If an electric vehicle catches fire, is it due to the poor quality of the battery cells from the battery manufacturer or the flawed design by the automaker?
Many people now have a misconception that when buying an NEV (New Energy Vehicle), choosing a battery from a well-known brand guarantees safety. Is this really the case?
A few days ago, Xiaomi hosted a live stream on battery safety, featuring a particularly interesting analogy. Their engineers compared the safety design of the entire vehicle battery pack to a "building."
Building a battery pack is very similar to constructing a house. A high-voltage battery pack is equivalent to a large building with 200 rooms (each battery cell equals one room).
If the entire building catches fire, is it a problem with each individual room, or is there a flaw in the building itself?
Does this analogy make things clearer? If a "room" (= battery cell) has a problem, it is the responsibility of the battery manufacturer; if the building catches fire, it is an issue with the building's safety design.
In the early stages of EV evolution, cell safety was indeed more critical. There was a demand for high energy density, fast charging speeds, and safe cell materials. However, materials with higher energy density tend to be more uncontrollable (= unsafe) when a battery catches fire. Cell manufacturers pursue the intrinsic safety of the cells, focusing on the selection of the cell's chemical material system.
As battery technology advances, the overall vehicle battery safety design is increasingly coming into the spotlight.
How to design the framework of the building (= the structure of the battery pack) to ensure safety despite decades of exposure to wind, sun, and rain.
How to design the safety exits of the building (= pressure relief valves), and how to design fireproofing and isolation (thermal safety) between rooms.
Whether an EV is safe depends not only on a single battery cell but also on the design of the entire battery pack, as well as structural safety, thermal safety, electrical safety, functional safety, big data safety, and more.
Part 1: Battery Pack Design
During Mr. Lei's live stream, many design details of the next-generation SU7 battery pack were revealed.
I have previously seen the physical teardown of this battery pack. My first impression at the time was that it was beautiful. A battery pack can indeed be described as beautiful; its design is highly regular, simple, and clear. From this schematic diagram, it can be seen that it is divided into four parts: the top cover, the modules on the left and right sides, the box assembly, and the bottom guard plate assembly.
● CTB
This battery pack adopts the CTB (Cell-to-Body) design, meaning the battery itself is an integral part of the vehicle body structure. The primary advantage of this design is its exceptional robustness. The die-cast part of the battery pack box uses the same material as the mega die-cast rear vehicle body: Xiaomi's self-developed Titan Alloy, which boasts extremely high strength.
The battery cells, the box, and the top cover are firmly bonded into a single unit. The top cover integrates four beams: two 2000MPa ultra-high-strength steel crossbeams and two 1500MPa seat crossbeams. Working in conjunction with the rocker panels, the design of these crossbeams is primarily aimed at resisting side impacts and improving torsional rigidity.
The torsional rigidity of the next-generation SU7 exceeds 51,000 Nm/deg, placing it in the top tier of mass-produced vehicles globally.
● Bottom Guard Plate
For EVs, especially sedans, the low chassis means the battery is placed underneath, making bottom protection particularly crucial. The bottom guard plate of this battery pack features a five-layer structure. From inside to outside, these five layers are:
It is particularly worth mentioning the outermost bulletproof coating. Do you remember the video from last year where Mr. Lei dropped a watermelon? The watermelon was dropped from the fifth floor and remained intact. The watermelon was coated with this exact same material, which offers scratch resistance 13 times higher than traditional PVC coatings.
In the demonstration, a small needle experiment was used for comparison. This is standard equipment on the next-generation SU7 and SU7.
● Structural Adhesive
The second highlight is the structural adhesive.
How are the battery cells, acting as rooms, integrated into the building? This is achieved through three types of structural adhesives:
Between the water cooling plate and the battery cells, a thermally conductive structural adhesive is used, serving the purpose of fixation and heat dissipation.
Between the top cover and the box, two layers of sealing structural adhesive are applied for insulation, waterproofing, and anti-aging. The dual layers primarily provide redundancy; in the event of water immersion, if the first layer fails, the second layer remains intact.
Between the top cover and the battery cells, a structural adhesive for fixing the cells is also used, forming a CtoB sandwich structure.
These structural adhesives, along with the entire shell, provide structural protection, and the application of the adhesives requires meticulous handling.
Another detail is that the battery pack uses adhesives from major international manufacturers. Such details often go unnoticed by consumers, but different adhesives vary significantly in cost. While ordinary users may not perceive this, these differences in material selection determine the service life.
● Aerogel
The aerogel thermal insulation layer is also one of the most critical designs in current battery design. Aerogel is one of the solid materials with the lowest known thermal conductivity, and its function is to prevent heat propagation.
In this battery pack, the aerogel coverage area exceeds 12 square meters. This is a relatively massive scale. A layer of aerogel film is applied between each battery cell.
When a battery cell undergoes thermal runaway, the heat will not transfer to other cells. The runaway is confined to a single cell, achieving thermal isolation.
● BDU
Aerogel provides physical thermal insulation, while fuses provide physical power disconnection. Both designs are crucial during an accident.
Traditional designs employ a solution combining relays with passive fuses, relying on physical melting after the current exceeds the limit to achieve protection. To make the high-voltage electrical system safer, Xiaomi has integrated an active fuse in the BDU (Battery Disconnect Unit). This acts as a smart circuit breaker; once abnormal high voltage is detected, it can instantly cut off the high-voltage power within 4 milliseconds.
How is this achieved? It primarily relies on the BMU (Battery Management Unit) battery management system. This system simultaneously monitors the voltage, current, temperature, and gas pressure signals of all 200 battery cells. Moreover, it is monitored 24/7 through AI (Artificial Intelligence) cloud big data and is designed according to the highest functional safety level in the automotive industry, ASIL-D.
As EVs become more widespread, a large amount of empirical data combined with AI judgments can be used to detect potential battery anomalies in advance. Early screening helps determine if there are any manufacturing defects in the battery cells. Therefore, whether it is the improvement of manufacturing capabilities by cell manufacturers, or the enhancement of both factory and on-vehicle testing methods, they have all become stronger.
Part 2: Xiaomi Auto Safety Standards vs. National Standards Comparison Table
Based on the information from the live stream, we have made a simple comparison:
| Test Item | National Standard (GB) | Xiaomi Auto Enterprise Standard | Improvement / Difference Description |
| Vehicle Frontal Collision Speed | 55 km/h | 64 km/h | Collision energy increased by 35% |
| Side Oblique Pole Collision | Standard point testing | Sequential collision testing at all positions on the full side | Achieves side collision testing with no blind spots |
| Vehicle Collision System | National Standard | C-NCAP + Euro NCAP + IIHS | Synchronized with the three major international highest safety standards |
| Battery Thermal Diffusion Test Temperature | According to GB 38031 | 55℃ | Verification under higher temperature and harsher conditions |
| Battery Thermal Diffusion Test SOC | Specified by national standard | Ternary lithium 97% / LFP 100% fully charged | Closer to real extreme usage scenarios |
| Battery Bottom Impact Sphere Head | Greater than 25mm | 25mm smaller sphere head | Higher unit pressure, more stringent |
| Battery Bottom Impact Energy | 150J | 150J | Consistent with the national standard, but more stringent when combined with the smaller sphere head |
| 800V System Insulation Requirements | National standard baseline value | 5 times the national standard | Significant improvement in insulation safety |
| 800V System Protection | Insulation requirements only | Insulation + additional waterproofing requirements | More comprehensive protection |
| Cell Short Circuit Test After Fast Charging Cycles | After 300 fast charging cycles | After 1000 fast charging cycles | Number of cycles increased to approximately 3.3 times |
| Cell Safety Requirements | No fire, no explosion | No fire, no explosion | Durability significantly improved on the basis of meeting the national standard |
It can be seen that Xiaomi has comprehensively upgraded its standards based on the national standards.
For example, the frontal collision speed is increased from the national standard's 55 km/h to 64 km/h, resulting in 35% more energy;
the testing points for the side oblique pole collision are more comprehensive, which is synchronized with the three major international highest safety standards.
The battery thermal diffusion test temperature is increased from 22℃±5℃ to 55℃; the SOC is also increased from the national standard's 95% to 97% for ternary lithium batteries and 100% fully charged for LFP batteries.
The 800V insulation requirement is 5 times the national standard, with additional waterproofing.
The number of fast charging cycles is also increased from 300 to 1000 times.
Summary
This live stream was conducted at Xiaomi's battery Pack factory, where some production and testing details were also shown. No matter how good the design is, it still requires the support of a reliable manufacturing process. For example, the entire PACK has 1,301 welding spots, and the finished battery pack has undergone 1,655 tests; anti-overflow adhesive strips are used to seal the edges of the cell gaps to prevent adhesive from seeping into the crevices, and so on.
Returning to the initial question:
When buying an EV, many people always like to ask, "Whose battery cells are you using?"
This question is certainly important, but after reviewing these details of Xiaomi's battery design, we should realize that EV safety is no longer just about looking at the "rooms," but rather the entire "building."
The quality of a single battery cell represents a single-point risk;
but the structural design of the battery pack, thermal isolation solutions, active power disconnection capabilities, and manufacturing consistency are capabilities led by the automakers.
True safety is not about "never having problems," but rather ensuring that even if a single-point problem occurs, the entire system can still confine the risk to the smallest possible scope.
Today's NEVs have reached a mature industrial chain. What will be competed on in the future is the structural design capability, manufacturing control capability, and data capability of the vehicle manufacturers.
The anxiety over energy density is declining, while the competition in systemic capabilities is rising.