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.

Conventional battery pack design has taken the form:

Cell -> Module -> Pack

This means we add material to make the module strong enough to be handled, it needs fixings and space around the modules for build tolerances. Hence, modules have been growing in size:

Cell -> Large Module -> Pack

The 30+ modules in some of the early EV designs has been reduced to 12 as shown in the Mercedes EQS design below or even just 4 in the Tesla Model 3.

C2P or CTP

What CATL and BYD propose is to place the cells directly into the battery pack. Search and you will see the abbreviations C2P and CTP everywhere. The CATL proposal suggests they can achieve >250Wh/kg at pack level. A significant increase on the Tesla 2021 Model 3 171Wh/kg.

CATL claim the following benefits

• 20 to 30% increase in volume utilization of the battery pack
• 40% reduction in the number of parts in a battery pack
• 50% increase in production efficiency

CATL have just announced their next generation of cell to pack design known a Qilin. This has prismatic cells that point downwards with flexible cooling plates between the cells.

This is a significant step change in energy density, however, a battery pack in a vehicle still needs to deliver some fundamental requirements:

• Safety
  • Spacing between cells and modules has traditionally been used to manage cell to cell propagation.
  • Lithium Iron Phosphate (LFP) is a more stable chemistry in cell to cell propagation.
• Mechanics
  • Structural beams within the battery packs help to manage crash loads and durability of the pack and vehicle. This structure needs to be kept or at least replaced.
  • In the BYD Blade design the cell itself adds to the overall stiffness of the pack with the cell being bonded to the top and bottom of the pack case.
• Service
  • Normally a module or modules can be replaced if anything goes wrong.
  • Does this move to a pack replacement approach at service?
  • What is the cost and environmental impact?

This cell to pack approach works very well for the lower cost LFP chemistry that has traditionally not been used in BEV’s as it’s lower energy density has made it difficult to package enough energy. However, the stability of the chemistry means that placing all of the cells next to each other is manageable at a system level. This has given lithium iron phosphate (LFP) a second life in electric vehicles.

The BYD blade cell to pack design is perhaps more interesting as it has been designed by a company that understands vehicle design. Also, this pack design is used in production in the BYD Han.

Next Step

The next step is to remove the battery case and go Cell to Vehicle (C2V or CTV).

References

1. CATL C2P – Robin Zeng: Seize the Certain Opportunity, Keep a Long-term Perspective, and Achieve a Win-win Situation

MG Rubik’s Cube Battery

The SAIC MG Rubik’s Cube battery is an interesting battery pack design. A rectangular and flat section pack designed to work in the Nebular vehicle platform. This platform covers a lot of vehicles from A-class to D-class and saloon, SUV, MPV and sports.

Key Pack Metrics

When designing a battery pack you will always be asked to benchmark it. For this there are a number of key metrics, including: Pack Gravimetric Energy Density and Cell to Pack mass ratio.