Revealing the core technology of Dry Battery

　　As one of the core components of pure electric vehicles, Dry Battery are closely related to the vehicle's cruising range, maintenance quality, power performance, control performance, etc. In terms of the manufacturing cost of pure electric vehicles, batteries also account for the highest proportion, generally above 30%, which results in higher selling prices and later maintenance costs for electric vehicles. Therefore, reducing the unit cost of batteries and increasing the energy density of batteries have always been the main directions for the development of electric vehicle technology.

　　For BYD, which originally started with batteries, high-performance batteries are one of BYD's trump cards. Especially after the replacement of ternary lithium batteries 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. We will conduct a comprehensive disassembly of the battery pack of the BYD Qin Pro EV500 model, and analyze BYD's innovative and management technologies such as battery pack safety design and thermal management design.

　　Square aluminum shell integration process

　　After uncovering the ultra-thin non-metal upper 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. The most intuitive thing is the integration of the battery pack. Craftsmanship. Integrated technology is very important in the research and development of Dry Battery. It must meet all safety requirements such as mechanical protection, thermal safety protection, thermal management, and environmental protection, while pursuing lightweight and cost optimization.

　　Different from the cylindrical battery cell method used by Tesla, BYD uses a square aluminum shell that is more popular in China, which has the advantages of high energy density and low integration difficulty. In addition, the square packaging process also helps to narrow the gaps between cells and make the overall size more compact. Cylindrical cells must leave triangular gaps between cells, which reduces space utilization.

　　The battery cell casing made of magnesium-aluminum alloy is lighter and lower in cost than the stainless steel casing used in cylindrical batteries, which helps to increase the energy density of the battery cell and has lower manufacturing costs. Moreover, the structure of the square shell can accommodate more electrolyte, lower the expansion stress of the cell poles, and the battery life is more than 2 times longer than that of the cylindrical shape.

　　battery module

　　The Qin Pro EV500 uses a nickel-cobalt-manganese ternary battery independently developed by BYD, which is based on lithium cobalt oxide and has been improved to use nickel-cobalt-manganese as the battery cathode material and a reasonable ratio of nickel-cobalt-manganese. While optimizing costs and ensuring safety, the battery has excellent electrochemical properties such as high capacity, good thermal stability, and wide charge and discharge voltage.

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

　　The battery module is assembled in a way that fully takes into account the needs for heat dissipation and lightweighting. It uses aluminum short plates on both sides and elastic steel straps to adapt to the expansion of the battery during the charging and discharging process. At the same time, modules of various specifications can achieve flexible layout to adapt to the needs of different models. The middle part of the car body should be as flat as possible, with a single-layer layout to increase the height and space inside the car.

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

　　However, we found that a copper busbar is used instead of an aluminum busbar on the lead-out pole. This is because the hardness of the aluminum busbar is low. Under high temperature and high stress conditions, aluminum will collapse, and it is not easy to rebound after collapse. When hot or cold, the gap will increase and the contact resistance will increase, causing safety hazards.

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

　　Between each battery pole and pole, the aluminum bus bar and pole are also laser welded together to ensure reliability. And there is a depression designed on the busbar to absorb the stress caused by mechanical vibration and electric shock expansion. If it is a straight aluminum bar, as the battery ages and expands, the distance between the poles of adjacent batteries will increase, and tensile stress will affect the reliability of the solder joints.

　　In the signal connection part, BYD uses a flexible circuit board, which is more integrated and thinner than the traditional sampling wire harness solution. If you look carefully, you will find filament-like wiring on the flexible circuit board, which we call the sampling line fuse. Its function is that during a collision, the sampling wire harness may be squeezed to cause a short circuit, which may cause the sampling wire to catch fire. These filaments will fuse due to overcurrent during the short circuit, thereby cutting off the short circuit loop and ensuring the safety of the entire wire harness and the battery. Module security.

　　battery management system

　　Due to the use of lithium batteries, in order to ensure that the battery always works within a more suitable temperature range, BYD has equipped it with an independent battery intelligent temperature control management system to ensure that the power battery can operate in complex temperature environments. Get stable and reliable performance. This intelligent temperature control management system can effectively ensure battery temperature uniformity through liquid medium insulation and cooling.

　　In terms of cooling method, 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 placed at the inlet and outlet of the battery and the battery stage ears. The power of the air conditioning compressor is adjusted in real time based on the battery temperature. The battery inlet water temperature and flow rate are used to control the battery temperature at a suitable operating temperature.

　　In terms of heating method, BYD connects a PTC water heater in series with the battery heat dissipation circuit. By adjusting the power of the water heater, it controls the inlet water temperature and flow, thereby controlling the battery to work at a suitable temperature in winter, ensuring charging speed and Discharge dynamics. And through the battery management system BMS, the battery status is monitored in real time and protected against low temperature, overcharge, over-discharge, over-temperature, etc., thereby extending battery life. When the temperature is too low or too high, charging and discharging power will be limited, and when the temperature is seriously too low or too high, charging and discharging will be prohibited to protect the battery.

　　Serpentine water-cooled flat tube

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

　　What is more distinctive is that the horizontally bending snake-shaped design used on Qin Pro EV500 can be said to adopt the same technical route as Tesla, but it is more difficult from a process perspective, especially in The outer ring of the curved part has a relatively large difference in stretch rate between the inner and outer sides of the material, which is prone to wrinkles and cracks. The requirements for materials and processes are very high.

　　The benefit of this is also obvious. The Tesla pipe is to cover the battery from the side, but the problem is that the contact surface between the cylindrical battery and the heat dissipation pipe is almost a straight line, and the efficiency is poor. This is why in the latest 21700 (Model 3 adopts) The battery module uses an integral glue filling method, which can only sacrifice weight for heat exchange. BYD's pipeline design works well with square batteries. 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 core can be cooled, but also achieves a very good lightweight effect compared to the cooling water channel designed with a whole aluminum plate. This is a leading technology in the entire industry and has completed a challenge for BYD.

　　Assembly process

　　During the final assembly of the entire battery pack, the process control is perfect. In particular, there are basically two or three confirmations on every water-cooling pipe connection point, every connector connection point, every high-voltage electrical connection point, and structural fixation points.

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

　　Generally, connectors only have one lock, and after locking, there will be a locking sound as a reminder. BYD not only has a sound as confirmation, but also has a secondary lock. The secondary lock can only be closed when the first-level lock is connected in place. The two-level locking design is in place.

　　In addition, the connection of high-voltage electrical appliances is also the core and most critical point in the entire battery pack assembly, especially 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 the long-distance connection of the main circuit, and is designed with many three-dimensional bends, so that when it is subject to vibration or thermal expansion, it can pass through these bends. to absorb changes in length and avoid transferring loads 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, thus greatly improving reliability. And on the screw tightening confirmation, we found that there are three colors of color marks, which means that three confirmations were made. The first pass is for automatically tightening the shaft and is marked with a red mark. The last two passes are for manual rechecking using a torque wrench and are marked with yellow and white marks respectively.

　　In addition, most of the pipelines in the entire battery pack are made of nylon mesh braided pipe sleeves, especially the pipelines in contact with the battery pack shell and internal components. This not only protects the wiring harness from wear and tear, but also reduces noise. .

　　Summarize

　　In general, BYD Qin Pro EV500 has made a lot of efforts to reduce the weight and reliability of the entire battery pack, and has improved the energy density of the battery by improving the cell ratio, optimizing the battery management system, and active thermal management technology. , thereby improving the vehicle's power, control and endurance performance.

　　Especially in terms of safety design, BYD engineers have considered it more carefully to protect users' 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 can be said to lead the industry's technological development direction.

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