702535 polymer battery.Lithium battery management system maintains and cools down through liquid media

　　As one of the core components of pure electric vehicles, power batteries 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 ProEV500 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-metallic 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, among which the most intuitively visible convenience It is the integration process of battery pack. Integrated technology 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 in line with the cylindrical battery used by Tesla. The core method is different. BYD uses a square aluminum shell with a higher domestic popularity, 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 electrodes, and the battery life is more than 2 times longer than that of the cylindrical shape. The battery module Qin ProEV500 uses the nickel-cobalt-manganese ternary battery independently developed by BYD, that is, Based on lithium cobalt oxide, it has been improved to use nickel cobalt manganese 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 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. Since the battery management system uses 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. Stable and reliable performance can be obtained below. 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.

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