning the initial SoC and measurement accuracy can be a challenging process.

This method is highly dependent on the measured current. As time goes by, the accumulated error will seriously affect the accuracy of SoC estimation. Determining an accurate initial SoC in the real world is also challenging (for example, when the battery is only charged in the range of less than 10% to 90%).

On the other hand, the OCV-based method has high estimation accuracy and has been recognized as an effective and popular method for SoC calculation. There is a non-linear relationship between the battery's SoC and OCV. This process requires enough batteries to be put on hold (the battery needs to be disconnected from the charger and load). The main disadvantage of this method is the quiet time. After the battery is disconnected and charged, it usually takes a long time to reach stability (it may take more than two hours in low temperature conditions).

The OCV-SoC relationship also depends on the battery life and temperature.

2. Battery temperature

Battery temperature is an important factor that affects battery performance, life, performance and safety. The thermal sensor is suitable for measuring the external temperature of the battery.

However, this information alone is not enough, because the internal temperature of the battery is a key parameter for proper battery management. The internal high temperature will stimulate battery aging and cause safety hazards (such as fire). The internal battery temperature usually changes significantly from the surface temperature (up to 12°C in high-power applications).

Provide an appropriate method for internal battery temperature evaluation to prevent accelerated battery aging, and support BMS algorithm to optimize battery energy discharge.

3. Classification of battery models

Generally, battery models can be divided into three main types:

(1). Electrical

(2). Thermal

(3). Coupling model (other models are rarely used in BMS design, such as dynamic models)

The battery electrical model involves an electrochemical model, a reduced-order model, a proportional circuit model, and a data-driven model. The electrochemical model provides information about the electrochemical behavior of the battery. The model can be very accurate, but requires advanced simulation and calculations. As a result, it is a challenge to fully adopt this model in real-time applications.

Therefore, the reduced-order electrical model is generated as a simplified physics-based electrochemical model to determine the state of charge (SoC) of the lithium-ion battery. A simple reduced-order electrical model provides less insight, but is convenient for real-time battery applications.

The key is to monitor the battery temperature, which is part of a successful BMS. If it is operated at higher or lower temperatures, the performance of the battery may decrease. A separate cooling system is usually used to maintain the proper battery temperature. For example, Tesla uses a patented battery pack configuration and a plate-based cooling system to dissipate heat and monitor battery temperature.

The battery-coupled electro-thermal model considers the battery's electrical (current, voltage, SoC) and thermal (surface and internal temperature) operations at the same time. Several coupled electrothermal models have been developed.