Therefore, reducing the unit cost of batteries and increasing the energy density of batteries have always been important directions for the development of electric vehicle technology.
For BYD, a company that originally started with batteries, high-performance batteries are one of its key strengths. Especially after switching to ternary lithium-ion 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 enhanced.
In this issue, we will conduct a comprehensive disassembly of the battery pack of the BYD Qin Pro EV500 and analyze the innovative and management technologies that BYD has mastered, such as battery pack safety design and thermal management design.
Square aluminum shell integration process
After removing the ultra-thin non-metallic top cover and the silica aerogel fireproof and heat-insulating layer of the battery pack, we can clearly see the overall layout and structure of the battery pack, the most obvious of which is the integration process. Integration technology is crucial in the development of power lithium-ion batteries. It must meet comprehensive safety requirements, including mechanical protection, thermal safety protection, thermal management, and environmental protection, while simultaneously pursuing lightweight design and cost optimization.
Unlike Tesla's cylindrical battery cells, BYD uses square aluminum casings, which are more common in China and offer advantages such as high energy density and lower integration difficulty. Furthermore, the square packaging process helps reduce the gaps between cells, making the overall size more compact. Cylindrical cells, on the other hand, inevitably require triangular gaps between cells, reducing space utilization.
The magnesium-aluminum alloy cell casing is lighter and cheaper than the stainless steel casing used in cylindrical batteries, which helps to increase the energy density of the cells and also reduces manufacturing costs. Furthermore, the prismatic structure can accommodate more electrolyte, reduces the expansion stress of the cell electrodes, and results in a battery life more than twice that of cylindrical cells.
Battery Module
The Qin Pro EV500 uses BYD's independently developed nickel-cobalt-manganese ternary battery, which is an improvement on lithium cobalt oxide, using nickel-cobalt-manganese as the positive electrode material and a reasonable ratio of nickel, cobalt, and manganese. While optimizing costs and ensuring safety, this results in excellent electrochemical performance such as high capacity, good thermal stability, and wide charge-discharge voltage range.
Furthermore, it effectively improves battery energy density to 160.9Wh/kg, combined with a capacity of 56.4kWh. This achieves an NEDC range of 420km and a constant speed range of 500km at 60km/h, effectively alleviating users' concerns about range. Moreover, thanks to the high energy density of the battery pack, the amount of battery material loaded in the car is effectively reduced, thus reducing the car's weight.
The battery module assembly method fully considers the needs of heat dissipation and lightweighting, employing short aluminum plates on both sides and elastic steel straps for binding, adapting to the battery's expansion during charging and discharging. Simultaneously, various module sizes allow for flexible layouts to suit different vehicle models. A flat, single-layer layout in the middle of the vehicle body maximizes interior height.
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 to using copper material, and the cost can also be controlled.
However, we found that copper busbars were used instead of aluminum busbars on the lead-out electrode. This is because aluminum busbars have lower hardness. Under high temperature and high stress conditions, aluminum will collapse and is not easy to spring back after collapse. The hot and cold cycles will cause the gaps to widen, increase the contact resistance, and bring safety hazards.
For joining copper and aluminum, which are made of different materials, BYD uses a technology called electromagnetic pulse welding. Compared with the commonly used direct pressing or ultrasonic welding techniques for copper and aluminum, electromagnetic pulse welding is more difficult to perform and, although it increases the cost, it offers the best results and is currently the most advanced technology.
Between each battery terminal, an aluminum busbar and terminal are laser-welded together to ensure reliability. A recess is designed into the busbar to absorb stress from mechanical vibration and electrical expansion. In contrast, with a straight aluminum busbar, as the battery ages and expands, the distance between adjacent battery terminals increases, and tensile stress can affect the reliability of the weld joint.
For the signal connection, BYD uses a flexible circuit board, which offers higher integration and is thinner than traditional sampling harnesses. Upon closer inspection, you'll find fine, thread-like wiring on the flexible circuit board; these are called sampling line fuses. Their purpose is to prevent short circuits caused by impacts, which could lead to the sampling harness catching fire. These fine threads will melt due to overcurrent during the short circuit, breaking the circuit and ensuring the safety of the entire harness and the battery module.
Battery Management System
Because it uses lithium-ion batteries, BYD has equipped them with an independent intelligent battery temperature control management system to ensure that the batteries operate within a suitable temperature range. This system guarantees stable and reliable performance of the power lithium-ion batteries under complex temperature environments. The intelligent temperature control management system effectively ensures battery temperature uniformity through liquid medium insulation and cooling.
In terms of cooling, BYD has added a heat dissipation circuit inside the battery, which is connected to the air conditioning circuit through a plate heat exchanger. Temperature sensors are placed at the battery inlet and outlet water and at the battery level ears. The power of the air conditioning compressor is adjusted in real time based on the battery temperature to control the battery inlet water temperature and flow rate, thereby controlling the battery temperature at a suitable operating temperature.
In terms of heating methods, BYD connects a PTC water heater in series in the battery heat dissipation circuit. By adjusting the power of the water heater, the inlet water temperature and flow rate are controlled, thereby ensuring that the battery can work at a suitable temperature in winter and ensuring charging speed and discharge power.
Furthermore, the battery management system (BMS) monitors the battery status in real time and protects against low temperatures, overcharging, over-discharging, and overheating, thereby extending battery life. When the temperature is too low or too high, the charging and discharging power is limited, and when the temperature is severely too low or too high, charging and discharging are prohibited, thus protecting the battery.
serpentine water-cooled flat tube
The water pipes used for cooling and heating are arranged at the bottom or side of different battery modules. We also noticed that the water pipes in the battery pack use the same harmonica pipes as Tesla. These harmonica pipes are very thin, with a wall thickness of 0.8-1mm, which is much lighter than the traditional aluminum alloy water pipes with a wall thickness of 1.6-2mm.
What's particularly noteworthy is that the horizontally folding serpentine design used on the Qin Pro EV500, while employing the same technical approach as Tesla, is more challenging from a manufacturing perspective. This is especially true for the outer ring of the curved section, where the difference in tensile strength between the inner and outer sides of the material makes it prone to wrinkles and cracks, placing extremely high demands on both materials and manufacturing processes.
The advantages of this approach are obvious. Tesla's piping is designed to "wrap" the battery from the side, but the problem is that the contact surface between the cylindrical battery and the cooling piping is almost a straight line, resulting in poor efficiency. This is why the latest 21700 (used in Model 3) battery module uses an integral potting method, sacrificing "weight" for "heat." BYD's piping design works well with the square battery, with the piping completely attached to the battery sidewall, maximizing the contact area.
This design ensures that each battery cell is cooled, and the cooling channels, designed using a single piece of aluminum, achieve excellent weight reduction. This is a leading technology in the industry, representing a significant achievement for BYD.
Assembly process
Throughout the battery pack assembly process, the process control was impeccable. In particular, there were two to three checks at each connection point of the water-cooling pipe, each connection point of the connector, each connection point of the high-voltage electrical components, and each point of structural fixation.
For example, some low-voltage connectors are responsible for battery signal acquisition. If the BMS system loses the individual cell voltage signal or the individual cell temperature signal, it cannot continue to work reliably, and therefore cannot fully guarantee the safety of the battery.
Typical connectors have only one locking latch, which makes a locking sound when tightened. BYD connectors, however, not only have a sound for confirmation, but also a secondary locking latch. The secondary latch can only be closed when the primary latch is fully engaged, making this two-stage locking design very effective.
In addition, the connection of high-voltage electrical components is the most crucial and critical aspect of the entire battery pack assembly, especially in terms of the reliability of the main circuit connection and the design of low internal resistance. BYD's battery pack uses high-temperature resistant polyimide-sealed copper busbars for long-distance connections in the main circuit, and incorporates numerous three-dimensional bends. These bends absorb changes in length when subjected to vibration or thermal expansion, preventing the load from being transferred to the connecting screws.
Although a single screw's contact resistance would be sufficient to meet the heat dissipation requirements from a contact resistance perspective, BYD still insists on using a double-screw design to significantly improve reliability. Furthermore, we observed three color-coded markings for screw tightening confirmation, indicating three rounds of verification. The first round involved automatic tightening of the shaft, marked with a red mark; the subsequent two rounds were manual checks using a torque wrench, marked with yellow and white marks respectively.
In addition, most of the pipelines inside the battery pack are covered with nylon mesh braided tubing, especially the pipelines that come into contact with the battery pack casing and internal components. This not only protects the wiring harness and prevents wear, but also reduces noise.
Summarize
In summary, BYD has made great efforts in the lightweighting and reliability of the entire battery pack for the Qin Pro EV500. By improving the cell ratio, optimizing the battery management system and active thermal management technology, the energy density of the battery has been increased, thereby improving the vehicle's power, handling and range performance.
Especially in terms of safety design, BYD engineers have paid meticulous attention to detail, maximizing the protection of users' driving safety. All of these aspects demonstrate BYD's technological advantages and development potential in the field of battery research and development, and it can be said that BYD is leading the industry's technological development direction.
