Introduction | security redundancy is important
On December 8, a vehicle smoke incident occurred in the underground parking lot of a residential area in Chengdu, and the vehicle involved was a Volvo S90 plug-in hybrid model. Counting the XC90 fire in Nanjing on November 30, Volvo has had two consecutive new energy vehicle safety incidents in ten days.
Although the final conclusion of the fire department has not yet been given, it still makes people ask: as a safety benchmark in the era of fuel vehicles, why have they repeatedly provoked fire after entering the era of electrification?
In fact, it is not only Volvo that has a fire incident, whether it is traditional car companies such as BAIC, GAC, ANDD, or new forces such as Tesla, Weilai, and Weima, there are records of fires. According to the incomplete statistics of the public document "New Energy Vehicle Fire Record Group", as of October 7, 2021, there were 68 known fire incidents.
In addition to the LG chemical used by Volvo, there are also mainstream battery manufacturers such as Ningde Times, BYD, and Panasonic. Battery safety, it seems, is an industry issue. So how can it be improved?
Consider the battery design of the universal Ultium platform, perhaps multiple ideas.
1 More than needle punch, battery safety should cover the entire life cycle
When it comes to the safety of power batteries, many people know that lithium iron phosphate is safer than lithium ternary lithium, but the stability is much weaker. Some people also know that BYD's "blade battery" has withstood needle prick tests. In fact, the power battery is quite complicated, and the acupuncture test is only a way to test for thermal runaway.
There are three kinds of thermal runaway inspection of Ultium batteries: first of all, it is also a needle pricking method, using steel needles or ceramic needles to puncture the battery, and the temperature change curve of the battery in a short period of time must meet the relevant requirements. The second is the external heating method, through external heating, to trigger a series of negative reactions inside the battery, under the test conditions can keep the nature of the battery stable. The third is the overcharge method, which is more practical, because the proportion of fire in the charging state is quite large, because it continues to recharge the battery after the battery is fully charged, which is easy to trigger the internal chain vector reaction of the battery.
Power batteries that have passed these three tests can enter the general supply chain.
In addition to thermal runaway, Ultium batteries also have special thermal diffusion evaluation indicators, which strictly follow the "thermal expansion five-level" evaluation system in the new national standard. First, the battery is triggered by thermal runaway, scenario 1: no thermal runaway is caused, and the test passes. Scenario 2: Thermal runaway occurs, but no heat spread or heat diffusion is detected, and the test passes. Scenario three: There is thermal runaway, and there is also heat spread, but it does not cause any harm, and it is also counted as passing.
The worst case scenario is that there is heat runaway, heat spread occurs, and it causes harm, and there are two evaluation indicators at this time:
Scenario 4: If the time of the alarm is more than 5 minutes from the time of the occurrence of the hazard, it is enough time for the driver and passenger to escape, and it can also pass the test. Scenario Five: If the interval is less than 5 minutes, then the test does not pass. This is the ladder of five-level evaluation indicators.
The universal Ultium battery does not avoid the possibility of problems with the power battery, after all, there are any working conditions in reality; but it emphasizes that there is enough safety redundancy and add a layer of protection to reduce the risk of fire.
There is one more issue that cannot be ignored. The current power batteries are grouped together by modules, that is, they are formed by cells and then packaged by molds. Therefore, in addition to the safety of the battery cell itself, the reliability of the auxiliary material is also very important.
For example, the thermal conductive gel and thermal conductive gasket for battery thermal management, the insulating tape wrapped in the battery cell, the insulating hot pressing film and high-strength cold film used in the battery module, etc., their wear resistance and stability should meet the requirements of at least 15 years. It can be said that the safety design of the universal Ultium battery covers the entire life cycle of the battery.
Most manufacturers have similar practices, but not many have raised it to the strategic level, and the general calculation is one.
2The non-module design is attractive, but the safety redundancy needs to be left sufficient
A series of new battery packs are emerging, the most popular of which is the CTP battery pack. Cell To Pack means that the battery cell directly forms a power battery pack, omits or simplifies the intermediate module structure, allows more cells to be placed in the unit space, and physically improves the energy density of the entire battery pack.
The benefits of this are: significantly reducing material costs, manufacturing costs, and increasing energy density and cruising range. So many people think that CTP is a symbol of advancement, but not necessarily.
CTP is essentially an extreme large module solution, eliminating the need for intermediate modules, so that the entire battery pack becomes a large battery cell. It's like a tea factory that puts tea leaves in small bags and then puts a few bags of tea leaves into a jar; now instead of small bags, put the tea leaves directly into the jar.
Obviously, this has a larger operating capacity and lower cost, but the problem is that the freshness of the tea will be reduced. High-grade tea leaves are more in the form of "first bags and then cans". As for the size and number of bags, it depends on the respective needs.
Taking the development of general power battery modules as an example, the earliest use of 355 modules, gradually transitioned to 390 and 590 modules. These numbers refer to the size, the 390 and 590 modules have been enhanced in the lateral size, especially the 590 module is more efficient and larger, which can further reduce costs.
For example, the energy density of the 355 module is 192kWh/kg, and the group efficiency is 91%; the energy density of the 390 module is improved, reaching 195kWh/kg, and the energy density of 590 can be significantly increased to 221kWh/kg. The number of parts is further reduced by reducing the number of modules, thereby increasing the energy density.
This puts forward higher requirements for the consistency of the battery, and some structural parts and even monitoring devices are missing, resulting in the cost of later maintenance and disassembly also increased. From 355 to module-free, the invention and application of any new automotive technology should prioritize safety, and it needs to balance innovation and cost.
The general view is that module-free design does not necessarily mean the most advanced, and only technological innovation under the premise of ensuring safety is really advanced. These solutions have been tried in the early stages, and finally the most suitable solution for the current market has been selected: both to achieve the maximum energy density of the battery pack and to ensure maximum safety.
Its new energy test building in Tangzhen, Shanghai, opened in early 2021. It is built in accordance with the highest standards and process systems of the General Motors Laboratory, covering the main verification and test capabilities of the core three electric components, batteries, electric drives and electronic control systems of new energy models.
At present, the laboratory has a special cell/module performance bench, which can support the performance and durability testing of batteries, modules and whole packages, and is currently the only main engine factory with battery safety development capabilities in domestic joint ventures. So the universal Ultium battery solution isn't necessarily the funniest, but it should be reliable.
3wBMS wireless battery management system, farewell to the big pot of rice
In addition to making full efforts in battery testing and grouping, GM has also specially built a set of advanced wBMS wireless battery management system.
The internal connection of the traditional power battery is very complicated, and the corresponding wiring harness connection between the battery modules not only makes the structure more bloated, but also brings safety hazards such as wear and aging. Through years of technical research, GM launched the wBMS wireless battery management system, which integrates power management, battery management, RF communication management and battery system functions into a set of telecommunications systems.
The new design reduces the low-voltage wiring harness by 90%, reduces the linking between the controllers, and avoids the possibility of loose or short circuits in the connector during vibration. Moreover, the scalability of the system is better, the replacement module is easier, and the installation and maintenance of the battery system is more convenient.
In addition, the health and efficiency management of the battery is more meticulous, and each module can carry out charge and discharge management and thermal management according to the actual situation of the module, which is safer and more efficient.
The essence of the wBMS system is to divide the original complex set of battery control and connection systems into a brain and several intelligent nodes. The intelligent node not only plays a role in connection, but also can view the battery's power, heat, fault, etc. at any time, and then optimize the fault part through short-range wireless communication technology to ensure that each module is individually optimal and overall stable.
With the blessing of this system, the general Ultium battery does not have to be managed like an integrated pot of rice - only when the entire battery pack has a problem, the system can "perceive". Instead, through dot matrix monitoring, the risk is eliminated before it occurs, and the efficiency is significantly higher.
And the wBMS system also has a "magic use", that is, once this technology is popularized, users can evaluate the performance of the battery of the second-hand car according to its data, and promote the standardization of the second-hand electric vehicle market.
Car gathering summary
The development of new energy vehicles is too fast, and various new concepts and new technologies not only dazzle consumers, but also easy to get lost by practitioners. Looking back, battery safety grasps two points: safety redundancy and advanced technology.
GM has been early and not fast in electrification, and has always had the stability of large manufacturers, and has not relaxed its exploration of new technologies. Ultium Aotene battery is a sample of this idea, I look forward to its market performance in the end?
No one can guarantee that no battery will be in trouble, but the attitude of pursuing a safer one still has to be there.