There is no absolutely safe battery in the world, there are only risks that are not fully identified and prevented. Make full use of the people-oriented product safety development concept. Although the preventive measures are insufficient, the safety risks can be controlled.

Take the model accident that occurred on the Seattle highway in 2013 as an example. There is a relatively independent space between each battery module in the battery pack, which is isolated by a fireproof structure. When the car at the bottom of the battery protection cover is pierced by a hard object (the impact force reaches 25 t and the thickness of the decomposed bottom panel is about 6.35 mm and the hole diameter is 76.2 mm), the battery module is thermally out of control and fires. At the same time, its three-level management system can Activate the safety mechanism in time to warn the driver to leave the vehicle as soon as possible, and ultimately protect the driver from injury. The details of the safety design used in Tesla’s electric vehicles are unclear. Therefore, we have checked the related patents of Tesla’s electric vehicle electric energy storage system, combined with the existing technical information, and conducted a preliminary understanding, hoping that others are wrong. We hope that we can learn from its mistakes and prevent the repetition of mistakes. At the same time, we can give full play to the spirit of copycats and achieve absorption and innovation.

TeslaRoadster battery pack

This sports car is Tesla’s first mass-produced pure electric sports car in 2008, with a global limited production of 2500. The battery pack carried by this model is located in the luggage compartment behind the seat (as shown in Figure 1). The entire battery pack weighs about 450kg, has a volume of about 300L, available energy of 53kWh, and a total voltage of 366V.

The TeslaRoadster series battery pack consists of 11 modules (as shown in Figure 2). Inside the module, 69 individual cells are connected in parallel to form a brick (or “cell brick”), followed by nine bricks connected in series to form a module A battery pack with a total of 6831 individual cells. The module is a replaceable unit. If one of the batteries is broken, it must be replaced.

The module containing the battery can be replaced; at the same time, the independent module can separate the single battery according to the module. At present, its single cell is an important choice for Japan’s Sanyo 18650 production.

In the words of Academician Chen Liquan of the Chinese Academy of Sciences, the debate on the choice of the single cell capacity of the electric vehicle energy storage system is a debate on the development path of electric vehicles. At present, due to the limitations of battery management technology and other factors, my country’s electric vehicle energy storage systems mostly use large-capacity prismatic batteries. However, similar to Tesla, there are few electric vehicle energy storage systems assembled from small-capacity single batteries, including Hangzhou Technology. Professor Li Gechen of Harbin University of Science and Technology put forward a new term “intrinsic safety”, which has been recognized by some experts in the battery industry. Two conditions are met: one is the lowest capacity battery, the energy limit is not enough to cause serious consequences, if it burns or explodes when used alone or in storage; second, in the battery module, if a battery with the lowest capacity burns or explodes, Will not cause other cell chains to burn or explode. Taking into account the current level of safety of lithium batteries, Hangzhou Technology also uses small-capacity cylindrical lithium batteries, and uses modular parallel and series methods to assemble battery packs (please refer to CN101369649). The battery connection device and assembly diagram are shown in Figure 3.

There is also a protrusion on the head of the battery pack (area P8 in FIG. 5, corresponding to the protrusion on the right side of FIG. 4). Install two battery modules for stacking and discharging operations. The battery pack has a total of 5,920 single cells.

The 8 areas (including the protrusions) in the battery pack are completely isolated from each other. First of all, the isolation plate increases the overall structural strength of the battery pack, making the entire battery pack structure stronger. Second, when a battery in one area catches fire, it can be effectively blocked to prevent batteries in other areas from catching fire. The inside of the gasket can be filled with materials with high melting point and low thermal conductivity (such as glass fiber) or water.

The battery module (as shown in Figure 6) is divided into 7 areas (m1-M7 areas in Figure 6) by the inside of the s-shaped separator. The s-shaped isolation plate provides cooling channels for the battery modules and is connected to the thermal management system of the battery pack.

Compared with the Roadster battery pack, although the model battery pack has obvious changes in appearance, the structural design of independent partitions to prevent the spread of thermal runaway continues.

Different from the Roadster battery pack, the single battery lies flat in the car, and the individual batteries of the Model Model battery pack are arranged vertically. Since the single battery is subjected to squeezing force during a collision, the axial force is more prone to thermal stress along the core winding than the radial force. Because the internal short circuit is out of control, theoretically, the sports car battery pack is more likely to be in a side collision than in other directions. Stress and thermal runaway are prone to occur. When the model battery pack is squeezed and collided at the bottom, thermal runaway is more likely to occur.

three-level battery management system

Unlike most manufacturers pursuing more advanced battery technology, Tesla chose a more mature 18650 lithium battery instead of a larger square battery with its three-level battery management system. With hierarchical management design, thousands of batteries can be managed at the same time. The framework of the battery management system is shown in Figure 7. Take Tesla’s oadster three-level battery management system as an example:

1) At the module level, set up a battery monitor (BatteryMonitorboard, BMB) to monitor the voltage of the single battery in each brick in the module (as the smallest management unit), the temperature of each brick, and the output voltage of the entire module.

2) Set up BatterySystemMonitor (BSM) at the battery pack level to monitor the operating status of the battery pack, including current, voltage, temperature, humidity, position, smoke, etc.

3) At the vehicle level, set up a VSM to monitor the BSM.

In addition, technologies such as overcurrent protection, overvoltage protection, and insulation resistance monitoring are embodied in US patents US20130179012, US20120105015, and US20130049971A1, respectively.