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On September 23rd, Weilai Automobile officially announced the standard battery pack of lithium iron trioxide (75kWh). The battery pack adopts the mixed arrangement of lithium iron trioxide and lithium iron phosphate cells, and applies the new generation of CTP technology. It is reported that the model carrying the three-yuan lithium iron standard battery pack (75kWh) has been booked and will be delivered to users in November this year.

As we all know, lithium iron phosphate battery and lithium ternary battery have been regarded as the two main battery technology routes in the global electric vehicle industry. The lithium iron phosphate battery package launched by Weilai is obviously different from the two. As the name implies, the ternary lithium-iron battery pack is a battery pack that uses the mixed arrangement of lithium ternary and lithium iron phosphate cells, while the last time the mixed arrangement was adopted was CATL. In July of this year, CATL announced the solution of AB battery at the same time as the sodium ion battery. This method combines the sodium ion battery and lithium battery to realize the advantages and disadvantages. However, as a car company, why did Weilai choose the ternary lithium iron technology route this time? What are the difficulties in the implementation of this technology? How about the performance of the battery pack with this technology?

Why choose lithium iron ternary technology?

With regard to lithium ternary and lithium iron phosphate, the industry has been paying considerable attention. From the perspective of materials, lithium iron phosphate battery and lithium ternary battery are the two mainstream battery technology routes in the global electric vehicle industry. They also have their own advantages and disadvantages in terms of energy density, low temperature performance and battery cost. Among them, the ternary battery has high energy density and good low temperature performance; Lithium iron phosphate battery has low cost, good high-temperature performance, long service life and high safety. Based on this, their competition in the market has never stopped.

According to the observation and analysis of the Gesch Automotive Research Institute, from 2016 to 2021, lithium iron phosphate battery and ternary battery showed an obvious trend of ebb and flow. In 2016, lithium iron phosphate battery once won 73% market share with 19.98GWh installed capacity. Since then, ternary lithium battery has risen rapidly, and its market share has soared from 27% in 2016 to 65% in 2019. Lithium iron phosphate battery began to decline all the way, and its market share was only 32% in 2019.

However, this situation has shown a reversal momentum in 2020. In 2021, the recovery trend of lithium iron phosphate battery will become more and more obvious. In May and July this year, lithium iron phosphate battery has achieved the reverse of lithium ternary battery in terms of production and installed capacity, and the growth rate in a single month is far greater than that of lithium ternary battery.

As for the strong return of lithium iron phosphate battery, the industry generally believes that it is inseparable from its own two advantages, one is cost performance, the other is safety. Based on this, with the rising price of raw materials and the further strengthening of the safety of power lithium batteries, many automobile enterprises began to widely use lithium iron phosphate battery packs. It is understood that at present, many star automobile enterprises and new forces of automobile manufacturing have launched lithium iron phosphate models, and international automobile brands are also interested in using lithium iron phosphate batteries for water test. In addition, the proportion of lithium iron phosphate vehicles in the new energy vehicle application catalogue of the Ministry of Industry and Information Technology has increased, and the proportion of passenger vehicles installed has increased more significantly.

When many participants have adopted lithium iron phosphate batteries, Weilai has no idea. It is understood that in the early stage, Weilai also had plans to use lithium iron phosphate, and had also developed a 68kWh lithium iron phosphate battery pack. However, compared with lithium ternary, there is a gap in the low temperature performance and SoC estimation of lithium iron phosphate in terms of user experience, so Weilai did not choose to mass produce the battery pack, but began to study new solutions.

After more than one year of research and development, Weilai launched a lithium iron battery pack (75kWh) with higher energy density and longer endurance through the combination of lithium iron battery and lithium iron phosphate. As a result, Weilai became the world's first automobile enterprise to mass produce ternary lithium iron technology.

How about the performance of lithium iron battery pack?

As mentioned above, lithium iron phosphate battery has the advantages of less use of rare metals and lower production costs, while its disadvantages are poor low-temperature performance and inaccurate SoC estimation. In the face of this, Weilai innovatively adopts the mixed arrangement of lithium ternary and lithium iron phosphate batteries, and applies the new generation of CTP (CelltoPack) technology to effectively solve the above pain points.

According to Zeng Shizhe, vice president of Weilai battery system, through Weilai patented technology, Weilai standard battery pack (75kWh) has achieved excellent low-temperature endurance performance and accurate power estimation ability, and can provide users with the same level of performance as the lithium battery pack.

25% reduction in low temperature endurance loss

First of all, the Ulai lithium iron ternary standard battery pack (75kWh) adopts a comprehensive thermal insulation design, which reduces the low temperature battery life loss by 25% compared with lithium iron phosphate battery pack. The realization of this performance depends on the complete thermal management software and hardware system design of Weilai:

Dual system control algorithm:

The original dual-system algorithm performs modeling control based on the low-temperature characteristics of ternary and lithium iron batteries. After multiple rounds of calibration, it effectively improves the energy use efficiency of the battery system at low temperature and ensures low-temperature performance.

Coupled battery heat generation intelligent heat management:

According to the characteristics of the increase in internal resistance of the battery at low temperature, develop the heat generation model of the battery, combine the thermal management of the whole vehicle, dynamically adjust the battery control objectives, achieve the balance between the minimum energy consumption and the driving experience, and improve the low-temperature performance.

Physical barrier of full cooling path:

The heat flow analysis of all heat dissipation paths of the battery pack is carried out, and the source of heat loss in extremely cold weather is obtained according to the big data analysis. The low thermal conductivity materials and innovative structural design are used to design the heat barrier at the key junction points of all paths, effectively improving the temperature of the battery when parking, and preventing the energy loss caused by the low temperature of the battery.

Radiative active thermal compensation:

In the extremely low temperature environment for a long time, the radiation heating system is activated actively, taking into account the energy consumption and temperature uniformity of the battery, to ensure that the battery quickly reaches the optimal working temperature. In a 12-hour extremely cold environment, the minimum temperature of the battery is increased by 40%, and the temperature uniformity is improved by 60%.

Double-system SoC estimation method

Considering that the accuracy of lithium iron phosphate and lithium iron phosphate is different in different sections, Weilai uses lithium iron phosphate as a scale to calibrate the SoC of lithium iron phosphate in the platform section in real time; In the high and low stages, the advantage of lithium iron phosphate is used to calibrate the SoC of lithium ternary. To this end, the dual-system SoC algorithm is developed, taking full advantage of the advantages of the dual-system lithium iron ternary.

At the same time, based on the characteristics of lithium ternary and lithium iron phosphate, such as the difference of self-discharge, Weilai first developed a high and low voltage DCDC conversion system in a high-power battery pack to achieve fast, real-time and balanced SoC calibration. At the same time, it also has many characteristics such as reducing static power consumption and extending battery life.

Through the organic combination of algorithm, hardware and dual-system cell, the accurate estimation of SoC is achieved, and the calculation error of traditional Fe-Li SoC is reduced from 10% to 3%.

New generation CTP technology

It is worth mentioning that the new generation of CTP technology has been applied to the Ulai ternary lithium iron standard battery pack (75kWh), that is, the battery pack is directly composed of a single battery cell, which simplifies the manufacturing and assembly by 10%; At the same time, the volume utilization rate of the battery pack is increased by 5%, and the energy density is increased by 14%, reaching 142Wh/kg.

Battery decay period

In the first year of service life, the battery attenuation of LiFe3 standard battery pack (75kWh) will be slightly higher than that of the original LiFe3 battery pack (70kWh); From the perspective of medium and long term use, the attenuation degree of the two is the same, and they are better than the traditional lithium iron phosphate battery as a whole.

The new battery pack has been opened for reservation, and the power exchange rights remain unchanged

With the official announcement of the Li-Ion standard battery pack (75kWh), the models carrying the battery pack have been online for booking at 14:00 on the same day. The new car buyers of Weilai can choose the Li-Ion standard battery pack (75kWh) or Li-Ion standard battery pack (100kWh). Users who choose the standard lithium iron battery pack (75kWh) will start delivering vehicles in November this year.

It should be pointed out that the standard battery pack (75kWh) model of lithium iron ternary battery is the same as the original 70kWh battery pack model of lithium iron ternary battery in terms of its price, BaaS price, power exchange rights, and upgrade rights. That is to say, in the Weilai current exchange system, the 75kWh battery pack and the 70kWh battery pack will be regarded as the same level battery pack, and both will be displayed as "standard battery pack (70/75kWh)" in the power exchange system, without distinction. This means that users who choose the original lithium-iron battery pack of 70kWh will also have the opportunity to use the lithium-iron battery pack (75kWh) through the power exchange system.

Earthly summary:

In order to solve the problem of poor low temperature performance and inaccurate SoC estimation, Weilai did not directly adopt the technical route of lithium iron phosphate, but created a new way to mix lithium ternary and lithium iron phosphate batteries. It can be said that this technical route not only improves the user's experience of using new energy vehicles, but also increases the cost of batteries in the current situation of rare metal shortage, Such a set of battery packs mainly consisting of lithium iron phosphate cells also reduced the cost of the company in terms of cell procurement to a certain extent. It is conceivable that once this technology is popularized, lithium iron phosphate battery will usher in greater development space Copyright notice: This article is an original article of Geshi Automobile. If you want to reprint it, please comply with the relevant provisions of the reprint instructions. Those who violate the reprint instructions will be investigated for legal responsibility by Geshi Automobile according to law!

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