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1. Lithium ion battery route vs. nickel hydrogen battery route

Just as pure electric vehicles cannot avoid Tesla, when it comes to hybrid power, it is also necessary to compare it with the Toyota Prius.

The hybrid system configuration of Toyota Prius is perfect, and it has been in use for nearly 20 years without major changes. However, in terms of battery type selection, it is a different scene. Toyota has been mainly using nickel hydrogen batteries until the fourth generation Prius provided two versions: lithium forklift batteries and nickel hydrogen batteries. The lithium-ion battery pack only has 56 cells, while the nickel hydrogen battery pack has 168 cells. Therefore, the cost and price of the two are not far apart, and the output voltage is also in the middle, but the weight of the lithium-ion battery pack is 16 kilograms lighter than the nickel hydrogen battery pack. Compared with lithium forklift batteries, nickel hydrogen batteries have inherent disadvantages, and it can be foreseen that Toyota will have to switch to lithium forklift batteries in the near future.

As a comparison, from the most primitive BAS system (belt assisted micro hybrid) to the plug-in Volanda and SPARK electric vehicles, all firmly adopt the lithium-ion battery route. I recently read an interesting article titled "Tesla's electric vehicle technology comes from TA", which mentioned that General Motors is an advocate for the lithium-ion battery route, and the person in charge of the General Motors EV1 project also has some connection with Tesla Roadster. The history of General Motors' involvement in electric vehicles may be traced back to 1990 (or even earlier), when General Motors launched an electric vehicle called Impact and provided it for users to try out.

Although the project later ran aground and all the trial vehicles were recycled, a large amount of experience and technology related to electric vehicles were accumulated through the project, which were used in subsequent VOLT, BEV, and BOLT. Starting from VOLT, lithium-ion battery technology will be widely used in the future. Even the previous generation of LaCrosse eAssist, which only had a 0.5kWh battery, used lithium forklift batteries. It should be noted that other micro hybrid cars in the same generation are also using lead-acid. Therefore, General Motors has taken the lead in the field of lithium-ion battery applications and is the first manufacturer to use lithium forklift batteries in everything from hybrid to pure electric.

It can be said that General Motors was the first person in the field of lithium-ion battery vehicles to eat crab meat, which is also why General Motors was singled out as a typical lithium-ion battery faction.

a) Energy density:

The important use of batteries is to store energy, so energy density is the most important parameter of batteries. In this regard, lithium forklift batteries have significant advantages over nickel hydrogen batteries. Taking the third-generation Prius nickel hydrogen battery launched in 2012 as an example, with a capacity of 1.3 kWh, it weighs 53.3 kg and an energy density of only 24.4 Wh/kg; The VOLT launched by General Motors of the same era uses lithium forklift batteries, with a battery pack capacity of 16kWh, a weight of 181.4kg, and an energy density of 88.2Wh/kg, which is about four times the difference in energy density between the two. The latest product from General Motors, the BOLT, uses LG's layered battery pack of 60kWh and 435kg, with an energy density of 138Wh/kg. The recently launched LaCrosse hybrid version uses the second-generation Voltech and a 1.5kWh battery pack, which can power two motors of 60kW and 54kW, with a noticeable power density. From this perspective, Toyota is still in the stage of moderate hybrid power, with the goal of reducing fuel consumption in a short period of time, and its future strategy is not clear; And General Motors is heading towards deep hybrid power, even laying the technical foundation for pure electric. The technical route of hybrid plug-in pure electric is already very clear.

b) Battery capacity:

In terms of battery capacity, the Prius can only provide less than 10 kilometers of pure electric driving, while the General Volt can provide up to 60 kilometers of pure electric driving range. Therefore, Toyota's approach to batteries is to assist systems, while General Motors places the battery in an equally important position as the engine. That is to say, Toyota's hybrid power still relies mainly on engines, such as the development of Atkinson cycle engines, while General Motors' hybrid power places batteries in a more important position.

c) Battery management technology level

From a technical perspective, lithium-ion battery systems are more complex and technically challenging than nickel hydrogen battery systems. Although lithium forklift batteries have high energy density and long service life, they are more sensitive to temperature and require the design of complex thermal management systems. For example, General Electric's second-generation VOLT has designed a multi-mode thermal management system that can heat batteries with waste heat or cool them with air conditioning. The design is very energy-efficient and ingenious, effectively increasing battery life and performance. However, nickel hydrogen batteries have poor temperature sensitivity, and a simple thermal management system can be designed. On the other hand, the universal lithium-ion battery pack is relatively large (such as the capacity of the VOLT battery being more than ten times that of the Prius), and more advanced balancing technology, charge and discharge control technology, etc. are used to control the temperature difference inside the Volt battery pack within 2C, effectively supporting an 8-year battery pack life guarantee period.

The universal Cadillac CT6 battery management technology is derived from the technical foundation of the second-generation Volt battery management, which fully demonstrates the highest level of battery integration and management: composed of 3 segments of 96S2P batteries (96 cell pairs, each containing 2 pairs of cells), protected by high-pressure die-casting aluminum plates on the outside, and also includes necessary high and low voltage wiring harnesses and thermal Plumbing pipes on the inside, And water-cooled fins were added between the battery cells, with a modular design that allows for flexible configuration of the battery pack's shape and capacity.

A battery pack composed of multiple cells

As an engineer, in my opinion, Tesla's battery management level is not low, but in the eyes of experienced drivers like General Motors who show off their skills, it is still somewhat immature. Not only Tesla, but Toyota also invests less in battery management technology. The difference in battery management level between lithium-ion battery and nickel hydrogen battery is mainly due to Toyota's belief that batteries are in an auxiliary position, and the naturally invested research and development costs are relatively low, with a focus on cost-effectiveness; General Motors, on the other hand, believes that batteries are half of the power system and has conducted in-depth research on batteries and their management technologies.

I don't know why, but seeing General Motors' dual planetary gear system and battery management system is dazzling and awe inspiring, which always reminds me of the other two companies that engineers aspire to: Motorola and Sony. Anyway, this technical level is impressive.

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

Although the topic is about the debate between hybrid schools, I can't help but compare it with Tesla's batteries. Whether hybrid or pure electric, battery technology is generally similar, so it should not be considered off topic.

In the past two or three years, people have often asked how powerful Tesla's battery management algorithm is, which can make electric cars run 400 kilometers, while ordinary electric cars can't run one or two hundred kilometers. So, how powerful is their battery management algorithm?

Firstly, it must be acknowledged that using a battery pack consisting of thousands of batteries is indeed impressive, but Tesla's ability to run far really has little to do with battery management algorithms. The important thing is that the batteries are packed well&hellip& Hellip; And such an answer generally cannot convince the questioner, which must be said to be the success of Tesla's marketing, making the general public believe that algorithms can generate electricity.

Compared to layered batteries, cylindrical batteries have significant disadvantages:

a) Volume utilization: When cylindrical batteries are grouped, they will inevitably occupy more space than layered batteries, thereby affecting the overall interior space of the vehicle.

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

c) Consistency and reliability: Tesla ModelS has a total of over 7000 units, while only 288 units were used on the VOLT. The addition of failures and consistency issues caused by too many units may gradually become apparent over time

In fact, Tesla's use of 18650 batteries was more due to price and business considerations than technical considerations. At that time, Panasonic was eager to find a partner, so it quoted a price lower than cost. And GM's global supplier system is relatively complete and has more say, so there is greater space to choose suppliers with more advanced technology.

3. The core issue of the battery route dispute: battery safety

Here we need to specifically mention the core issue of the battery route dispute: battery safety. Despite the booming development of electric vehicles, frequent fire and explosion accidents may lead to the downfall of this industry; Don't be fooled by Tesla's soaring success, if a concentrated safety issue triggers a concentrated recall and compensation, the company will find it difficult to sustain itself.

In short, safety issues are the black swan of car companies. Neglecting security issues may seem like breaking free from shackles and developing faster, but it also faces the risk of a sudden halt to the development path (or the risk of not breaking out and gaining an advantage, which is called opportunistic approach). And in this regard, Tesla has to be said to be a bit aggressive:

Figure 1 shows Tesla's BMS, and it can be seen that neither of the left and right connectors is automotive grade. The middle two connectors are more like debugging interfaces rather than products. These interfaces are often used in IT, but whether they can adapt to the harsh environment of vehicles remains to be verified.

In terms of chips used, Tesla's IT genes have prompted them to use some IT industry chips, such as DSP, ARM, FPGA, etc. General Motors, on the other hand, completely uses traditional automotive grade chips, and microcontrollers are generally Freescale. The hardware design meets the ASILC level of ISO26262.

In terms of high voltage safety, the common method is to use MSD (Manual Service Disconnect), which is the yellow connector shown in the figure below. This yellow connector can physically disconnect the battery pack from the external high voltage, ensuring the safety of maintenance personnel. All signals related to high-voltage safety are processed with dual backup, and even dual CAN bus redundancy backup is designed specifically for the battery management system to ensure high-voltage safety without any loss.

Tesla, in order to save costs, omitted this component and relied on high-voltage relays for disconnection. This method poses a safety risk as it cannot be disconnected when the high-voltage relay experiences adhesion failure. However, considering Tesla's production capacity is only a few thousand units per month, and the production time is not long, the probability of problems occurring is also low. If the quantity is increased, there may be problems.

Of course, if Musk can launch the rocket into the sky and retrieve it, his understanding of safety and reliability will not be inferior. The safety and reliability issues of Tesla mentioned above may only be one of its development strategies. Since it can only sell for a few thousand units now, design safety based on a few thousand units; When I can sell hundreds of thousands of vehicles, I naturally design safety according to the quantity of hundreds of thousands. If that's the case, it has to be said that Tesla is still much more flexible than traditional car companies; If that's not the case, it's very likely that there will be a problem after measuring.

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