Why does Tesla insist on using lithium cobalt oxide batteries?

As one of the core components of electric vehicles, Tesla's batteries have been ridiculed and criticized. Even after the explosive growth of sales, there are still many industry experts who call it "battery technology is old" and "no core." Competitiveness". The reason is that Tesla is the only company that uses the 18650 lithium cobalt oxide battery. This battery has been used in laptops, and it is difficult to get into the "elegant" of electric vehicles, and there are security risks. Is this really the case?

According to reports, as a backing of power output, the role of batteries on electric vehicles is self-evident. Because of its stable performance, high safety factor and high cycle chargeability, lithium iron phosphate battery is the first choice for power batteries on the market, such as Chevrolet Volt, Nissan Leaf, BYD E6 and Fisker Karma.

Using a lithium cobalt oxide battery in a car, Tesla counts as the first

Tesla is somewhat different, and his Roadster and ModelS use a 18650 lithium cobalt oxide battery. Compared with the lithium iron phosphate battery, the battery has mature technology, high power, high energy density and high consistency, but the safety factor is low, the thermal and electrical characteristics are poor, and the cost is relatively high.

According to industry insiders, the external voltage of the 18650 battery will be overheated if it is lower than 2.7V or higher than 3.3V. If the battery pack is large and the temperature gradient within the group is not well controlled, there is a high risk of fire. The key to battery technology is precisely voltage and current and heat control. No wonder Tesla has been criticized for not being reliable with 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. According to the latest research report of Haitong International Securities Co., the energy density (170 wh/kg) of Tesla battery is about twice that of BYD electric vehicle lithium iron phosphate battery.

Why does Tesla insist on using lithium cobalt oxide batteries?

The 18650 battery used by Tesla was

in the 1970s. Ms. Whittingham of Binghamton University in the UK invented the 18650 battery. This battery is often used in digital products such as notebook computers and glare flashlights. A cylindrical lithium battery with a diameter of 18mm and a height of 65mm is used in cars. Tesla is the first person to eat crabs.

Tesla's battery technology director Kurt Kelty once said: Tesla also tried more than 300 kinds of batteries on the market, including plate and square batteries, and finally chose Panasonic's 18650 battery. On the one hand, the 18650 has a higher energy density. And stability and consistency are better. On the other hand, the 18650 can effectively reduce the cost of the battery system. Moreover, although the size of each battery unit is small, the energy of each unit can be controlled to a small range, and the fault zone can be reduced even if a unit of the battery pack fails due to the use of a large-sized battery unit. In addition, billions of 18650 battery cells are produced every year around the world, and the level of security is constantly improving.

Panasonic official website shows that the lithium battery model NCR18650 is a high-energy model (High Capacity) 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 CTO JBStraubel: since the transition from Roadster to ModelS for four years, the battery component has dropped by about 44%, and it will continue to decline. Matsushita invested $30 million in Tesla in 2010 and became one of its shareholders. And in 2011, a strategic agreement will be reached, which will be responsible for Tesla's battery supply for all vehicles in the next five years. According to Tesla's current forecast of 20,000 units per year, the Panasonic 18650 will be assembled on more than 80,000 Models.

The magical reorganization of the 6831 lithium battery

18650 has a security risk. It is an indisputable fact. How does Tesla "get it"? The secret weapon lies in its battery management system. The solution is to combine the Panasonic 18650 packaged batteries of 6831 ampere-hours around 20 and 8 hours in series and parallel.

Why does Tesla insist on using lithium cobalt oxide batteries?

Tesla Roadster electric sports car power system

To drive an electric car, a large number of 18650 batteries are required. The TeslaRoadster battery system contains 6831 small batteries, while the TeslaModelS is up to 8000 knots. It is especially important to arrange and assemble these large numbers of small batteries.

At this time, Marc Tarpenning, one of Tesla's founders, came in handy. He was an expert in this field and successfully sold a company. He moved the network control field to a model of hundreds of thousands of servers and moved to the Tesla battery system control field. Through a year of DOE (design of experiment, experimental design), he successfully controlled this 6831 section with hierarchical management. Small battery and voltage and temperature:

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

99 battery bricks are connected in series to form one battery;

11 battery sheets form a battery pack, totaling 6831 knots.

Only these levels are not enough. For each level, we must monitor them. So they have fuses on each battery unit, each battery brick, and each battery. Once the battery is overheated or the current is too high, it will be immediately melt the fuse, and disconnect the output.

Only a fuse is not enough, so:

BMB (Battery Monitor board) is a battery monitoring board on each cell to monitor the voltage, temperature and output voltage of each cell.

On the entire battery pack, a BSM (Battery System Monitor) is provided to monitor the working environment of the entire battery pack, including battery pack current, voltage, temperature, humidity, bearing, smoke, etc.

At the vehicle level, a VSM (Vehicle System Monitor) is provided to monitor the BSM.

Such a battery control system became the core technology of Tesla. When Tesla just announced the costly system (rumors are higher than 20,000 US dollars), many people in the industry unanimously sang it, thinking that 7000 batteries will be placed. The behavior in the car is ridiculous. But the facts gave them a strong response. The Chevrolet Volt caught fire and the FiskerKarma model had three spontaneous combustion events in a year. In contrast, neither Roadster nor TeslaModelS of Telsa, had never had a spontaneous combustion event.

Why does Tesla insist on using lithium cobalt oxide batteries?

TeslaModelS panel and frame design

Tesla has cooperated with Toyota, Daimler and other manufacturers in the field of electric vehicles, including battery power for smart, Mercedes-Benz A-class, Mercedes-Benz B-class and other electric models. The Toyota RAV4 electric vehicle provides battery packs and motors. Currently Daimler holds a 4.3% stake in Tesla and Toyota also holds a 2.9% stake in Tesla. However, due to the short period of time and the small number of products that Tesla has officially put into the market, the theoretically perfect Tesla battery system needs to continue to face practical tests.