We recently conducted a simple user questionnaire survey on the safety of power batteries. Among the 195 users who filled out the survey, 145 were certified electric vehicle owners. 96 of them (49.4%) indicated that the safety of power batteries is a factor that can significantly affect the final purchasing decision. 98 people (50.5%) believed that the priority of battery safety is equivalent to the price and performance of the vehicle when purchasing, while 59 people (30.4%) even regarded this as the first priority in purchasing.
In addition, 127 people (65.4%) in our survey believed that strict battery safety testing is a bonus to demonstrate the safety of the battery, such as needle puncture and fire burning.
However, 74 people (38.1%) believed that although lithium iron phosphate batteries are safer, lithium batteries perform better, and they prefer the latter.
From this questionnaire survey, we can see that battery safety is still an important consideration for consumers when purchasing pure electric vehicles. Under the influence of the trend of automotive electrification, consumers have established a preliminary understanding of this aspect. Even though lithium iron phosphate batteries are safer, there are still quite a few users who prefer ternary lithium batteries due to their better performance.
Inevitable Battery Safety
Safety has standards
When it comes to power battery safety, what everyone fears most is battery fires, which are usually caused by systematic thermal runaway resulting from factors such as system short-circuiting, high system temperature, and battery structure deformation.
Therefore, if certain measures can be taken to prevent systematic thermal runaway, the overall safety of the battery system can be greatly improved.
In recent years, while vigorously promoting new energy in policy, our country has also actively promoted industry’s attention to battery safety. The GB 38031-2020 issued last year made mandatory provisions for battery thermal runaway. The document states:
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Thermal runaway refers to the phenomenon that the temperature of the battery becomes uncontrollably high due to the exothermic chain reaction of a single battery cell.
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Thermal diffusion refers to the phenomenon that thermal runaway of a single battery cell in the battery pack or system triggers the successive occurrence of thermal runaway of other battery cells.
Starting from January 1, 2021, non-nickel-hydrogen battery packs should undergo “thermal diffusion passenger cabin protection analysis and verification” in accordance with 8.2.7.2.
The national standard proposes that “a thermal event alarm signal should be provided before 5 minutes of battery pack or individual cell thermal runaway causing thermal diffusion and endangering the passenger cabin occurs.”
If the temperature diffusion within these 5 minutes does not pose a danger to passengers, such as causing an open flame, then the system’s safety is considered to meet the requirements.
This mandatory safety standard issued by the government clearly and rigorously sets the bottom line for the safety of power batteries and has been publicly disclosed. Many excellent companies in our industry have also responded positively.
Last year, BYD conducted a needle-puncture test on lithium iron phosphate battery cells, and no fire occurred during the entire test. SAIC Roewe also stated at a press conference that its battery pack can ensure no thermal runaway for 30 minutes in fire and hot performance tests, far higher than the 5-minute mandatory national standard.
NIO also announced at its 100-degree battery pack release conference that its battery pack only produces smoke and doesn’t catch fire after thermal runaway. Recently, GAC also performed needle-puncture tests using ternary lithium battery cells in its “magazine battery,” and the battery system also only produced smoke and didn’t catch fire.
Also recently, the Voyah brand claimed that its ternary lithium battery pack can even achieve “no smoke” in thermal runaway tests. With curiosity and skepticism, I carefully examined their test content.
Smokeless Thermal Runaway Test
The battery pack used by Voyah for the thermal runaway test is the one that will be used in the mass-produced Voyah FREE pure electric vehicle, with NCA nickel-cobalt-aluminum ternary lithium battery cell chemistry material and 2170 cylindrical battery cells provided by Samsung.
I was somewhat surprised when I learned this information: on the one hand, the battery used for the test is an 811 high-energy density formula, which raises higher requirements for the thermal runaway measures of the battery according to my common sense.
On the other hand, Voyah actually uses cylindrical battery cells. The battery system composed of cylindrical battery cells is only confident for a few manufacturers to use due to process problems and difficulties in battery management, and one of the most famous ones is Tesla. In addition, according to Tesla, the 2170 battery cell used by Voyah is the second-generation cylindrical battery cell optimized through heat dissipation and specific energy considerations after optimization.
Returning to the thermal runaway test, Voyah used the method specified in the national standard to heat the battery cell until thermal runaway occurred. During the battery pack production and assembly, a miniature heating device was installed on one of the cylindrical battery cells, and temperature sensors were arranged around it as required by the national standard.
The battery pack’s state of charge was at 100% SOC during the test. The Voyah staff members controlled the pre-installed miniature heating device to heat the battery cell up to a maximum of 300℃, triggering thermal runaway in a single battery, and monitored the battery pack’s status.
The final result is that no further chain thermal runaway occurred in the battery cells other than the overheated one, even the innermost cells closest to the overheated one were controlled to within about 70℃ during the test.
From the outside of the battery pack, there were almost no changes during the entire test, no explosion, no fire, and as the official statement said, not even smoke.
I am quite surprised by these results, and my biggest question is how did this battery system cut off the chain thermal runaway in individual cells?
Non-“Fire” Battery Pack
Comprehensive Fireproof Barrier at Cell Level
Currently, most of the industry’s prismatic cells are grouped by cell – module – pack, and in order to increase the packing density, cells are often adjacent to each other. Although cells have their own flame retardant design, more powerful flame retardant structures are usually not present on every single cell.
On the other hand, since the volume of prismatic cells is large and the thickness of battery packs is limited, flame retardant structures are usually also in the horizontal and vertical directions.
In the earlier mention of the battery pack tested by Lanto, it was found that 2170 cylindrical cells were used, and I learned from further information that Lanto had implemented cell-level flame retardant protection during the process of encapsulating cells in the battery pack: except for the parts of the cells that are connected to the battery pack circuit system and the battery pack liquid cooling and temperature control system, each cell in the battery pack was completely wrapped and isolated by polymer flame retardant material.
When each cell is isolated in the “isolated chamber” by polymer flame retardant material, it is difficult for the thermal runaway of a single cell to spread to adjacent cells.
However, what is discussed here is the extreme case of placing a heating device on the battery cell and causing thermal runaway. This is the final safeguard for battery safety. To ensure the overall safety of the battery system, we definitely cannot rely on just one measure.
IP X9K seal, 200 kN compression: this battery pack is almost unbreakable
To prevent thermal runaway in the battery cell, the battery pack system is a critical component in terms of hardware. First of all, sealing is important. If the battery pack at the bottom of the chassis is not well-sealed, it is highly likely to let water into the battery pack when wading.
To ensure good sealing, in the development stage of the battery, Voyah conducted two rigorous tests on the battery pack. One test was to immerse the battery pack in one meter deep seawater while it was in the powered-on state for two hours. The other was a jet-sealing test using hot water at 85°C under an equivalent pressure of 100 atmospheres.
After passing the above tests and achieving the industry-leading IP X9K seal standard, the battery pack also underwent a salt spray test to ensure that the sealing effect of the battery pack can withstand the test of time by simulating natural corrosion during use.
After sealing, there is also the structural strength of the battery. As there have been multiple cases of the battery pack catching fire due to deformation caused by collisions with the chassis, Voyah has done a lot of considerations regarding the structural strength of the battery pack. High-strength aluminum alloy is used as the material, and multiple reinforced ribs are designed in the structure. The final result is that it can withstand 200 kN of extrusion, which is equivalent to 20 tons of extrusion force.
Based on such strength design, even after the battery pack underwent the most severe 75°, 32 km/h vehicle column impact test, it still did not deform to the point of extruding the internal battery cells. This demonstrates the protective ability of the battery pack.
Furthermore, vibration, insulation, water immersion, multi-directional mechanical impact, high and low-temperature tests, etc., are often performed on the same battery pack during the development stage.In the eyes of the R&D personnel of the Voyah automobile company, vehicles in the real world will face various environments and challenges, so the battery pack needs to be tested in multiple dimensions and combinations on the same battery pack to assess the overall protection level.
Safety Comes at a Cost
After countless efforts in research and testing, the battery pack equipped in the Voyah Free electric vehicle has exceeded 150 national safety standards after undergoing 150 days of complete testing during the development period.
For example, as for the flame-retardant and insulation walls of the cell mentioned earlier, it took a year to develop the heat expansion performance, and dozens of test modules were created in order to fill the gaps sufficiently and reduce air bubbles. More than ten different flame-retardant materials were also tested hundreds of times, consuming dozens of battery packs in the process.
The entire battery pack had to undergo a complete testing process, which exceeded 150 days. As a result, they had to pay a price of 50% more on the research and development costs and an extension of 8 months on the R&D period.
For Voyah, as a state-owned company, money is not a problem, and the R&D budget may exceed the planned amount. However, for any new car company, time is invaluable in the current industry development speed. Voyah realizes this, but to them, the price for higher-level safety is worth it.
Remember, This is a Lithium-Ion Battery Pack
Safety is undoubtedly important, but it is not everything for the power battery. Side effects of the manufacturer’s considerable time and money invested in battery safety design may not be very noticeable during daily use by users, but battery performance is.
It must be said that compared to lithium iron phosphate batteries, this is a strong aspect of the ternary lithium-ion battery’s chemical characteristics. Ternary lithium-ion batteries typically have better charging and discharging characteristics than lithium iron phosphate batteries, which allows for a greater power output of the battery system. Compared to lithium iron phosphate batteries, the ternary lithium-ion battery’s discharge curve is smoother, which is more conducive to BMS monitoring and prediction. Higher energy density is beneficial to both lightweight vehicle design and improved driving range.
Moreover, for cold regions, the low-temperature performance of ternary lithium-ion batteries is better than that of lithium iron phosphate batteries.
Voyah does not want to compromise on battery safety, but this high-density ternary lithium-ion battery pack is not compromised in terms of performance.
Conclusion
Looking back at the development of the new energy industry in the past few years, the progress in battery technology is visible with the continuously increasing driving range. However, the rapid development has also exposed some impetuous companies.
The many incidents of electric vehicle fires and explosions not only affected the automaker and battery supplier but also caused society’s concerns and rejection of new energy vehicles.The negative impact mentioned above requires long-term joint efforts from the whole industry to eliminate. Leading enterprises in the forefront of this process should be respected. At this point, I am pleased to see that LanTu, which enters the high-end new energy field with the identity of central enterprises, is among them.
Finally, I would like to quote the following words: “Top-notch experts compete in slowness, clumsiness and steadfastness, smart people do hard work. By choosing a narrow path and working steadfastly, value can be created for society and users, and the whole world will make way for you.”
This article is a translation by ChatGPT of a Chinese report from 42HOW. If you have any questions about it, please email bd@42how.com.