Zhang Fangnan of Honeycomb Energy: Concept of safe design of power battery system

The subsidy policy for new energy vehicles has been declining since 2016, and the high energy density route for power batteries has been proposed. The recent series of electric vehicle spontaneous combustion incidents have aroused the public's further attention to the safety of electric vehicles. On July 5, the 2019 New Energy Power Battery Safety Technology Forum hosted by Gasgoo was held in Shanghai. Zhang Fangnan, director of the Pack Development Center of Honeycomb Energy Technology Co., Ltd., gave a keynote speech on the safety design of power battery systems. The content is as follows:

Zhang Fangnan: The topic I will share with you is "The Concept of Honeycomb Energy's Power Battery System Safety."

There are five parts in total. The first part is an introduction to safety issues. Mr. Wang also mentioned that consumers are gradually shifting from "mileage anxiety" to "safety anxiety". We have also made a preliminary classification of safety accidents. There are many ways to classify safety in the industry. The classification methods are not the same, but the overall classification is similar. We have also made a preliminary analysis of the causes of failure, including fire and explosion, high-voltage electric shock, and chemical safety. The causes of failure are also divided into three categories: one is the control strategy reason, one is the external short circuit reason, and the other is the internal reason. Control strategy failure includes overcharging, over-discharge, over-temperature, and insulation failure. External short circuit causes include mechanical damage caused by over-discharge or line aging. Internal short circuits are mainly defects in the manufacturing process and the dissolution of the insulating film during later use.

We have made a preliminary division of system safety, dividing the entire life cycle of the battery into three stages. The first stage is to pay attention to safety during battery production and design, then safety in battery application, and cascade recycling safety, making three major areas of division.

The product design process is also divided into four levels. The first level is mainly for raw materials, battery cells, vehicles, and the entire system process to achieve multiple active and passive designs, and to achieve safe control through the optimization of control strategies, that is, relying on automated production error prevention, using advanced manufacturing technology to ensure product consistency and achieve a zero-defect manufacturing process.

The second level is early warning and intervention. Through the control strategy combined with high-precision sensors, remote cloud analysis, and the front-end BMS, it is possible to identify such abnormal data in the early stages of an accident and provide early warning.

The third level is before the safety risk occurs, basically immediately after the accident happens, and the driver is reminded to stop the car through sensors and BMS to ensure personal safety.

The fourth level is to control and delay the spread of thermal runaway through passive safety and fire-fighting measures after the accident occurs.

Regarding the design of the safety of the entire system, from materials, systems, complete vehicles, battery cells and complete vehicles, we believe that the most important thing in the process is the demand. The current system development demand basically comes from the SOR of the vehicle manufacturer. The demand may not be complete and comprehensive, and the decomposition process of the entire level may not be very professional. Secondly, the vehicle is decomposed into the system, from the system to the battery cell, and then to the material. The completeness of the decomposition of this demand needs to be further strengthened. We believe that the future design should first focus on improving the proposal of demand and the process of demand decomposition.

At present, the national standards for all safety test items are basically comprehensive, but the national standards are basic and entry-level standards after all. Real road conditions and more stringent road conditions may not be reflected in the standards. Therefore, it is necessary to identify requirements and test items other than the standards during system development. For example, the honeycomb has identified a total of 104 test standards, including safety, reliability, and performance durability. For example, some temperature shocks, vibrations, etc. required by the national standards are all related tests for individual products. After a single product is completed, the same product can be used for other verification projects, such as temperature shocks, vibrations, and water immersion tests, to verify the sealing failure caused by aging during actual use. There will be a detailed introduction later.

According to the structural design concept, the battery pack is under the chassis. The battery pack plays a very important role in receiving the force from the side of the vehicle or strengthening the structural strength of the vehicle. In this case, it is necessary to add crossbeams between modules or inside the battery pack during the battery pack design process to improve the side impact of the vehicle. Of course, increasing the space between the battery pack and the edge of the vehicle is also beneficial to collision safety.

These are some special test items that we have identified in addition to the national standard. For example, when washing the whole vehicle, the flushing of the high-pressure water gun will have a certain impact on the sealing of the battery pack. When the whole vehicle passes through a flooded road at high speed or an icy road in winter, the requirements for the battery pack sealing are different. This includes long-term immersion in water, while the national standard has a short-term immersion requirement. In this case, the requirements for the sealing design will be more stringent, and the test items and test methods will be significantly different.

For example, after a vehicle collides, a rollover test is performed. After a simulated vehicle collision experiment, some specific experimental methods are added to meet the differences in actual use.

These are some special conditions we have identified at the vehicle level, such as falling during driving, or some tests when the vehicle passes through obstacles, including crossing ditches, simulating sideslip and hitting the roadside, various bumpy road tests, including scanning road conditions, etc. In fact, in the needs of the whole vehicle, the requirements for battery pack design and system development are not very clear, and the battery factory or system development unit needs to have a very in-depth understanding of the needs of the whole vehicle.

At present, most of the concerns are probably after the accident. Due to design reasons, people are more concerned about accidents caused by design defects, but the real durability and life after aging are not paid much attention in the industry. Reliable and durable design requires in-depth research. For example, in the Pack-level life test, most of the tests on the structure are conducted through vibration experiments, but the actual load conditions are not very consistent. Therefore, when conducting the whole package vibration test, it is necessary to collect data in combination with the actual use conditions of the whole vehicle to perform corresponding simulations and tests to ensure the durability of the product.

At the same time, the development of fasteners and the quality of sealing performance are all achieved through fastening, so the selection specifications and corresponding verification need to be studied. The reliability of the sensor connection in the battery pack and the possibility of impact also need to be addressed.

Regarding electrical safety, the selection of internal high-voltage components, as well as the withstand voltage of high-voltage insulation, are better than the national standards. For example, when selecting fuses and contactors, we need to combine the battery cells, especially the internal fuses (OSD) of the square battery cells themselves, and the matching relationship between the main fuses and contactors of the Pack to develop the system and design the experiments. The design of the reliability of electrical components needs to consider the use of the vehicle in certain extreme situations, such as playing with keys, which causes the battery pack to be repeatedly powered on and off, which may have a certain impact on the life of the relay and pre-charge resistor. At the same time, combined with the power-on control strategy, we can optimize the relevant requirements of our selection.

Regarding the safety of battery pack thermal management, especially the selection of sensors in the early stages of thermal runaway, the main approach now is to use BMS strategies and strategy optimization, combined with pressure, smoke, or gas detection to jointly judge and identify thermal runaway so that timely feedback can be provided to passengers in the early stages of thermal runaway to remind them to pay attention to safety.

Pack-level design, from battery cells to modules to the entire package, requires multiple protection designs for thermal runaway, such as adding heat-insulating and insulating materials between battery cells. At the same time, this material has a certain degree of compressibility. This material is currently difficult to find, and it also requires colleagues in the industry to work together to develop this new material. Use heat-insulating and insulating materials to prevent or prolong the spread of thermal runaway between battery cells, and add isolation measures such as anti-collision beams between modules, as well as this insulating and fire-proof material, to prolong the spread of thermal runaway between modules. At the same time, add corresponding exhaust channels during the design process of the Pack to allow it to exhaust in a directional manner to avoid harm to relevant personnel, especially the driver.