From cell to pack, and what cell-to-pack took out
A car battery is built in three layers, and each one adds mass that stores no energy. Removing the middle layer is why a pack figure and a cell figure have moved apart.
A battery pack is not one object. It is three nested ones, and knowing which layer a number refers to is the difference between a meaningful comparison and a misleading one.
The three layers
The cell is the electrochemical unit: anode, cathode, separator and electrolyte. It comes as a cylinder, a rectangular prism or a flat pouch, and it is the only part of the battery that stores anything.
The module is a group of cells in a housing, with its own wiring, sensing and sometimes its own cooling. It exists to make the pack serviceable and to contain a failure in one cell.
The pack is every module plus the enclosure, the cooling circuit, the battery management system, the contactors and the crash structure. This is what is bolted into the car.
Why the layers cost you
Everything above the cell adds mass and volume without adding capacity. So the pack's energy density is always lower than the cell's, often by a wide margin, and a cell figure quoted as though it were a pack figure overstates what the car actually carries.
That gap is the reason the intermediate layer came under attack. Cell-to-pack designs delete the module: cells are bonded directly into the pack structure, and in the more aggressive versions the pack becomes part of the car's floor and takes structural load. BYD's Blade design and CATL's cell-to-pack generations are the two most visible examples.
It also moves the cost per kilowatt-hour, which is the figure the whole category is judged on.
The trade is serviceability. A modular pack can have one module replaced; a structural pack is far harder to repair, which shifts cost from manufacturing to the repair and insurance side.
What to check on a spec sheet
When a density figure appears, the first question is which layer it describes. When two batteries are compared, the second question is whether both figures describe the same layer.
A cell-level number against a pack-level number is not a comparison at all, and it is a common enough error in press material to be worth checking every time.
Sources
Terms defined here
- Cell
- The electrochemical unit: anode, cathode, separator and electrolyte in one container. The only part of a battery that stores energy.
- Module
- A group of cells in a shared housing with its own wiring and monitoring. The intermediate layer that cell-to-pack designs remove.
- Gravimetric energy density
- Energy stored per unit mass, in watt-hours per kilogram. Quoted at cell or pack level, and the two differ substantially.
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