How to optimize the EVs Lithium-ion battery packs weight?
The lithium-ion battery pack is one of the core components of electric vehicles. The weight of the battery pack accounts for about 20-30% of the total weight of the vehicle, and it is also one of the components with the highest cost in vehicle production.
As we all know, the battery pack has very strict safety requirements, and it also determines the performance of the vehicle’s power and cruising range.
In order to improve performance, building a lightweight battery pack has become very crucial.
Overview
Why should the battery pack be lightweight?
How to make a lightweight battery pack?
1. Optimize the layout of battery packs
2. Optimize the battery module
4. Adopt Lightweight material for battery cases
Why should the battery pack be lightweight?
Research data shows that traditional fuel vehicles can reduce their weight by 10%, and their economy can be improved by 6-8%; Under constant speed driving conditions, the self-weight of electric vehicles is reduced by 10%, which can increase the driving range of the whole vehicle by about 10%.
From the perspective of the battery pack, due to the relatively large restrictions on the composition and size of the battery core materials, the only way to reduce weight is to start with the structure. Lightweight boxes and compact modules have become the key direction of research for electric vehicle companies.
How to make a lightweight battery pack?
The lightweight of the battery pack can roughly be divided into two levels – the system design level and the detailed design level. The following figure illustrates the subdivision level:
In addition to its lightweight, the structure of the battery pack still needs to meet safety requirements such as mechanical safety, sealing insulation, and fire prevention. The strength, stiffness, crashworthiness, and stability of the box structure will all have an impact on the performance of the battery pack.
There are five effective ways to make a lightweight lithium-ion battery pack for EVs:
1. Optimize the layout of battery packs
In the limited space of the battery pack box, a certain number of battery cells form a battery module through specific mechanical and electrical connections.
Based on the shape, lithium battery cells include cylindrical cells, prismatic cells and pouch cells. Except for the cells, auxiliary functional components such as BMS and high-voltage wiring harnesses are arranged inside the battery pack.
According to the shape and load-bearing characteristics of the vehicle battery pack, the battery cells are arranged in series and parallel to form a powerful battery system. The inside cells or module layout and structure of the battery pack are different.
The layout of the power battery pack is usually determined by the space characteristics of the vehicle. Factors such as the driving mode of the vehicle, the position of the centre gravity of the vehicle, and ground clearance need to be considered.
In order to meet the needs of vehicle manufacturers, EV battery pack manufacturers have developed EV battery packs that have a variety of module arrangements, battery box shapes, and mounting lug positions. The most commonly used structural layouts for battery packs are underbody suspension type, distributed standard boxes and integrated body structures.
Battery Pack in Underbody suspension type
Early electric vehicles were mostly converted from traditional fuel vehicles. EV battery packs were usually installed in the front cabin, trunk, and bottom of the floor of the car, such as the Nissan Leaf’s “concave” battery pack shown in the figure below.
* Figure Nissan Leaf battery pack
The suspension battery pack is under the vehicle body and is connected to the bottom of the car frame by bolts. It has the advantages of an efficient and flexible design and high manufacturing independence. It is an EV battery pack structure widely used in passenger cars, such as Nissan Leaf, Geely Emgrand EV, and other electric vehicles.
Distributed standard battery boxes
Standard box distribution is to form a battery system through series and parallel connections of several standard battery box with the same or similar structure. This system has the characteristics of a flexible layout and various installation positions.
This structure is mostly used in larger and regular passenger cars or special-purpose vehicles, such as Yutong E10 pure electric buses.
* Figure Standard box battery pack
Body structure integrated battery pack
With the continuous increase in the demand for the cruising range of electric vehicles, the space-constrained traditional vehicle structure cannot meet the optimal design requirements, and the structural layout of the body structure integrated battery pack has gradually attracted attention.
The special design platform for electric vehicles of Rivian as shown in the figure integrates the battery pack into the body structure.
The increasing demand for the driving range of electric vehicles, combined with the development of vehicle forward design technology, has prompted the coordinated development of body design and battery pack structure, aiming for a compact body structure and better battery pack performance.
Platform-based and modular body structure integrated power battery packs are increasing, such as the Audi Q4 e-tron, which uses the VW MEB platform, as well as the Model S and Model X.
2. Optimize the battery module
At the system design level, the lightweight design of battery packs starts with the selection of battery cell parameters and cell size.
There are matching design problems between lithium-ion power cells and power battery systems under different chemical systems and size parameters, which usually need to be calculated and determined at the conceptual design stage of the battery system.
Finally, by optimizing the internal layout of the battery pack box and reducing the design level, maximum utilization of box space is achieved.
For example, the CTP design technology (Cell To Pack, CTP) proposed by CATL. The picture shows the structural design of a CTP system at CATL.
* Figure CATL CTP design application example
In this CTP design, the cell and battery management system are directly fixed in the battery pack shell, the battery cells are built in the upper and lower shells, and the shell is filled with thermal conductive glue.
Due to this design form does not use a module structure, the volume utilization rate of the battery pack is increased by 15-20%; the batteries are assembled separately, which reduces the difficulty of assembly and increases production efficiency by about 50%
3. Improved packing method
Large module design
Increasing the size and capacity of the cell results in a reduction in the mass of structural parts shared by each cell. For example, the large module design structure of CATL:
* Figure CATL large module design example
The design of the large module replaced those battery cases being installed in vehicles. The battery module is directly installed onto the vehicle through the support sleeve and installation beam via the fixing piece. This way not only realized the lightweight of the battery pack but also improves the connection strength of the battery pack on the vehicle.
Integrated design
Reduce intermediate levels such as modules, optimize the size of battery cells, and improve the utilization of box space.
As shown in the figure below, BYD’s “blade battery” battery pack design scheme uses a flat and large-sized battery cell, which is arranged in an array inside the battery pack box and inserts the battery as a blade.
* Figure BYD Blade battery structure
It is reported that this design can increase the specific energy of the battery pack by about 50%, and reduce the production cost by about 30%.
4. Adopt Lightweight material for battery cases
The effect of using lightweight materials to reduce the overall weight of the case is very obvious.
At present, relatively mature lightweight materials are divided into two categories: aluminium-magnesium alloys and composite materials.
Among metal materials, aluminium alloy is not only light in weight and highly resistant to corrosion, but it is also conducive to recycling. It is widely used in battery packs.
Considering structural strength, die-cast aluminium boxes and extruded-welded aluminium boxes are used mostly in lower box bodies, while pressed-welded aluminium boxes are generally used in upper box covers.
When it comes to non-metallic materials, composite materials are currently a hot topic. Its advantages of being very lightweight, performing well as an insulator, and requiring little processing and moulding make it suitable for battery packs and even complete vehicles.
The data shows that the battery pack upper cover made of SMC is about 38% lighter than the traditional metal material upper cover. In addition, the application of carbon fibre composite materials is gradually increasing, and the weight reduction effect of composite materials is obvious.
Some companies try to apply composite materials to the lower floor of electric vehicles. However, the stiffness characteristics of composite materials are poor. Therefore, it is necessary to thicken the size or use a sandwich structure to improve the bending resistance of the structure.
At the same time, the box under the battery pack will be designed as a sandwich structure, and a metal or honeycomb aluminum structure will be added to the middle layer to combine metal and non-metal, which has many advantages such as lightweight, high strength, and good crashworthiness.
5. Ultimate Design
An ultimate design is the optimization of performance during the detailed design stage or the improvement of the design at later stages.
The ultimate design needs to know the critical value of the design. This is not only to meet the performance requirements but also to meet the parts processing and product assembly process requirements.
In the ultimate design, CAE simulation analysis technology identifies critical production process parameters and product performance critical values.
For example, the design of the load-bearing parts of the battery pack box is strengthened, and the non-load-bearing parts are made of thin-walled materials. The thickness of different positions of the box is changed to achieve structural performance that meets the design requirements while reducing weight as much as possible.
* Fig. Example of the ultimate design of an aluminium box structure
Creating a lightweight EV battery pack is one of the smartest and most cost-effective ways to maximize performance and maintain safety. It has many advantages, such as improved range, increased acceleration, greater sustainability, and reduced costs in terms of energy consumption. By the above five effective methods outlined in this article, you’ll be able to construct an efficient lithium-ion battery pack that will exceed your expectations. With proper design and optimal materials—and with Bonnen’s assistance—you can rest assured knowing that your EV will run reliably for years to come. Start building your vehicle’s lightweight EV battery pack now! And when you’re ready for lithium battery needs, don’t forget to connect with Bonnen for best selection!
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