Why does Tesla insist on using lithium cobalt acid batteries?

Tesla's (TSLA) batteries, one of the core components of electric vehicles, have been derided and criticized, and even after an explosion in sales, many industry experts have called them "old battery technology" and "non-core competitiveness." The reason is that Tesla is the only company that uses the 18650 cobalt acid lithium-ion battery, which has been used in notebook computers and is difficult to access to electric cars, and there are safety risks. Is that really the case?

According to reports, as the backing of the power output, the role of batteries on electric vehicles is self-evident. Because of its relatively stable performance, high safety factor and a high number of recyclable charges, lithium iron phosphate batteries are currently the first choice for power batteries on the market, such as Chevrolet Volt, Nissan Leaf, BYD E6 and FiskerKarma.

Using lithium cobalt oxide batteries in cars, Tesla counts first

Tesla is a bit of an alternative. His Roadster and ModelS use 18650 cobalt acid lithium batteries. Compared with lithium iron phosphate batteries, this type of battery has relatively mature technology, high power, high energy density, and high consistency, but low safety factor, poor thermal and electrical characteristics, and relatively high cost.

According to industry insiders, the 18650 battery external voltage will show symptoms of overheating if it is lower than 2.7 V or higher than 3.3 V. If the battery is large and the temperature gradient in the group is not controlled well, there will be a great risk of fire. The key to battery technology is voltage current and heat control. No wonder Tesla has been criticized as unreliable battery technology.

But in fact, lithium iron phosphate batteries that are considered safer and more reliable are not foolproof. In the preparation of the sintering process, iron oxide is likely to be reduced to elemental iron in a high-temperature environment, and elemental iron causes a micro short circuit of the battery, which is a very taboo substance in the battery. In addition, the lithium iron phosphate battery has a large difference in charge and discharge curves in actual production, poor consistency and low energy density, which directly affects the sensitive endurance of electric vehicles. The latest research report of Haitong International Securities Co., Ltd. shows that the energy density of the Tesla battery (170 Wh / kg) is about twice the energy density of the BYD electric vehicle lithium iron phosphate battery.

Why does Tesla insist on using lithium cobalt acid batteries?

Tesla's 18650 battery

Back in the 1970s, Ms. Whittingham of the University of Binghamton in the United Kingdom invented the 18650 battery, which is commonly used in digital products such as notebook computers and bright lights but uses this 18mm diameter, 65mm high cylindrical lithium battery on cars. Tesla was the first person to eat crabs.

Tesla's battery technical director Kurt Kelty once said: Tesla initially tried more than 300 types of batteries on the market, including board and square batteries, and eventually chose Panasonic's 18650 battery. On the one hand, the 18650 energy density is greater. And stability and consistency are better. On the other hand, 18650 can effectively reduce the cost of battery systems. Moreover, although the size of each cell unit is small, the energy of each cell can be controlled in a smaller range. Compared with the use of large cell units, even if one unit of the battery fails, the effect of the failure can be reduced. In addition, the world produces billions of 18650 battery units each year, and the safety level is constantly improving.

Panasonic official website shows that the lithium battery model NCR18650 is a high-energy model (HighCapacity) battery, the nominal volts of the battery is 3.6V, the nominal minimum capacity is 2750 mAh, and the weight is 45.5 grams. Also, the 18650 used on Tesla's second-generation ModelS is 30% more energy efficient than the previous Roadster.

According to Tesla's chief technology officer, JBStraubel, the cost of batteries has dropped by about 44 % during the four-year transition from Roadster to ModelS and will continue to decline. Panasonic invested 30 million U.S. dollars in Tesla in 2010 and became one of its shareholders. And reached a strategic agreement in 2011 that will be responsible for the supply of batteries for all Tesla vehicles in the next five years. According to Tesla's current estimated production of 20,000 vehicles per year, Matsushita 18650 will be assembled on more than 80,000 Models.

Section 6831 Magical reorganization of lithium batteries

It is an indisputable fact that there is a security risk in 18650. How can Tesla "fix" this hard injury? The secret weapon is its battery management system. The solution given is to combine the Panasonic 18650 encapsulated batteries of 6831 knots or so in series and parallel.

Why does Tesla insist on using lithium cobalt acid batteries?

Tesla Roadster Electric Sports Car Power System

To drive an electric car, it takes a large number of 18650 batteries. Tesla Roadster's battery system contains 6,831 small batteries, and TeslaModels is up to 8,000 knots. It is particularly important to arrange and assemble these large numbers of small batteries.

At this time, the past experience of Tesla's founder, Marc Tarpenning, came into use. He was an expert in the field and successfully sold a company. He moved the network control field to program the control of millions of servers to the Tesla battery system control field. Through a year of DOE (design, experimental design), he successfully controlled the 6831 small batteries with hierarchical management. Voltage and temperature:

69 18650 batteries were packaged in parallel into one battery brick;

99 battery bricks in series into one battery chip;

Eleven battery chips form a battery pack, totaling 6831 knots.

These levels alone are not enough. Each layer is monitored, so they have fuses at each cell, each battery brick, and each battery piece at both ends. Once the battery is too hot or the current is too large, it immediately melts. Disconnect output.

Just having a fuse isn't enough, so:

On each battery piece, a BMB (Battery Monitor), the battery monitor, is installed to monitor the voltage, temperature, and output voltage of each battery piece.

On the entire battery pack, a BSM (Battery System Monitor) is set to monitor the working environment of the entire battery pack. Including battery pack current, voltage, temperature, humidity, orientation, smoke and so on.

At the vehicle level, there is a VSM (vehicle system Monitor) to monitor the BSM.

Such a battery control system became the core of Tesla's technology. When Tesla just announced the expensive system (rumored to be more than 20,000 US dollars), many industry insiders agreed with it that it was ridiculous to put 7,000 batteries in cars. However, the facts gave them a strong response. The Chevrolet Volt fire and the FiskerKarma model had three spontaneous combustion events in a year. In contrast, Tesla, whether it was Roadster or TeslaModels, had never had a fire spontaneous combustion incident.

Why does Tesla insist on using lithium cobalt acid batteries?

Design of TeslaModelS Battery Plate and Frame Integration

Tesla has teamed up with companies such as Toyota and Daimler in the field of electric vehicles, including providing battery power for electric models such as smart, Benz Class A, and Benz Class B, and providing batteries and motors for Toyota RAV4 electric vehicles. Daimler currently holds a 4.3 % stake in Tesla and Toyota also holds a 2.9 % stake in Tesla. However, due to the fact that Tesla has not been officially put into the market for a long time and the number of products is not large, the perfect Tesla battery system, in theory, needs to continue to face practical tests.

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