Battery pack consistency is the bottleneck for the popularization of electric cars

As the most important part of electric vehicles, the price, performance, life and safety of the power lithium battery pack directly determine the performance of the whole vehicle. For power lithium batteries, the consistency of the battery pack has always troubled vehicle manufacturers engaged in the research and development of electric vehicles at home and abroad. Controlling the raw materials, production process and factory inspection of power batteries can effectively improve the consistency of the battery pack. In addition, reasonable monitoring and use during driving, as well as the best combination with other drive control systems will also help improve the consistency of the battery pack.

Materials are key

Due to the subtle differences between the battery cells in the battery pack, the performance of the battery pack is definitely not the algebraic addition of the performance of a single battery. The differences in the capacity, internal resistance and other characteristics of each battery cell will cause the large capacity to always be in a state of shallow charge and discharge with a small current, while the small capacity will always be in a state of overcharge and over discharge with a large current. The performance parameter difference between the two is getting bigger and bigger, and the small capacity will fail prematurely; or the large internal resistance will become an energy-consuming series resistor, and it is easy to overheat and shorten the life. During the use of the battery pack, the temperature, voltage, current, etc. of each battery cell are different. After working for a period of time, the consistency of the battery pack will inevitably continue to deteriorate, and finally the energy efficiency of the battery will be rapidly reduced to a level that is unacceptable and cannot be continued to be used.

The type of battery cell has a great impact on the consistency of the battery pack. In terms of power batteries, lithium batteries will replace nickel-metal hydride batteries and become the mainstream of new energy vehicles in the future. After 2012, as lithium iron phosphate (LiFePO4) batteries have established their absolute advantages in technology, cost, and market, their demand growth rate will increase significantly. Compared with bulky nickel-metal hydride batteries with memory effect, lead-acid batteries with pollution in the production process and the product itself, and expensive, short-lived and unsafe cobalt-lithium batteries, lithium iron phosphate batteries not only have the advantages of safety, long life, light weight, small size, high single cell voltage and low self-discharge, but also are far better than other batteries in terms of battery cell consistency. Therefore, they will gradually become the main development direction of automotive power batteries in the future.

Since the conductivity of the lithium iron phosphate battery anode is not very good, it directly limits its energy density and other characteristics. Different companies have different solutions to make the integration and separation of lithium ions and anodes more efficient while minimizing the loss of the anode.

1) A123, a US company, has shown that reducing the size of the positive electrode crystal particles to a certain extent will have a great impact on battery performance. When the positive electrode crystal particles can be controlled below 100nm to more than ten nanometers, and the electrode is denatured by adding other metals, when lithium ions enter and exit the crystalline positive electrode material (charging and discharging), the changes in its crystal structure will be significantly smaller (less residue on the positive electrode material), and lithium ions will enter and exit the positive electrode crystal array more quickly. In this way, not only the battery discharge power is greatly improved, but the battery can be repeatedly charged and discharged without causing the battery unit to fail due to damage to the electrode material.

2) Phostech's method is to deposit a very thin layer of carbon atoms with a honeycomb structure (Carbon Coating) on ​​the anode, which serves as a nested window for lithium ions in the dielectric to enter and exit the anode, and also greatly improves the efficiency of the intercalation and separation of lithium ions and the anode.

Both A123 and Phostech claim that their products can be charged and discharged for more than 5,000 times, which can ensure the life and cost-effectiveness of the battery cells.

Quality control during production

First of all, strict monitoring and inspection of the source, purity, composition, ratio, storage conditions, and quality of the diaphragm of the masterbatch is the first checkpoint to ensure the consistency of the battery cell. At present, the raw material supply of positive and negative electrode materials and dielectrics of high-end electric vehicles is basically controlled by several large manufacturers in the United States and Japan with relevant patented technologies. Domestic BYD and Tianjin Slant also have the ability to mass produce raw materials.

Fluctuations in the manufacturing process of battery cells may be more difficult to control than battery materials. For example, in the battery coating process, the uniformity and thickness of the slurry coating need to be adjusted in real time by automated monitoring equipment during the production process to ensure uniform coating. Solid-state process, liquid (wetting) process, casting process, control of the purity, size and crystal morphology of nano-scale particles, selection of solvents, addition of reactants, batching and ensuring uniform batching, recycling of process products and waste (reducing costs), etc., quality control of each key link is very critical. Slight deviations in the process parameters and reaction conditions of any step will directly affect the consistency of the finished product, resulting in the failure of the qualified rate to reach the level of mass production, thereby affecting the commercial production of battery cells. Therefore, large-scale dedicated precision production lines are necessary to ensure process stability.

Only a few companies at home and abroad have the technical ability to produce complete sets of lithium iron phosphate battery equipment, such as A123 and Valence in the United States, Phostech in Canada, Toshiba, Hitachi and Sanyo in Japan, LG in South Korea, etc. There are about 20 other companies that can provide equipment required for some processes of lithium iron phosphate battery production lines. The price of a complete set of equipment is about 150 million to 330 million yuan, which is relatively easy for local new energy companies with government backgrounds to find financing channels. However, if the production system is not developed by itself, it is difficult to master the core technology of controlling process parameters. As a result, in terms of daily operation, maintenance, and system upgrades, it is completely controlled by others, and the maintenance cost will be as high as tens of millions of yuan per month. BYD's iron battery production line is designed and manufactured using its own technology, but the degree of automation, online control level and finished product yield have not been disclosed.

For battery cells that have already been produced, if effective quality control can be carried out on the battery cells before leaving the factory, including strict control of the battery cells allocated to the same battery pack from the same factory and the same batch, and related electrical function tests such as deep discharge, defective products can be detected as much as possible and the differences within the group can be reduced, which is also beneficial to improving the consistency of the battery pack.

Dynamic balance and system linkage

For battery packs that have been assembled using the series connection principle, the dynamic balancing principle can still be used to improve their consistency: for each single cell in the battery pack, its voltage, internal resistance and state of charge (SOC) and other parameters can be monitored at any time. If this information can be reflected to the battery management system in a timely and effective manner, the battery management system will control the charge and discharge state and flow rate of each battery according to its actual condition, as well as the corresponding cooling and heat dissipation adjustment to ensure that the battery unit works in the most suitable state, which can naturally protect the battery unit to the maximum extent, extend its life, and ensure the consistency of the entire battery pack during use.

Since the actual technical difficulties of battery pack consistency are difficult to resolve for the time being, vehicle manufacturers, battery pack and drive system suppliers are now more likely to adopt a "curve-saving" approach to minimize the trouble caused by this problem.

Undoubtedly, the ultimate goal of improving the consistency of battery packs is to improve the cost-effectiveness, life and safety of battery packs. Whether the battery pack can play its best performance depends not only on its own characteristics, but also on the coordination between the battery pack and other systems. If we can design the corresponding battery management system, starting system, engine system and charging system based on the actual characteristics of the battery pack to be used while studying to improve the efficiency of the battery pack, we can achieve the best matching effect. In this way, even if it is difficult to improve the consistency of the battery pack cells, other systems that are specially designed to cooperate with it can accurately detect and control the temperature, electrical performance parameters and life of the battery pack and each of its cells on the one hand, and on the other hand, they can adjust its temperature/humidity in time, reasonably arrange the charging and discharging of the battery cells, and prevent some battery cells from being overloaded and overcharged, while other battery cells are shallowly discharged and charged.

Such a comprehensive and systematic design of the battery drive system lowers the threshold for using battery modules, but places higher requirements on the integration capabilities of system equipment suppliers. To design an ideal and balanced system, equipment manufacturers need to conduct in-depth research in basic sciences such as chemistry, electricity, and materials science, and have mature technologies in related electronic and electrical components and barebones.

One feasible approach is that vehicle manufacturers should try to have a single capable equipment manufacturer provide individual "quasi-equipment" for the drive system, including the power battery pack, rather than multiple equipment manufacturers. Daimler and Evonik jointly built an automobile battery factory, and General Motors and South Korea's LG jointly developed electric vehicles, all of which valued each other's strong technical strength. Domestically, Chery has just cooperated with BetterPlace to develop electric vehicles with replaceable batteries, while BYD continues to insist on independent design and production. The results are still to be expected.

Formulation of national standards for electric vehicles

Since China's automotive technology comes from Japan, the United States, Germany, and France, and new energy technology comes from even more sources, there are still large differences in the understanding of the development direction of electric vehicles among various regions and departments in China. This has led to the inability to issue national recommended standards for electric vehicles, and there is no way to talk about the specifications, safety standards, and charging methods of electric vehicle power lithium batteries. For such an industry that needs strong government support and the joint development of industry companies and scientific research institutions, the unclear macro-purpose will naturally directly affect the overall level of the industry, as well as the progress of the research and development and listing of related products of each independent enterprise.

Japan is currently working with the Ministry of Economy, Trade and Industry, the Ministry of Transport, five major automakers and more than 10 related companies to jointly study and launch its own power battery standards. This is worth considering for relevant Chinese companies.