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Discussion on different technical routes of electric vehicle power lithium battery

Discussion on different technical routes of electric vehicle power lithium battery


1. Lithium-ion battery route vs. NiMH battery route

Just as a pure electric car can't avoid a Tesla, a hybrid car can also be compared to a Toyota Prius.

Toyota Prius hybrid system configuration is perfect, nearly 20 years without major changes, the use of today. However, it is a different story in the choice of battery types. Toyota has been mainly using nickel-metal hydride batteries until the 4th generation of Prius. Lithium ion battery has only 56 Cells, while nickel-metal hydride battery has 168 Cells, so the cost and price of the two are not far from each other. The output voltage is about the same, but the lithium-ion battery packs weigh 16 kilograms less than the nickel-metal hydride ones. Nickel-metal hydride batteries have an inherent disadvantage compared to lithium-ion batteries, and it is foreseeable that Toyota will have to switch to lithium-ion batteries in the near future.

GM, by contrast, has resolutely adopted the lithium-ion battery route, from the original BAS system (belt-assisted microhybrid) to the plug-in Volt and Spark electric cars. Recently I read an interesting article "Tesla's electric vehicle technology comes from TA". It said that GM is the advocate of lithium ion battery route, and the person in charge of GM's EV1 project also has a long history with Tesla Roadster. GM's involvement in electric vehicles probably goes back to 1990, if not earlier, when it introduced an electric car, the Impact, and offered it to consumers for testing.

Although the project was suspended and all the prototype vehicles were recalled, it yielded a wealth of EV experience and technology that would be used in the Volt, Bev, and Bolt. Follow-up with the Volt, large-scale use of lithium-ion battery technology. Even the previous generation of LaCrosse eAssist with a 0.5kWh battery was powered by lithium-ion batteries, considering that leading acid was used in other microhybrids of the era. As a result, GE has a head start in the field of lithium-ion batteries, being the first to use them in everything from hybrids to pure electric batteries.

It is fair to say that GM was the first in the field of lithium-ion batteries, which is why it is singled out as a typical example of lithium-ion batteries.

A) Energy density:

The important purpose of batteries is to store energy, so energy density is the most important parameter of batteries. In this respect, lithium ion batteries have greater advantages than nickel metal hydride batteries. The third-generation Prius nickel-metal hydride battery launched in 2012, for example, has a capacity of 1.3kWh, weighs 53.3kg, and has an energy density of 24.4Wh/kg. The Volt, launched by GM at the same time, uses a lithium-ion battery pack with a capacity of 16kWh, a weight of 181.4kg and an energy density of 88.2Wh/kg, a difference of nearly four times between the two. GM's latest product, the Bolt, uses an LG layered battery pack of 60kWh and 435kg, which has an energy density of 138Wh/kg. The recently launched LaCrosse hybrid version, which uses the second generation VolTech, uses a 1.5kWh battery pack that can power both 60kW and 54kW motors. You can see the power density. From this point of view, Toyota is still in the medium hybrid stage, aiming to reduce fuel consumption in a short time, and its strategy for the future is not clear. While GM is aiming at the direction of deep hybrid power, even laying the technical groundwork for pure electric power. The technical route of hybrid - plug-in - pure electric power has been very clear.

B) Battery capacity:

In terms of battery capacity, the Prius has just under 10 kilometers of all-electric range, while the Volt has as much as 60 kilometers of all-electric range. So while Toyota thought about the battery as an auxiliary system, GM put the battery as important as the engine. In other words, Toyota's hybrid is still engine-based, such as the Atkinson cycle engine, while GM's hybrid puts the battery in a more important position.

C) Battery management technology level

From the technical point of view, the lithium ion battery system is more complex than the nickel metal hydride battery system, and the technical difficulty is greater. Although lithium-ion batteries have high energy density and long service life, they are more sensitive to temperature and require complex thermal management systems. For example, GM's second generation Volt has designed a multi-mode thermal management system that uses waste heat to heat the battery or air conditioning to cool the battery. The design is very energy efficient and sophisticated, effectively increasing battery life and performance. The nickel metal hydride battery is less sensitive to temperature and a simple thermal management system can be designed. On the other hand, the general lithium ion battery pack is larger (for example, the VOLT battery capacity is more than ten times that of the prius), the use of more advanced equalization technology, charge and discharge control technology, so that the temperature difference in the VOLT battery pack can be controlled within 2C, effectively supporting the battery pack life guarantee period of 8 years.

GM's Cadillac CT6 battery management technology is based on the second generation Volt battery management technology foundation and demonstrates the highest level of battery integration and management: It is composed of 3 sections of 96S2P batteries (96 Cell cells, each Cell includes 2 Cell pairs). The external part is protected by high-pressure die-cast aluminum plate, and the internal part also contains necessary high and low pressure wiring harness and ThermalPlumbing. In addition, water-cooled fins are added to the cells. The modular design allows flexible configuration of the shape and capacity of the battery pack.

A battery pack consisting of multiple cells

As an engineer, in my opinion, Tesla's battery management level is not low, but it is still a little immature in the view of GM's old drivers showing off their skills. Not only Tesla, but Toyota has invested less in battery management technology. The difference in battery management level between lithium-ion battery and nickel metal hydride battery is mainly due to Toyota's view that batteries are in an auxiliary position, and the R&D cost of natural investment is low, and the input cost performance is the main one. General Motors, on the other hand, considers the battery to be half of the power system and has conducted in-depth research on the battery and its management technology.

I don't know why, but looking at General Motors' dual-planet toothed system and battery management system is so dazzling and amazing that it always reminds me of two other companies engineers aspire to: Motorola and Sony. Anyway, the skill level is very high.

2. Layered lithium-ion batteries vs. cylindrical lithium-ion batteries

Despite the title of the hybrid genre debate, I couldn't help but compare it to Tesla's battery. Whether hybrid or pure electric, battery technology is broadly the same, so it shouldn't be off topic.

In the past two or three years, people often ask that Tesla's battery management algorithm is so good that it can make electric cars run 400 kilometers, while ordinary electric cars can't run 100 or 200 kilometers. Then how is their battery management algorithm so good?

First of all, we have to admit that it's cool to pack thousands of batteries into a battery pack, but the Tesla's distance really has less to do with battery management algorithms and more to do with how many batteries are crammed in. … That response, which generally fails to convince questioners, is a marketing coup for Tesla, which has convinced the general public that algorithms can generate electricity.

The disadvantages of cylindrical batteries over layered batteries are very obvious:

A) Volume utilization: when cylindrical batteries are grouped, they will necessarily take up more space than layered batteries, thus affecting the overall space of the car.

B) Difficulty in thermal management: the contact area of the layered battery is larger, which is more conducive to heat dissipation and heat balance between the battery cells. The battery pack of BOLT can control the temperature difference between the cells within 2 degrees, which greatly improves the consistency and durability of the battery.

C) Consistency and reliability: There are more than 7,000 units in the Tesla ModelS, while only 288 are used on the Volt. Increased failures due to too many units and consistency issues may emerge over time

In fact, Tesla's use of the 18650 battery was driven more by price and business than technology, and Panasonic, desperate to find a partner at the time, offered a price below cost. However, GM's global supplier system is relatively complete and has more power of speech, so it has more space to choose more technologically advanced suppliers.

3. The issue at the heart of the battery route debate: battery safety

In particular, there is the issue at the heart of the battery route debate: battery safety. Despite the development of electric vehicles in full swing, after the volume of frequent fire and explosion accidents may cause the industry to collapse; Despite Tesla's heyday, it would be difficult for the company to survive if a series of safety problems resulted in recalls and compensation.

Safety, in short, is a black swan for car companies. Ignoring security may be like breaking off the shackles and developing faster, but it also risks coming to a screeching halt on the development path (or risks not breaking out, so as to gain opportunities, which is called opportunistic route). Tesla, on the other hand, is a little aggressive:

Figure 1 below shows the BMS of Tesla. It can be seen that the left and right connectors are not automobile level, and the middle two connectors are more like debugging interfaces rather than products. Such interfaces are often used in IT, but whether they can adapt to the harsh environment of vehicles remains to be verified.

In terms of the chips used, Tesla's IT gene prompts them to use some chips in the IT industry, such as DSP, ARM, FPGA, etc. While GM completely uses the traditional automotive chip, the microcontroller is generally Freescale, the hardware design to meet the ASILC level of ISO26262.

For high voltage safety, a common approach is to use MSD (ManualServiceDisconnect), the yellow connector shown below, which physically disconnects the battery pack from the external high voltage to ensure the safety of maintenance personnel. All the signals related to the high voltage safety are processed by dual-channel backup, and even the redundant backup of dual-channel CAN bus is specially designed for the battery management system to ensure the high voltage safety without loss.

Tesla, on the other hand, has cut costs by eliminating the part and relying instead on high-voltage relays to disconnect. This method, when the high-voltage relay adhesion fault, can not be disconnected, there are safety risks. But given that Tesla only makes a few thousand cars a month, the time to mass produce them isn't long enough for problems to occur. And if the quantity comes up, it may encounter a problem.

Of course, Musk can send a rocket up and back again, and his understanding of safety and reliability is not bad either. The above safety and reliability problems of Tesla may just be a development strategy of Tesla. Since it can only sell a few thousand cars at present, we should design safety according to the quantity of several thousand cars. When I can sell hundreds of thousands of cars, it's natural to design safety for hundreds of thousands of cars. If that's the case, it has to be said that Tesla is still much more nimble than traditional car companies; If this is not the case, it is likely to be measured after the problem.

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