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. In this issue, 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. The serpentine water-cooled flat tubes 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 packs use the same
harmonica tube as Tesla. This harmonica tube is very Thin, with a wall thickness
of 0.8-1mm, it 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
bending snake-shaped design used on Qin ProEV500 can be said to adopt the same
technical route as Tesla, but it is more difficult from a process perspective,
especially in bending. For part of the outer ring, the stretch rate of the inner
and outer parts of the material is quite different, and wrinkles and cracks are
prone to occur, and the requirements for materials and processes are very high.
The benefits of this are also obvious. The Tesla pipeline is to "cover" 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,
and the efficiency is poor. This is why in the latest The 21700 (used in Model
3) battery module adopts the overall glue filling method, which can only
sacrifice "weight" for "heat". 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 very 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, thereby
greatly improving reliability. And when confirming the tightening of the screws,
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 and avoids wear and tear, but also reduces noise. . Summarize
BYD Qin ProEV500 has made great 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 power of
the vehicle. , control and battery life 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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