Is this the end of the world? Analysis of Tesla's battery safety technology

1 Battery structure and control

    "Oh my god, man, that's a Tesla, it's brand new." If you didn't see it with your own eyes, no one would believe that the car on fire was a Tesla Model S. Before this, including the Roadster model, the cumulative mileage of mass-produced Tesla electric vehicles has exceeded 181 million kilometers. Tesla seems to have proved the reliability of its products in this way. It's like everyone believed that the Titanic was an unsinkable cruise ship. Of course, in terms of technology, Musk did give us a lot of reasons to believe him.

Battery assembly structure

    Since the battery is to blame, let's first understand the situation about it. As we all know, the battery of TESLA electric vehicles uses the NCA series (nickel cobalt aluminum system) 18650 lithium cobalt oxide battery provided by Panasonic, and the capacity of a single battery is 3100 mAh (mAh, the unit we usually see on the battery is "ampere-hour", which is mainly selected according to the different capacity of the battery).

    We are not unfamiliar with this type of battery. Electronic digital devices such as laptops use this type of battery. Compared with other types of batteries used in electric vehicles, the technology of 18650 batteries is more mature. Its specific energy (the amount of electrical energy released by the unit mass of electrode materials participating in the electrode reaction) is almost twice that of lithium iron phosphate batteries (BYD e6). In other words, under the same volume, the battery unit composed of 18650 batteries can store more electrical energy. This is one of the reasons why Tesla uses this battery.

    Despite this, it is still difficult to use this type of battery in electric vehicles. For example, in order to meet the needs of an electric vehicle, many 18650 lithium batteries are needed, which brings up a problem that needs to be solved: how to combine them together.

    The battery unit of the 85kWh MODEL S uses a total of 8142 18650 lithium batteries. Engineers first evenly distribute these batteries into bricks and sheets to form a complete battery pack, which is located on the bottom plate of the vehicle body.

    Although the way these thousands of batteries are combined together is considered incredible by the industry, for Tesla engineers, it is just a slightly more complicated arithmetic problem compared to the next problem to be solved.

Stability of 18650 batteries

    Although the 18650 lithium cobalt oxide battery is the key to meeting a higher driving range, its stability at high temperatures is slightly worse than that of lithium nickel cobalt manganese oxide (NCM) and lithium iron phosphate batteries. Therefore, strong technical support is needed in terms of safety.

    Its violent character has caused it a lot of trouble. I remember that a few years ago, Sony recalled its notebook batteries due to explosions. However, the current 18650 batteries can technically avoid spontaneous combustion or explosions without cause. However, after a strong impact, this type of battery still has a high possibility of explosion. In addition, its adaptability to low temperature environments is not very stable. In low temperature environments, cobalt-acid lithium batteries are prone to overheating due to excessive discharge. In this way, how to manage these batteries has become a very important matter.

2 How to monitor the status of the battery pack for vehicle body structural safety

    The safety device in the battery pack is distributed to each 18650 lithium cobalt oxide battery. Each 18650 lithium cobalt oxide battery is equipped with a fuse at both ends. When the battery is overheated or the current is too large, the fuse will be cut off to avoid the entire battery pack being affected by an abnormal situation (overheating or excessive current) in a certain battery.

    In the above introduction to the battery pack structure, we mentioned that each battery pack is composed of several battery cells, one battery cell is composed of several battery bricks, and one battery brick is composed of several 18650 lithium cobalt oxide batteries. In addition to each 18650 battery having a safety device, each battery cell and each battery brick also has a safety device. Once a problem is found inside a unit, the safety device will cut off its connection with other battery cells to avoid the situation where others are affected. In addition, each battery cell has a relatively independent space separated by a firewall. Even if a fire occurs inside a single battery cell, the fire can be controlled to a certain extent and will not spread quickly to the entire battery pack.

    Of course, the fuse is the last barrier. When it is cut off, it means that there is a problem with a battery unit. If it involves replacement, the entire battery pack can be replaced in units of "pieces". Each battery is connected in parallel, while the battery bricks and battery pieces are connected in series. That is to say, in actual use, when a battery has a problem, the vehicle will not break down, and only the vehicle's range will be affected.

    Maintaining the working status of the entire battery pack and monitoring the system of each battery cell is key to the performance of TESLA electric vehicles (it is also very important for other brands of electric vehicles). The battery monitoring device (Battery System Monitor) in each battery cell (which consists of battery bricks containing several 18650 lithium cobalt oxide batteries) monitors the status of each battery brick in the battery cell, including not only current and voltage, but also the operating temperature of the battery, the relative position of each battery brick, and whether smoke is generated.

    Battery temperature is a key factor affecting battery performance. Too high or too low will cause fluctuations in its working state, which in turn affects the overall performance of the vehicle. Therefore, the system needs to constantly balance the temperature of each battery. The above-mentioned guarantees on safety and battery performance are all due to the design of this battery hierarchical management mode.

The body structure ensures the safety of the battery assembly

    Previously, Tesla Model S was crash-tested by NHTSA (National Highway Traffic Safety Association) and received a five-star rating. This result made Elon Musk a little self-inflated. Anyway, the Model S's protection of dummies and battery status in various crash tests are commendable. Let's take a look at the performance of this car in passive safety through the collision situation at that time.

    NHTSA's crash tests include 100% frontal collision, 27° side collision, 75° side pole collision, rollover collision, and roof strength tests. If you want to learn more about the NHTSA crash test method, you can read our previous article.

    In the 100% frontal collision test, MODEL S crashed into a rigid barrier at a speed of 56km/h. Judging from the body posture during the collision, compared with what we saw in the C-NCAP collision test, the rear end of MODEL S did not lift up significantly. This is related to the weight distribution of the entire body. The battery of MODEL S is distributed under the vehicle, the motor is located at the rear axle, and the front of the vehicle is not equipped with a powertrain like a traditional car. Therefore, the weight of the vehicle is basically concentrated at the bottom and rear of the vehicle and the center of gravity is low. In this way, in this collision test, the body amplitude is relatively small.

    Since there is no need to install a powertrain in the "engine compartment", there is enough space to arrange the structure used to increase the strength of the vehicle body. In such a collision situation, it is difficult to damage the battery pack located under the passenger compartment.

    In contrast, side collisions pose a greater threat to battery safety. Therefore, it is very important to use high-strength materials to prevent the battery pack from being over-extruded during a collision. As mentioned above, 18650 lithium cobalt oxide batteries do have the possibility of explosion during a collision. However, blindly increasing strength is not a good thing for the occupants. The energy absorption and buffering concept we mentioned in the previous analysis of collision tests is also applicable here. It is easy to imagine how difficult it is to prevent the battery pack from being squeezed and to protect the occupants.

    From a document, we can see that the sill beam of MODEL S adopts a multi-layer structure composed of aluminum parts. This structure can gradually absorb the collision energy under the guidance of the structure while resisting the collision force. The structural concept is derived from the lander used by the Apollo spacecraft when landing on the moon. I don’t know if this is an idea that Elon Musk, who is determined to retire on Mars, came up with. According to this report, in the column collision test, the protection of the passenger compartment of MODEL S (63.5% of the cockpit is retained) is better than that of Volvo V60 (7.8% of the cockpit is retained).

    Taking into account the third row of seats in the rear, in the design of the rear anti-collision structure, MODEL S is equipped with two anti-collision beams at the rear of the car to reduce the possibility of injury to the third row passengers in the event of a rear collision.

Why is it still on fire?

    After reading the introduction of battery safety technology and passive safety of the vehicle body, why did the Model S catch fire after the collision? Some time after the incident, Elon Musk issued a relevant statement, saying that the vehicle fire caused by the high-speed accident was caused by the battery protection cover at the bottom of the vehicle being pierced by a hard object. The strong impact caused the peak force of the piercing to exceed 25 tons and punched a hole with a diameter of 76.2 mm in the 6.35 mm underbody guard.

    Immediately, the vents at the front of the battery assembly began to release heat and caused a fire. When the firefighters arrived, they used traditional fire-fighting methods, which caused the fire area to expand. The fire was later controlled using a dry powder fire extinguisher. Because a separate firewall was built for each battery module inside the battery assembly, the fire did not spread to other places. The alarm system also promptly warned the driver to leave the vehicle as soon as possible through various sounds and icons, and the driver was ultimately spared injury.

Editor's summary:

    Because of the collision and fire accident, Tesla's stock price, which has quadrupled this year, fell by 8.92% on the same day. Indeed, compared with traditional car fires, electric car fires are more sensitive. BYD has experienced similar incidents before. These two incidents are both accidents. One is that the bottom of the car was pierced by metal, and the other is that it was hit by a high-speed sports car. In such extreme cases, even traditional cars cannot rule out the possibility of fire.

    It is undeniable that Musk has pushed electric vehicles to a new height. To a certain extent, he has colored the TESLA brand with his personal colors. Of course, this is only one aspect for the TESLA brand. More importantly, it explains to us what the future transportation will look like. MODEL S should be considered a real start. Next, in addition to continuing to create user experience, battery upgrades are also the key to product development. In Musk's eyes, only supercapacitors can be used for him (fast charging and discharging speed), but due to technical reasons, supercapacitors cannot fully meet the needs of electric vehicles. In short, in terms of safety, electric vehicle manufacturers will take every step very carefully.