Recent frequent electric vehicle fires have brought battery safety back into the spotlight. Besides the battery itself, battery management is crucial for ensuring battery safety. At the 2019 China (Xi'an) New Energy Vehicle Industry Ecosystem Conference, Xu Jun, Associate Professor and Doctoral Supervisor at the School of Mechanical Engineering, Xi'an Jiaotong University, delivered a keynote speech entitled "Analysis of Key Technologies for New Energy Vehicle Battery Management."
I. Necessity Analysis of Battery Management for New Energy Vehicles Xu Jun stated that while the development of new energy vehicles is rapid, they still face bottlenecks in areas such as driving range, safety, lifespan, and cost. Range anxiety manifests in several ways: firstly, the total range of electric vehicles cannot meet demand; secondly, the remaining battery capacity cannot be accurately provided.
To overcome these bottlenecks, electric vehicle battery systems face four key requirements: high specific energy, high safety, long lifespan, and high state-of-the-art accuracy. How can these four indicators be improved based on existing battery technology? This is the task of battery management.
The main functions of a battery management system are divided into four parts: first, data acquisition, which mainly includes the acquisition of information such as voltage, temperature, and current; second, output, which is whether the remaining range can be accurately calculated; third, balancing, which is how to make the performance of a large number of batteries in series better; and fourth, thermal management, which is to ensure that the battery operates at a suitable temperature for better performance.
II. Power Battery Status Estimation and Fault Diagnosis Analysis
Power battery state description indicators include SOC estimation, SOH estimation, SOP estimation, and SOE estimation. Xu Jun pointed out that battery state cannot be directly measured by sensors, and battery systems have strong nonlinearity and time-varying characteristics. At the same time, the complex and ever-changing usage environment and operating conditions increase the difficulty of state estimation.
Common SOC estimation methods include ampere-hour integration, data-driven methods, and model-based methods. According to Xu Jun, the main problem with ampere-hour integration is the difficulty in measuring the initial SOC. Current solutions involve ampere-hour integration with correction, a commonly used approach. Data-driven methods are numerous, such as neural network models. These methods require extensive experimental data for model training and high-performance computing, and lack universality, thus limiting their practical application. The main problem with model-based methods is that the model changes constantly with battery degradation, leading to inaccurate estimations. This method has received considerable research attention, and some have already been put into practical use.
Common methods for estimating State of Harm (SOH) include direct measurement, online estimation, and indirect methods. Direct measurement involves directly measuring the battery's characteristic parameters to evaluate SOH, primarily including capacity/energy measurements and impedance measurements, typically performed under laboratory conditions. The key challenge in online estimation is the accuracy of State of Charge (SOC). Indirect methods utilize the relationship between other quantities and the actual capacity.
Xu Jun stated that battery systems are highly complex, and the safety performance of high-energy-density and high-safety lithium batteries is still at a bottleneck. It is necessary to understand the fault-causing mechanisms of battery systems and achieve accurate fault detection and early warning to improve system safety.
III. Analysis of Power Battery Balancing Structure and Strategy
Balancing primarily addresses the issue of battery inconsistency, which can stem from various causes, including inconsistencies arising during manufacturing and usage. Battery inconsistency can easily lead to overcharging or over-discharging, potentially resulting in thermal runaway or even explosion.
Xu Jun stated that balancing and reconfiguration are effective methods to address battery inconsistencies. The balancing topology is the hardware foundation for achieving battery balancing, and its design is the initial step in the design of the battery balancing system, providing a design basis for subsequent balancing control strategies and the construction of experimental platforms.
Battery balancing can be divided into two types: passive balancing and active balancing. Passive balancing dissipates excess energy in the battery as heat through resistors until all batteries reach a consistent state. Its advantages are simple structure and low cost, but its disadvantages are low balancing efficiency and significant energy consumption.
Consistency control strategies include voltage-based, SOC-based, and capacity-based approaches. Among them, the voltage-based approach is convenient, intuitive, simple, and widely used, but its disadvantage is that the voltage difference at the battery terminals is small, resulting in poor balancing effect. The SOC-based approach can effectively avoid over-balancing, but it has high requirements for controller design and is more difficult to use. The capacity-based approach can obtain the maximum usable capacity, but the calculation is complex and the application is difficult.
IV. Power Battery Structural Design and Thermal Management Analysis
Xu Jun pointed out that batteries need to operate within a very suitable temperature range; excessively high or low temperatures will affect the battery's performance. Operating a battery in a high-temperature environment can cause it to overheat and lead to thermal runaway, which in severe cases can even cause the battery to explode. At very low temperatures, the amount of electricity the battery can release and charge is extremely limited, and under low-temperature conditions, the battery may experience internal short circuits, which can potentially lead to thermal runaway.
The purpose of thermal management is to ensure battery safety and enable the battery to perform better. The main functions of thermal management include:
(1) Effective heat exchange is performed when the battery temperature is too high to prevent thermal runaway accidents;
(2) Preheat the battery when the temperature is low to ensure charging and discharging performance;
(3) Reduce temperature differences within the battery pack and suppress the formation of localized hot spots. Therefore, battery thermal management is of great significance for improving vehicle performance.
Battery thermal management methods include air cooling, liquid cooling, phase change materials, and heat pipes. Xu Jun frankly stated that with substantial subsidies for new energy vehicles, many power batteries on the market currently lack thermal management. He believes that after subsidies are phased out, everyone will let their products speak for themselves; whoever has better technology will gain a larger market share, and the higher the actual user acceptance of their product will be.
Air cooling, also known as air cooling, uses air as the cooling medium. Xu Jun stated that the Prius air cooling system is currently quite well-known in this field, while some domestic manufacturers' so-called air cooling simply involves adding a few fans, and the effect is not particularly good.
Liquid cooling is gradually gaining acceptance in China, and more and more manufacturers are launching products that use liquid cooling.
Phase change material (PCM) cooling involves directly immersing the battery pack in PCM, or using a jacketed structure where a ring-shaped PCM is wrapped around a single cell to form a slightly larger single cell, which is then assembled into a battery pack. During battery discharge, the system stores heat in the PCM as latent heat of phase change, thereby absorbing the heat released by the battery and rapidly reducing its temperature.
Heat pipe cooling uses a sealed hollow tube structure and utilizes evaporation phase change for heat transfer. The main advantage of heat pipe cooling is its ability to instantly transfer heat from one side to the other. According to Xu Jun, heat pipes are already used in consumer electronics, but their application in electric vehicle battery management systems is still relatively limited.
Battery thermal management includes not only cooling but also low-temperature heating. Low-temperature heating methods include external heating and internal heating. External heating methods include air/liquid heating, membrane heating, and other heating methods, while internal heating methods include AC heating and internal self-heating.
