Battery safety is the core of new energy vehicle safety.
With the rapid popularization of pure electric vehicles, the number of them has increased greatly, and the quality problem of power batteries has become increasingly prominent. Among them, thermal runaway is the biggest inducement that affects the safety of power batteries. According to the accident investigation data of 20 1 1-20 19, the safety problems of power batteries caused by uncontrolled thermal diffusion account for more than 50%.
Generally speaking, the causes of power battery fire mainly include five aspects: battery module aging, external collision, high temperature weather, battery thermal runaway and high load. External collision and high temperature weather are external causes, and aging of battery components, thermal runaway of battery and high load are related to the quality of power battery and thermal management system, which are often the direct fuses of spontaneous combustion.
Source: EVS-GTR Conference.
What should be considered in the design of battery pack?
The size of the battery pack, there are many parts in the chassis of the whole vehicle, and the space for placing the battery pack is limited. To meet the space requirements of the vehicle, and secondly to meet the requirements of the vehicle's pure electric cruising range, this can be directly converted into how many kWh the battery pack needs to be designed. Then we choose the battery, including the form of batteries, square shell, soft bag or cylinder, and the capacity of each battery. Then we know the working voltage range of other electrical appliances in the whole vehicle, which determines whether the batteries in our battery pack are connected in parallel or in series. When monitoring the batteries in the battery pack, BMS will require the series and parallel connection of the batteries.
Composition of battery pack
Selection of raw materials
The main raw materials were optimized and modified effectively to improve the thermal stability of the battery and avoid thermal runaway.
Positive and negative electrode materials:
Anode materials with excellent kinetic properties are selected to reduce the risk of lithium precipitation.
Thermal stability coating on the surface of negative electrode particles, electrolyte negative electrode film-forming additives and material surface protection.
SEI/ECM protective film is formed on the surface of anode and cathode materials through the reaction of electrolyte solvent and additives, which prevents the materials from further reaction and deterioration and improves the stability and safety of the materials.
Electrolyte:
Electrolyte flame retardant additive
Improve the formula, increase the flash point and vapor pressure of electrolyte system, and achieve flame retardant effect.
Diaphragm:
High heat-resistant PET, aramid and other basement membranes, reducing the risk of battery short circuit.
Surface thermal stability coating for reducing thermal shrinkage of diaphragm.
structural design
The lithium battery pack is mainly composed of bearing frames (lower frame and upper frame), lithium batteries, high-voltage connectors (such as high-voltage connectors) and low-voltage connectors (such as low-voltage connectors), as shown in the following figure.
The lithium battery frame is not only the carrier of each component, but also the "bridge" connecting the whole vehicle. The lithium battery is assembled on the whole vehicle through the lithium battery frame installation structure.
In order to facilitate installation and maintenance, the bearing frame is generally divided into an upper frame and a lower frame. The lower frame mainly carries devices and bears more weight of the battery system; Generally, the upper frame mainly plays a protective role, with less load-bearing requirements.
Active safety design of battery pack
Thermal runaway detection: By monitoring the temperature and voltage combined with the function of timely wake-up, the vehicle can be given an alarm before the thermal runaway of the battery pack occurs, so as to ensure the personal safety of personnel.
Voltage detection: real-time battery voltage detection.
According to the battery performance, the voltage threshold and the voltage drop rate threshold are set to define whether thermal runaway occurs.
Temperature detection: real-time module temperature detection
According to the battery performance, the high temperature threshold and the heating rate threshold are set to define whether thermal runaway occurs.
Anti-false alarm design: redundancy design
In order to prevent false positives, the detection time and detection conditions are redundantly designed, which increases the reliability of strategy judgment.
Wake-up strategy: real-time wake-up strategy
After the BMS sleeps, it will wake up automatically at regular intervals. After waking up, detect the current temperature and voltage values.
Passive safety design of battery pack
Thermal runaway route of the battery: Through the "five prevention" design of thermal runaway, the "0" thermal diffusion of the battery pack is finally realized (that is, the thermal runaway of a single cell will not spread to adjacent batteries or modules).
Electrical insulation withstand voltage design: If insulation failure will cause serious short circuit, in order to avoid secondary insulation failure, the corresponding insulation design shall be carried out according to the customer's maximum working voltage Vmax and working altitude.
Double insulation design: the module design adopts double insulation protection: the battery cell itself has a layer of insulating battery cell blue film and battery cell top patch to meet the requirements of insulation and voltage resistance, and there is insulating paper protection between the end plate and the battery cell and between the battery cell and the bottom installation surface, which all meet the requirements of insulation and voltage resistance.
Structural safety test: Problems such as vibration and impact, including collision, which can be monitored or may occur in the short term, are relatively easy to solve, such as how to carry out problems that can only be monitored in the long term. Therefore, through the welding test of end plate and side plate module, according to the relationship between module cycle and expansion force, the welding strength requirements and indexes of module end plate are designed.
Thermal runaway protection scheme: Through the thermal runaway protection design, the five-fold protection of battery pack thermal runaway is realized: sensor early warning, thermal design of battery interval, increasing thermal interval of module interval, guiding thermal runaway exhaust gas to be discharged according to specific channels, and optimizing the selection of explosion-proof valves.
conclusion
This paper analyzes the current design process and simulation research status of battery structure, the concept of package-level passive safety design, and how to carry out all-inclusive passive safety protection (design, simulation and strategy of all-inclusive protection according to the thermal runaway performance of batteries). In addition, the research on thermal analysis, dynamic analysis and collision analysis of battery pack will also be the focus of the next research.