The battery becomes part of the frame
A structural battery pack is not a box of cells bolted underneath a car. It is built into the frame itself, so the pack carries some of the load the frame used to carry alone. Two carmakers ship a version of this today, built two different ways. In the Tesla Model Y, cylindrical battery cells are bonded directly to metal castings at the front and back of the car, and the floor pan, the metal sheet that normally forms the floor, is removed entirely. A peer reviewed engineering review states this removes more than 70 parts from the floor structure. A teardown by Munro and Associates found the whole pack held to the body by only 38 bolts.
BYD adds stiffness with a different design
BYD took a different route. In the Seal sedan, the battery is a sandwich, an upper cover, BYD Blade Battery cells, and a tray protecting the underside, according to engineers at BYD. A peer reviewed engineering review, independent of BYD, states this design reaches a torsional stiffness, the resistance of a car body to twisting along its length, above 40,000 newton metres per degree, about 70% higher than a standard car body. Engineers at BYD measured the same design directly and found a 70% increase in torsional stiffness, a 57% increase in bending stiffness, and a pack that fills 66% of the space available inside the car.
Figures are measurements by engineers at BYD for the Seal sedan, published in a peer reviewed SAE International technical paper. The stiffness figures are gains over a standard car body.
Show the numbers
| Torsional stiffness gain | 70 |
| Bending stiffness gain | 57 |
| Pack volume used | 66 |
What the wider technology does
Cell to body integration, the approach BYD used, is not limited to one car. Across the category, the same independent review states it cuts the number of parts in a car by about 40%, fills 60% to 70% of the available pack volume against 40% to 50% for a conventional, module based pack, and cuts overall mass by 10% to 15%. These are figures for the technology in general, not a measurement of one named car, and no source among the 5 states an absolute stiffness figure for the Tesla Model Y to compare against the BYD figure directly.
General figures for cell to body integration as a category, from the independent review, not measurements of one named car. The mass reduction figure is itself a citation inside the review to a separate paper.
Show the numbers
| Cell to body pack volume used | 60% to 70% |
| Conventional pack volume used | 40% to 50% |
| Mass reduction from cell to body | 10% to 15% |
What building it in costs to fix
Bonding the battery into the frame carries a cost the stiffness numbers do not show. The independent review states that a bonded structural pack cannot be taken apart without destroying the joint, and warns this can turn a small underbody impact into a total loss. The Munro and Associates teardown reached a similar verdict on the Tesla pack, describing it as having essentially no repairability once sealed. The Drive reports that replacing the battery in a Tesla Model 3 costs around 16,000 dollars or more, on a part that can already account for nearly half the price of an electric car, and states that insurers are increasingly writing off cars over battery damage that would once have been repaired.