Revealing BYD's core power battery technology: square aluminum shell integration process

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

For BYD , which started out as a battery manufacturer , high-performance batteries are one of BYD's trump cards. Especially after replacing the 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.

In this issue, we will fully disassemble the battery pack of the BYD Qin Pro EV500 model, and analyze BYD's innovations and management technologies such as battery pack safety design and thermal management design.

Square aluminum shell integration process

After removing 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 of the battery pack. The most intuitive thing is the integration process of the battery pack. The integration process is very important in the research and development of power batteries. It must meet all safety requirements such as mechanical protection, thermal safety protection, thermal management, and environmental protection, while pursuing lightweight and cost optimization.

Unlike the cylindrical battery cell 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 reduce the gap between the battery cells, making the overall size more compact, while the cylindrical battery cells must leave triangular gaps between the battery cells, reducing the space utilization rate.

Compared with the stainless steel shell used in cylindrical batteries, the battery cell shell made of magnesium-aluminum alloy is lighter and cheaper, which is conducive to improving the energy density of the battery cell and reducing the manufacturing cost. In addition, the square shell structure can accommodate more electrolyte, the battery cell electrode expansion stress is lower, and the battery life is more than twice that of the cylindrical one.

Battery Module

Qin Pro EV500 uses BYD's independently developed nickel-cobalt-manganese ternary battery, which is based on lithium cobalt oxide and has been improved to use nickel-cobalt-manganese as the positive electrode material of the battery, and the ratio 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 charge and discharge voltage.

The battery energy density is effectively increased to 160.9Wh/kg, combined with a capacity of 56.4kWh. The NEDC range is 420km, and the range at a constant speed of 60km/h is 500km, which effectively alleviates 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 battery module grouping method fully considers the needs of heat dissipation and lightweight. It adopts the method of bundling with aluminum short plates on both sides and 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 inside the car.

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

However, we found that copper bars were used instead of aluminum bars on the lead-out poles. This is because the hardness of aluminum bars is lower. Under high temperature and high stress conditions, aluminum will collapse and is not easy to rebound after collapse. The heat and cold will cause the gap to increase and the contact resistance to increase, posing a safety hazard.

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

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

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 safety of the battery module.

Battery Management System

Since lithium batteries are used, BYD has equipped them with an independent battery intelligent temperature control management system to ensure that the battery always works within a suitable temperature range, so as to ensure that the power battery can obtain stable and reliable performance in a complex temperature environment. This intelligent temperature control management system can effectively ensure the uniformity of battery temperature through liquid medium heat preservation 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 installed at the battery water inlet and outlet and at the battery level ears. The power of the air-conditioning compressor is adjusted in real time according to the battery temperature to control the battery water inlet temperature and flow rate, thereby controlling the battery temperature at a suitable working temperature.

In terms of heating method, BYD connects a PTC water heater in series in the battery cooling circuit. By adjusting the power of the water heater, the water inlet temperature and flow rate are controlled, so that the battery can operate at a suitable temperature even in winter, ensuring the charging speed and discharge power.

The battery management system BMS monitors the battery status in real time and protects against low temperature, overcharge, over discharge, over temperature, etc., thereby extending the battery life. When the temperature is too low or too high, the charge and discharge power will be limited, and when the temperature is seriously too low or too high, the charge and discharge will be prohibited, thereby protecting the battery.

Serpentine water cooling 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 used the same harmonica tubes as Tesla. This harmonica tube is very thin, with a wall thickness of 0.8-1mm, which is much lighter than traditional aluminum alloy water pipes with a wall thickness of 1.6-2mm.

What is more distinctive is that the horizontally bent serpentine design used on the Qin Pro EV500 can be said to have adopted the same technical route as Tesla, but it is more difficult from a process perspective, especially in the outer circle 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 demands on materials and processes.

The benefits of this are obvious. Tesla's pipes are designed to "wrap" 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, which is inefficient. This is why the latest 21700 (used in Model 3) battery module uses an overall glue injection method, which can only sacrifice "weight" for "heat". BYD's pipe design works better with square batteries, and the pipes are 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 has completed a challenge for BYD.

Assembly process

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

For example, some low-voltage connectors are responsible for battery signal acquisition. If the BMS system loses the single-cell voltage signal or single-cell temperature signal, it can no longer work reliably and cannot fully guarantee the safety of the battery.

The general connector has only one lock, and there will be a locking sound as a reminder after locking. BYD not only has a sound as confirmation, but also has a secondary lock. Only when the primary lock is plugged in place can the secondary lock be closed. The two-stage locking design is very perfect.

In addition, the connection of high-voltage electrical appliances is also the core and most critical point in the entire battery pack assembly, especially in the reliability of the main circuit connection and the low internal resistance design. BYD's battery pack uses high-temperature resistant polyimide pressed 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 connection screws.

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

In addition, most of the pipelines in the entire battery pack are made of nylon mesh braided tube sleeves, especially the pipelines that are in contact with the battery pack shell and internal devices. While protecting the wiring harness and avoiding wear, it also plays a role in reducing noise.

Summarize

In general, BYD Qin Pro EV500 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, handling and endurance performance.

Especially in terms of safety design, BYD engineers have considered it in detail, so as to protect the driving safety of users 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.