Cell to Body

We have seen cell to pack battery designs from BYD and CATL. BYD with their ground breaking Blade design has brought LFP chemistry back into the world of EV’s.

Mercedes Benz EQS battery
Cell to Pack

Cell to Pack is all about reducing cost and increasing the volumetric density of battery packs. This is primarily aimed at road vehicle battery design. This can offer some significant increases in energy density and cost reductions. However, this does remove barriers between cells and hence brings into focus the task of how to stop cell to cell propagation.

The next step is to place the cells directly into the vehicle body structure: Cell to Body or CTB or C2B. At first you might think that this is just the next obvious step, however, there are a number of questions around manufacturing, repair and service that we need to understand.

The first claim to do this in production is BYD with the BYD Seal built on their e-Platform 3.0

BYD cell to body battery technology

Benefits of CTB

BYD [1] list the benefits of this approach as:

  • efficiency
  • aerodynamics
  • power
  • body rigidity
  • space utilisation

We need to look at each of these in more detail, specifically how the battery has enabled or benefitted from this.

Establishing the metrics and advantages might take some time as the battery pack shares a lot of structure with the Body in White.

This post has been built based on the support and sponsorship from: About:EnergyAVANT Future MobilityQuarto Technical ServicesTAE Power Solutionsh.e.l group and The Limiting Factor. 

The CAD generated images from BYD show a rectangular flat pack. This approach is a fundamental step to simplifying the design and reducing costs.

The plan view doesn’t give too much away as to how the front and rear crash forces are transferred into the overall structure. In the next image of actual parts there appears to be a substantial cross-car beam in front of the battery pack.

This shows the battery pack being loaded into the Body in White.

Here we can clearly see the lateral structure in the body.

The sills and the fillets that transfer forces in the cross-car beam in front of the battery into the sills are very substantial.

What we don’t see is how the top panel of the battery is fixed to the cross beams.

The mounting points and body structure suggest that there are further opportunities for integration.

The CTB design for the Xiaomi SU7 has a very detailed approach to safety: cells vent downwards, thermal barriers between cells and 3 independent monitors for thermal runaway.

Further Questions

Firstly, let us look at the questions this raises. Most of these questions are around manufacturing and assembly based processes.

  • Manufacturing process
    • when is the body treated and painted?
    • how is the top panel of the battery sealed to the body?
    • how is the sealing of the battery tested?
  • Service
    • is the battery pack extracted leaving the top panel attached to the body?
    • how easy is this pack to recycle?

Module to Body

Perhaps one of the closest in the Cell to Body design approaches is the work done as a global steel industry initiative aimed at developing the world’s first fully autonomous, electric vehicle body structure concept for ride sharing. This project was called Steel E-Motive, the full report covers the design and decision making process in depth [3].

This is module to body in design, but perhaps the biggest element is that the enclosure is the body structure.

We will extend this analysis with more data and examples as they become available. Please do drop us a line if you have further data or examples.

References

  1. BYD SEAL Dynamic and Intelligent, BYD
  2. Car Body Torsional Rigidity – A Comprehensive List, YouWheel
  3. Shaping the future of sustainable mobility through steel innovation, Steel E-Motive

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