Safer than seat belts! These batteries can be called black technology. What does the fire acupuncture mean?

What is the most vulnerable thing about pure electric vehicles? Battery. Ever since this kind of car became popular, social news about various accidents involving tram battery packs has never stopped. I guess everyone must have seen the earth-shattering scenes of spontaneous combustion and explosion of lithium batteries from various media. Therefore, it is normal for everyone to have doubts about the safety of battery packs, and major car companies have been working hard to dispel consumers' concerns about battery safety.

Since entering 2023, we seem to have discovered that the safety of vehicle power batteries has developed rapidly. Several domestic car companies have begun to conduct public experiments and subject batteries to various "inhumane" "tortures." Some are acupuncture, some are burning, some are squeezed by heavy objects, and some are like Geely's earlier, where acupuncture, fire, and falling come in turns. In fact, their purpose of doing this is just to prove how safe their batteries are.

So, how safe are current car batteries?

Maybe you have read our previous popular science articles about battery packs, and you may know that a battery pack is roughly divided into two major structures: battery cells and battery protective cases. Accessories such as battery heat dissipation and insulation are all installed in the battery protective case. And from the name "battery protective case", everyone can also know that this thing is used to protect the battery core.

So does this mean that the success of various extreme tests on batteries by car companies is entirely due to the battery protective case? Yes, not entirely. First of all, everyone needs to know that in most mainstream domestic car companies, there are two major items of overall safety of power batteries: protective case safety and battery cell safety.

Let’s talk about cell safety first. This is the last line of defense for the battery safety system. It is the means that the cell body alone can provide to prevent thermal runaway and cell short circuit after all external protection systems have failed. There are two types of battery cell safety: battery cell protective shell safety and battery cell structural safety. The structural safety of battery cells actually means working on the materials that make up the battery body, by replacing the positive and negative electrode materials, changing the material of the ICM film (that is, the thin film in the center of the battery that separates the positive and negative electrodes), and changing the electrolyte formula, etc. and other methods to try to slow down as much as possible the phenomenon of excessive lithium nesting between lithium electrons and positive or negative electrode materials under certain extreme conditions.

Lithium nesting refers to electrons that should be "running freely between the positive and negative electrodes". For various reasons, they are nested in the positive or negative electrode materials that can react continuously with them. Positively charged electrons will continue to react after being nested into the positive and negative electrode materials. These materials whose base material is lithium will have lithium crystal growth (industrially, the way to make solid lithium crystals is through electron bombardment). When the lithium crystal grows to a certain extent, it is likely to pierce the ICM film or the battery cell protective shell. This is how the battery cell short circuits.

So everyone can understand that the battery cell structure we mentioned above is safe, which is to fundamentally prevent internal short circuits caused by the excessive growth of lithium crystals. As for the safety of the battery cell protective shell, in fact, to put it bluntly, it is to increase the strength of the battery shell. Even if lithium does crystallize, try to ensure that these lithium crystals do not pierce the battery shell so quickly. Please note that we are talking about "not so fast", not "cannot puncture". This is because the puncture ability of lithium crystals is very strong. Unless the battery core protective shell is made of reinforced composite materials, it will basically There's nothing you can do to completely prevent lithium crystals from piercing the metal shell.

So to put it bluntly, what can be done at the level of battery cell protection is basically concentrated on positive and negative electrode materials and electrolyte materials. Whether launched by BYD or other car companies in the past two years, batteries that are similar to blade batteries and can pass acupuncture tests essentially replace the positive and negative electrode materials, and the electrolyte and ICM membrane are also structured. Improvement. Of course, how these car companies improve the cell structure, how to choose the positive and negative electrode materials, and the material ratio are all top-secret business secrets for every car company. It is impossible for us to learn from public channels. to obtain any relevant structural information.

So to put it bluntly, more and more car companies’ batteries can pass the acupuncture test. This is basically the effect of the improvement of the battery structure.

However, when designing the battery pack, an extreme condition called the "failure method" needs to be considered. Suppose that in a certain scenario, all the protection systems of the battery cells fail, all the cooling systems of the battery pack fail, and the battery management system fails. When all three systems fail, the battery pack is placed in an extreme external environment. How to ensure that the external combustion time of the battery is delayed as much as possible?

The so-called external combustion time is actually easy to explain. It is how long the battery protective case can withstand under extreme circumstances where the battery cell will inevitably run away from heat. This is actually a key problem in the design of battery protective cases. Around 2016-2019, major car companies had no good solution to this problem. The only way was to cover the inside of the battery protective case with more fireproof materials, replace the outer shell material with flame-retardant and flame-resistant materials, and replace the protective The sealant between the upper and lower shells was replaced with a very costly fireproof material.

From 2020 to 2022, major car companies began to improve the structure level of battery protective cases. There are roughly two "schools" here. One is the "lower fireproof cabin" structure promoted by Great Wall, Changan, and GAC, and the other is the "blade battery + armored lower floor" promoted by BYD. There is no absolute distinction between these two technologies. They only represent two different technical understandings.

As the name suggests, the lower fire compartment structure is to place an independent fire extinguishing system corresponding to each battery module under the battery protective shell and battery module. Each fire compartment is charged with high voltage. Once a battery module or a certain cell experiences thermal runaway, the high temperature will quickly burn the partition between the battery cell and the fire compartment. The special fire extinguishing agent in the fire compartment will be sprayed upward due to pressure leakage, and the battery will be destroyed as much as possible. Put out the fire caused by the module. Even if the fire extinguishing fails, due to the special nature of this structure, the fire will burn downward and concentrate in the fireproof cabin, and the materials in the fireproof cabin are all flame-retardant materials. In this way, the emergency escape time and space of the entire vehicle can be guaranteed for a period of time after a single battery cell or a single module undergoes thermal runaway.

To put it bluntly, this structure is a bit like a "variation" of the automatic fire extinguishing system of high-rise buildings. BYD and other car companies have a different approach. Their thinking is this: As long as I can guarantee the absolute safety of the battery core itself and ensure that the battery core itself will not catch fire, there is no need to damage the battery core protective shell. For fireproof cabin treatment, you only need to strengthen the protective shell that is exposed to the outside environment (bottom, side).

Therefore, the idea of ​​BYD and other car companies is basically to protect the battery itself by strengthening the physical strength of the bottom of the protective case. Although this structure lags behind the structures of several other car companies in terms of advancement, because BYD and other car companies can ensure that the battery core itself will not thermally runaway, the battery core will not thermally runaway + the bottom of the reinforced battery protective case Thanks to the protection, this solution is also very secure.

Looking up at the U8 before   (  parameters  |  inquiry  ) and hitting the desert is an example of this model. Because the bottom of the battery protective case is strong enough, U8 can directly ignore the ubiquitous bottoming phenomenon in desert environments.

If the trams of a few years ago still needed to pay special attention to protecting the battery pack during daily driving, today's trams basically do not have this worry. In general, you can drive it exactly as you would a gas-powered car. Even because the strength of the battery protective shells of these new generation trams is much stronger than the impact resistance of the chassis of many fuel vehicles, some extreme road conditions that fuel vehicles dare not pass, the current trams are basically stress-free.

but! It’s not that all current electric car battery packs are so awesome. Judging from the current situation, several well-known domestic brands are very strong in this regard. Basically, no matter how you "fiddle" with their cars, nothing will happen. Overall, joint venture vehicles have a generational disadvantage compared to domestically produced vehicles in terms of battery protection structures. Among joint venture cars, American, German and Korean models do relatively well in terms of battery pack safety.