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The Next-Generation Battery Pack Design: from the BYD Blade Cell to Module-Free Battery Pack

This story is contributed by Xinghua Meng and Eric Y. Zheng

  • With cell-to-pack technology, BYD designed the module-free battery pack using the Blade Cell.
  • The geometry of the Blade Cell is a key to the realization of the module-free battery pack.
  • With the module-free pack design, VCTPR and GCTPR can be enhanced to over 60% and 80%.

In the previous article, we described the concept, specifications, pros and cons of the BYD Blade Battery from cell level. Here, we explain how this novel design is realized in the module-free battery using cell-to-pack (CTP) technology.

What is CTP? Why module-free?

The conventional battery manufacturing process is from cell to module, and then from module to pack. This intermediate step divides the battery into separate modules, each of which can have its own independent battery management and diagnostic systems. This allows malfunctioning of cells to be controlled on the module level and allows for modules to be replaced individually as opposed to the entire pack. In addition, modules can provide some structural support for the pack. The trade-off with this design is that the terminal plates, side plates, and internal connectors of a module take up space and weight. In a conventional battery pack, this limits the GCTPR (gravimetric cell-to-pack ratio) to 77% or lower, and the VCTPR (volumetric cell-to-pack ratio) is typically around 50% but sometimes even lower than 40%. The additional steps needed for assembly also increases the manufacturing cost.

With the aid of advanced fabrication technology on the materials and cell levels as well as an updated battery management system (BMS), module-free batteries have become a hot topic. With CTP technology, battery packs are assembled directly from the cells without the need for modules. Many battery manufacturers, such as BYD Auto, CATL, LG Chem, and SVOLT, are exploring CTP technology. The Blade Battery is BYD’s realization of the CTP concept (Figure 1).

Figure 1. The structure of the Blade Battery from cell to pack.

BYD Blade Battery-Inspired by CTP Geometry

At the center of the design of the Blade Battery is the cell geometry, which has a much lower aspect ratio compared with conventional cylindrical or prismatic cells. According to BYD’s patents, the cell depth (Z axis) is 13.5 mm while the cell length (X axis) can range from 600 mm to 2500 mm. The inactive parts of the cell, including the lug, pole, and busbar , are all aligned with the X axis. For most EV cells, space in the Z direction is at a premium because of the limited room under passenger seats (Figure 2). The ultra-long X axis mitigates the volume lost to the inactive parts. In the CTP process, each cell is connected in series or parallel aligned with the X axis. The large surface area can increase the thermal dissipation of the cells.

Figure 2. A schematic of battery pack fixed under passenger seats in the vehicle.

Vehicle designers are seeking to reduce the space needed for the battery and the height is a prime target.

The height of the Blade Battery is reduced by ~50 mm, compared with regular LFP battery back with modules, providing more space to the passengers and decreasing the coefficient of drag (0.233 cd for BYD Han).

In the Z direction, the structure of the Blade Battery is completely different from conventional module-based battery packs (Figure 3). The lower profile of the Blade Battery offers more flexibility in optimizing between design and capacity. In addition, each cell is used for not only energy storage but also structural support of the battery pack. The array design provides extremely high strength in the Z axis. As shown in Figure 4, the strength of Blade Battery combined with the honey-combed structural panels provide sufficient support to the battery pack. This breakthrough enables the space to be utilized more effectively, solving the fixing and strength issues at the same time without modules.

Figure 3. Comparison of the design in Z-axis between conventional battery pack and BYD Blade Battery pack.
Figure 4. Schematics of honey-combed structural panels for battery pack.

CTP Performance Enhancements

In the past few years, LFP-based EVs have often been perceived as unattractive to high-end consumers due to their low volumetric and gravimetric energy density, which results in a shorter range. Even worse, this low volumetric energy density often requires car designers to make room for a larger pack.

The module-free Blade Battery, however, takes advantage of its blade cells to increase the volumetric energy density by up to 50%, suggesting a potential VCTPR and GCTPR of 62.4% and 84.5%, respectively.

Other CTP technology

Although the Blade Battery shows a lot of promise, the blade geometry is not perfect . For example, the Blade Battery has a challenging manufacturing process. With an electrode roll dimension larger than 500 mm, roll-to-roll alignment and lamination and quality control will be very difficult. Manufacturing inconsistencies in the cells could blunt many of the advantages of this CTP design. This module-free design is also not the only option for CTP. Another popular solution involves substituting traditional smaller modules with larger ones. CATL and SVOLT, among others, have chosen this path. Using large modules can save weight and volume by reducing the number of inactive connectors between modules.

Summary

Module-free or not, CTP technology seeks to improve energy density by reducing the weight and volume of the inactive materials, such as module shells and connectors. BYD’s Blade Battery design explored a bold CTP concept through its module-free pack. High quality control in materials and cell manufacturing, however, remain critical prerequisites of CTP.

Xinghua Meng is a battery research scientist in the United States. His research interests focus on next-generation cathode and anode materials for electric vehicles and renewable energy storage systems. He is actively applying nanotechnology in lithium-ion batteries. Dr. Meng earned his Ph.D. Degree at the Department of Chemical Engineering and Materials Science at Wayne State University in 2016.

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