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An in-depth analysis of Tesla's battery management system

An in-depth analysis of Tesla's battery management system


Tesla's battery system

Battery system is the power source of electric vehicles and the most core system component in the whole industrial chain. In the case of the Tesla Models, the battery system (lithium-ion battery + battery management system) accounts for 56% of the cost, while the engine of a traditional sedan accounts for about 15% to 25%. By 2016, the cost of battery systems had decreased and the cost structure had also changed, with individual batteries accounting for 83% of the cost, battery management systems accounting for about 13%, and battery cooling systems accounting for the remaining 4%.

Through a detailed combing of the composition of Tesla's battery system and Tesla's supporting charging facilities, we can have an intuitive and in-depth understanding of Tesla's battery industry chain, and other new energy vehicles can also be used for analogy. At present, the cost of battery system is one of the most important factors restricting the development of Tesla and other new energy vehicles. Understanding battery system is equivalent to having the key to understand the new energy vehicle industry.

For electric vehicles to be practical, it is necessary to consider its range after a charge and the convenience of charging. To understand these two points, it is necessary to pay attention to the structure of the battery and the charging speed and equipment distribution of the charging equipment.

Models have been available in 40, 60, 70, 75, 85, 90, and 100kWh with battery power. For ModelS up to 85kWh, there are performance versions that offer even better power performance, such as the Perf and Ludicrous ModelS. The maximum distance and horsepower that can be driven on a full charge vary from model to model. As technology advances and in order to better meet people's needs, Tesla has gradually eliminated some ModelS, with 75, 90, and 100kWh still available to order today.

ModelX has launched 60, 70, 75, 90, 100kWh models, and now only 70 and 100kWh models can be ordered.

According to Zhongguancun Online, the battery pack for the basic version of Tesla's Model3 electric car is about 60kWh. According to Musk's Twitter and related information, the highest version is only 75kWh. This is due to the fact that the Model3 has a smaller wheelbase than the Models and ModelX, and is also positioned as an entry-level model. According to a tweet from ElonMusk on July 9, the first Model3 rolled off the production line that day, and Tesla will hold a delivery ceremony for the first 30 customers who ordered the Model3 on July 28.

In addition, Model3 uses batteries differently than Models and ModelX. At a press briefing hosted by Tesla Motors Japan on July 15, 2015, KurtKelty, Chief Battery Technology Officer at the U.S. headquarters, announced that the Model3 will use a new 21700 lithium-ion battery, which will have a 30% higher energy density than the 18650 lithium-ion battery used in the Models and ModelX.

3. Structure of battery and panel

Unlike other electric cars, Tesla doesn't use a single, large battery. Instead, it uses thousands of small, cylindrical cells assembled together.

Models and ModelX currently use Panasonic's 18650NCA special cell, a cylindrical cell with a cross-sectional diameter of 18 mm and a height of 65 mm. The standard 18650 lithium-ion battery, which is widely used in laptop batteries, has the chemical formula LinicoAlO2.

The advantage of using a single battery is that the explosion power of a single 18650 battery is limited. Even if a battery unit in parallel fails, the range of a battery unit will shorten the driving distance supplied by a battery unit. And the technology is mature, suitable for mass production. At the same time, the battery consistency is good and the cost is low. Thanks to the Tesla's excellent thermal management system, Panasonic's specially designed Tesla 18650 battery is lighter and cheaper than the conventional model by removing some of the unnecessary safety features. Tesla also fitted a fuse to each cell, rather than the usual safety throughout the battery pack.

Because the powertrain uses small, single batteries, Tesla's battery system is extremely complex and sophisticated. Taking the model model 85kWh as an example, the battery panel is divided into 16 battery groups. As shown in the figure below, each rectangular block is a battery group, and there are two groups stacked on the far right. The battery packs are connected in series with a total voltage of 402 volts. Each Tesla battery pack consists of six individual battery packs connected in series, each of which consists of 74 18,650 cells connected in parallel. In order to facilitate the placement of the heat dissipation pipe in the battery pack, the single battery pack is arranged irregularly.

As a result, the ModelS85kWh model uses up to 7,104 batteries, with a working voltage of 3.6V and a capacity of 3.2Ah, for a total power of about 82kWh, slightly less than the model's stated power. The battery pack of more than 7,000 cells weighs nearly 700 kilograms, accounting for nearly half the weight of the entire car. Similarly, the 100kWh model uses a total of 8,256 individual cells, also divided into 16 battery packs.

In order to protect the battery pack, Tesla designs a waterproof and breathable valve on the top surface of the front of the battery pack, which makes use of the volume difference between the gas molecules and the liquid and dust particles to allow the gas molecules to pass through, while the liquid and dust can not, so as to achieve the purpose of waterproof and breathable, and prevent the condensation of water vapor inside the battery pack.

The positioning of the Models panel makes it the floor between the axles, a design that brings many benefits. Because the panels are the largest portion of the vehicle by weight (the ModelS85kWh battery pack weighs 544 kg), the center of gravity height of the Model is only 18 inches, which gives a large lateral acceleration (0.9g) and good roll-resistance.

To protect the panels at the bottom, Tesla has added a protective layer made of aluminum alloy (or steel, fiberglass, carbon fiber, plastic, etc., as currently used in the ModelS) that wraps the battery modules and keeps a buffer distance between them. And call it BallisticShield.

4. Battery management system

Battery management system (BMS) is the installation of battery security monitoring and effective management, improve the efficiency of battery use. Important functions include data acquisition, battery status calculation, energy management, thermal management, safety management, equalization control and communication functions. Through the effective control of the battery pack charge and discharge and heat release, the system can achieve the purpose of increasing the mileage, prolonging the service life, reducing the operation cost, and ensuring the safety and reliability of the battery pack application.

Tesla's BMS can provide accurate battery health prediction technology, battery balance management technology, battery residual power management technology, battery thermal management technology, diagnosis and early warning technology. Even for electric vehicles with the same battery capacity, the range, charging time, start acceleration time and battery life will vary greatly due to the difference in BMS.

Through its powerful BMS system, Tesla can effectively achieve the consistent management of more than 7,000 18650 batteries, achieving high safety and reliability goals. In addition, Tesla has applied for more than 140 core patents in battery cooling, safety, charge balance and other BMS related fields, so BMS technology is one of Tesla's core competencies.

Each Tesla battery pack has its own separate battery management system, located on the side of the battery pack, which is shown in green above. Sensors and chips monitor the temperature of each battery at any time and can turn off the battery in milliseconds in case of an emergency. 

5. Battery cooling system

In addition to the battery pack inside the panel, the most is the coolant pipeline. Each Tesla has a special liquid circulation temperature management system around each individual battery.

The coolant is green in color and consists of a mixture of 50 percent water and 50 percent glycol. Coolant is constantly flowing in the pipe, and eventually will be distributed in the vehicle head heat exchanger, so as to maintain a balance of battery temperature, prevent the battery local temperature is too high resulting in battery performance decline. Tesla's battery thermal management system keeps the temperature between the battery packs at 2 ° C, which effectively extends the battery's life by keeping the panels at a controlled temperature.

6. Supporting charging facilities

Tesla's charging methods are divided into three major ways: home charging, destination charging and supercharging station.

Home charging is Tesla's most important way of charging. Tesla cars can be charged directly from a 220V10A/16A home socket. Domestic ordinary household electricity is 220V40A meter, but the socket is 10A or 16A. The charging current of 10A or 16A can be selected through the charging cable of the Tesla car belt and the corresponding plug. It can be charged without installing a single electricity meter or any equipment modification, but the charging speed is slow, and it can be charged for about 8km per hour.

As most of the residential areas in China are used in the form of 220V single-phase power supply, the highest current can reach 40A. Each Model and ModelX vehicle can be charged with 40A charging current after being installed in their own parking space with the complimentary home charging pile. This allows charging to take place much faster than the 16A charging current on most electric cars in China.

A Tesla HighPowerWallConnector is also available. Apply for 80A electricity meter separately or increase the capacity of the existing electricity meter, and realize fast charging by installing a special charging wall for Tesla and installing a second charger (optional when purchasing a car) in the car. It can charge up to 17.6kWh per hour, travel 80-100km, and take 5-6 hours to go from no charge to full charge.

Destination charging is a charging method set for the convenience of travel. Tesla is partnering to install purposeful charging stations at locations including some restaurants, restaurants, shopping centers and resorts, where owners can also charge when they arrive. The charging pile at the destination charging station is identical to the charging pile or charging wall installed in the home charging station. Charges are also determined by the company that installs them. Tesla currently has 654 charging stations in 30 provinces, including municipalities and autonomous regions, according to its website.

In addition to the above two charging methods, Tesla also has a DC fast charging mode Super Charge. Tesla claims its supercharger stations are the fastest in the world, typically charging in just a few tens of minutes. Charging stations are often located near restaurants, shopping centers, WiFi hotspots, etc., where they can be charged during parking breaks. This is the first choice for long car trips.

According to Tesla's official website, it has established 861 supercharging stations around the world, with a total of 5,655 supercharging stations. According to Tesla's official microblog in June, it has built 117 supercharging stations and 554 supercharging piles in China. Meanwhile, in 2017, the scale of Tesla's global charging network will double; The expansion of the charging network in China will also follow the global expansion.

Some Tesla Superchargers also use solar panels to supply part of the charge, as well as shade from the sun. Each charging pile costs between $100,000 and $175,000, with more than half of the money going into remodeling the foundation.

ElonMusk also revealed on Twitter that Tesla is planning an update to the Supercharger, which will likely put the output in excess of 350kW, which will significantly speed up charging and reduce charging time.

The charging gun used in the super-charging pile is also an extremely technology-intensive component. Sensors in the charging gun detect changes in the temperature of the battery inside the car as the Tesla is charged in the supercharging pile. Once the battery temperature is too high, the charging gun will immediately send a signal to reduce the charging intensity, so that the battery temperature is lowered; At the same time, the cooling system in the panel, also synchronously make the corresponding response, so that the cooling strength is strengthened synchronously. The process of automatically adjusting the charging intensity is that the stun gun, the battery cooling system, and the current of the charging pile work together in a highly coordinated manner.

In addition to Tesla, there are other brands of electric vehicles on the market, which also have their own charging piles. State Grid, China Southern Power Grid and other power companies have also built some public charging piles. Tesla officially announced the new national standard charging adapter at the Guangzhou Auto Show in November 2016. Since then, Tesla electric vehicles can be charged not only in Tesla's charging network, but also by using charging facilities that meet China's new national standards.

Plug one end of the adaptor into the non-Tesla charging pile plug in line with the new national standard, and plug one end into the Tesla car to charge. But at present, some charging piles must also deal with the corresponding charging card to charge, charging speed is also uneven.

7. Battery charging management technology

The speed with which Tesla's electric cars can be charged is thanks to its quick-charging technology. Through variable current and three-stage charging method, reasonable distribution of charging current, with the highest charging efficiency of fast charging. The technology used to charge mobile phones quickly today is similar.

For lithium-ion batteries, the smaller the chance of deep discharge, the longer the life of the battery. On the contrary, frequent deep discharge (the battery power used to 20% or less) will lead to reduced battery life. Tesla batteries in its unique process, in 50-0% cycle mode (using no more than 50% of the battery power), the total battery life is doubled. In the deep cycle mode, the capacity decays to 50% after 900 cycles. So from the perspective of protecting the battery life, you can try to control each trip within 80% of the maximum battery life, to prevent the power over 80% after charging, but can regularly deep discharge to activate the battery.

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