Increasing the energy density of lithium 18650 li ion battery has always been the main direction of the development of electric vehicle technology.

As one of the core components of pure electric vehicles, power lithium 18650 li ion battery are closely related to the vehicle's cruising range, curb weight, power performance, and handling performance. In terms of the manufacturing cost of pure electric vehicles, lithium 18650 li ion battery also account for the highest proportion, generally more than 30%, which leads to higher prices and later maintenance costs for electric vehicles. Therefore, reducing the unit cost of lithium 18650 li ion battery and increasing the energy density of lithium 18650 li ion battery have always been the main direction of the development of electric vehicle technology.

For BYD, which originally started with lithium 18650 li ion battery, high-performance lithium 18650 li ion battery are one of BYD's killer features. Especially after replacing the ternary lithium battery with higher energy density, higher discharge voltage, and better low-temperature performance, the core competitiveness of BYD's EV model series has been greatly improved.

In this issue, we will fully disassemble the battery pack of BYD Qin ProEV500 and analyze BYD's innovations and management technologies such as battery pack safety design and thermal management design.

Square aluminum shell integration process

After uncovering the ultra-thin non-metallic cover of the battery pack and the silica aerogel fireproof and heat-insulating layer, we can clearly see the overall layout structure of the battery pack, of which the most intuitive is the integration process of the battery pack. The integration process is very important in the research and development of power lithium 18650 li ion battery. It must meet the comprehensive safety requirements of mechanical protection, thermal safety protection, thermal management, environmental protection, etc., and pursue lightweight and cost optimization

Different from the cylindrical battery cell method used by Tesla, BYD uses a square aluminum shell with a higher domestic popularity rate, which has the advantages of high energy density and low integration difficulty. In addition, the square packaging process also helps to reduce the gap between the battery cells, making the overall size more compact, while the cylindrical battery cells must leave a triangular gap between the battery cells, reducing the space utilization rate.

Compared with the stainless steel shell used in cylindrical lithium 18650 li ion battery, the battery cell shell made of magnesium-aluminum alloy is lighter and less expensive, which is conducive to improving the energy density of the battery cell and has a lower manufacturing cost. Moreover, the square shell structure can accommodate more electrolyte, the expansion stress of the battery cell pole piece is lower, and the battery life is more than 2 times longer than the cylindrical one.

Battery module

Qin ProEV500 uses BYD's independently developed nickel-cobalt-manganese ternary battery, which is based on lithium cobalt oxide. After improvement, nickel-cobalt-manganese is used as the positive electrode material of the battery, and the proportion of nickel-cobalt-manganese is reasonably matched. While optimizing costs and ensuring safety, the battery has excellent electrochemical properties such as high capacity, good thermal stability, and wide charging and discharging voltage.

And effectively improve the battery energy density to 160.9Wh/kg, combined with a capacity of 56.4kWh. Achieve a NEDC range of 420km and a constant speed range of 500km at 60km/h, thereby effectively alleviating users' concerns about the range. And thanks to the high energy density of the battery pack, the battery loading of the car is effectively reduced, thereby reducing the car's own weight.

The grouping method of the battery module fully considers the needs of heat dissipation and lightweight. It adopts the method of bundling aluminum short plates on both sides with elastic steel belts to adapt to the expansion of the battery during charging and discharging. At the same time, modules of various specifications can achieve flexible layout to meet the needs of different models. The middle part of the car body is as flat as possible, with a single-layer layout to increase the height space in the car.

In terms of detailed design, aluminum bars are used for the main circuit connection and its signal acquisition part. Under the same conductivity, the weight can be reduced by more than half compared with copper materials, and the cost can also be controlled.

However, we found that copper bars are used instead of aluminum bars on the lead-out pole. This is because the hardness of aluminum bars is low. Under high temperature and high stress, aluminum will collapse, and it is not easy to rebound after collapse. The heat and cold will cause the gap to increase, the contact resistance will increase, and bring safety hazards.

In the connection of different materials of copper and aluminum, BYD uses a technology called electromagnetic pulse welding. Compared with the commonly used copper-aluminum direct rolling connection or ultrasonic welding technology, the process of electromagnetic pulse welding is more difficult. Although the cost will also increase accordingly, the effect is the best and it is currently a more advanced technology.

Between each battery pole, the aluminum busbar and pole are also welded together with a laser to ensure reliability. A depression is designed on the busbar to absorb mechanical vibration and stress caused by electric shock expansion. If it is a straight aluminum busbar, as the battery ages and expands, the pole spacing between adjacent lithium 18650 li ion battery will increase, and the tensile stress will affect the reliability of the solder joint.

In the signal connection part, BYD uses a flexible circuit board, which is more integrated and thinner than the traditional sampling harness solution. If you look closely, you will find that there are filament-like wiring on the flexible circuit board, which we call the sampling line fuse. Its function is that in the event of a collision, it may squeeze the sampling harness to cause a short circuit, which in turn causes the sampling line to catch fire. These filaments will fuse due to overcurrent during a short circuit, thereby cutting off the short circuit loop and ensuring the safety of the entire harness and the battery module.

Battery management system

Due to the use of lithium lithium 18650 li ion battery, in order to ensure that the battery is always working in a relatively suitable temperature range, BYD has equipped it with an independent battery intelligent temperature control management system to ensure that the power battery can obtain stable and reliable performance under complex temperature environments. This intelligent temperature control management system can effectively ensure the uniformity of battery temperature through liquid medium insulation and cooling.

In terms of cooling, BYD has added a heat dissipation circuit in the battery, which is connected to the air conditioning circuit through a plate heat exchanger. Temperature sensors are installed at the battery water inlet and outlet and the battery level ears. The power of the air conditioning compressor is adjusted in real time in combination with the battery temperature to control the battery water inlet temperature and flow rate, so as to control the battery temperature at a suitable working temperature.

In terms of heating, BYD connects a PTC water heater in series in the battery heat dissipation circuit, and controls the water inlet temperature and flow rate by adjusting the power of the water heater, so as to control the battery to work at a suitable temperature in winter and ensure the charging speed and discharge power.

And through the battery management system BMS, the battery status is monitored in real time, and protection is provided for low temperature, overcharge, over discharge, over temperature, etc., thereby extending the battery life. When the temperature is too low or too high, the charging and discharging power will be limited, and when the temperature is seriously too low or too high, charging and discharging will be prohibited, thereby protecting the battery.

Snake-shaped water-cooled flat tube

The water pipes used for cooling and heating are arranged at the bottom or side of different battery modules. At the same time, we noticed that the water pipes in the battery pack use the same harmonica pipes as Tesla. This harmonica pipe is very thin, with a wall thickness of 0.8-1mm. Compared with the traditional aluminum alloy water pipes with a wall thickness of 1.6-2mm, it is much lighter in weight.

What is more distinctive is that the horizontal bending serpentine design used in Qin ProEV500 can be said to use the same technical route as Tesla, but it is more difficult from a process perspective, especially in the outer ring of the curved part. The elongation rate of the material inside and outside is quite different, and wrinkles and cracks are prone to occur, which places very high requirements on materials and processes.

The benefits of doing this are also obvious. Tesla's pipeline is to "wrap" the battery from the side, but the problem is that the contact surface between the cylindrical battery and the heat dissipation pipeline is almost a straight line, which is less efficient. This is why the overall glue filling method is used in the latest 21700 (Model3) battery module, which can only sacrifice "weight" for "heat". BYD's pipeline design works well with square lithium 18650 li ion battery. The pipeline is completely attached to the side wall of the battery to maximize the contact area.

This design not only ensures that each battery cell can be cooled, but also achieves a very good lightweight effect compared to the cooling water channel designed with a whole piece of aluminum plate. This is a leading technology in the entire industry and a challenge for BYD.

Assembly process

During the assembly process of the entire battery pack, the process control is very perfect. In particular, there are basically two or three confirmations on each connection point of the water cooling pipe, each connection point of the connector, each connection point of the high-voltage electrical connection, and the point where the structure is fixed.

For example, some low-voltage connectors are responsible for battery signal acquisition. If the BMS system loses the single cell voltage signal or the single cell temperature signal, it cannot continue to work reliably, and the safety of the battery cannot be fully guaranteed.

General connectors have only one lock, and there will be a locking sound as a prompt after locking. BYD not only has a sound as confirmation, but also a secondary lock. Only when the primary lock is plugged in place can the secondary lock be closed. The two-level locking design is very in place.

In addition, the connection of high-voltage electrical appliances is also the core and most critical point in the assembly of the entire battery pack, especially in the reliability of the main circuit connection and the low internal resistance design. BYD's battery pack uses high-temperature resistant polyimide sealed copper bars for long-distance connections in the main circuit, and designs many three-dimensional bends, so that when subjected to vibration or thermal expansion, these bends can absorb the change in length and avoid transferring the load to the connecting screws.

Although from the perspective of contact internal resistance, the contact internal resistance of a single screw meets the heating requirements. But BYD still insists on using a double screw design to greatly improve reliability. And in the tightening confirmation of the screws, we found three color codes, which means that three confirmations were made. The first time is tightened by the automatic tightening shaft and marked with a red mark, and the next two times are manually re-checked with a torque wrench, marked with yellow and white marks respectively.

In addition, most of the pipelines in the entire battery pack use nylon mesh braided sleeves, especially the pipelines that contact the battery pack shell and internal devices, which protect the wiring harness and avoid wear while also reducing noise.

Summary

In general, BYD Qin ProEV500 has made a lot of efforts in the lightweight and reliability of the entire battery pack, and has improved the energy density of the battery by improving the battery cell ratio, optimizing the battery management system and active thermal management technology, thereby improving the vehicle's power, control and endurance performance.

Especially in the design of safety, BYD's engineers have considered it more carefully, so as to protect the user's driving safety to the greatest extent. All of the above reflect BYD's technical advantages and development space in the field of battery research and development, and it can be said that it has led the direction of industry technology development.

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