BMW is proposing a solid-state battery module with deliberately empty spaces between its cell stacks. Those gaps give expanding cells room to move within an arrangement designed to keep them under pressure. The potential benefit for an owner is better resistance to the wear caused by repeated charging and driving.
The design, detailed in a patent filing discovered by Carmoses, places neighboring stacks at different angles within the battery module. One version resembles a checkerboard, with each group of layers turned 90 degrees relative to the next. Some stacks deliberately contain fewer cells, helping create the clearances that let the assembly change size.
Nothing in the filing names a specific model. We used the BMW i7 for these renders because BMW has already built a solid-state demonstrator around the sedan to investigate cell expansion and operating pressure.
Why The Stacks Face Different Ways
A solid-state cell replaces the liquid electrolyte with a solid material that carries ions between the electrodes. Its internal layers need close, consistent contact so those ions can cross the interfaces without excessive resistance. Compression helps maintain that contact.
The difficulty is that active materials expand and contract as the battery charges and discharges. If contact deteriorates, resistance can rise and usable capacity can decline. The pressure system has to accommodate that movement throughout ordinary charging and driving.
BMW’s planar arrangement puts the stacks in alternating orientations, with their broad cell faces pointing in two perpendicular directions. Plates around the module’s perimeter press inward. Neighboring blocks touch, transmitting that force through the array, while rigid compensation plates help distribute the load across their contact surfaces.
The result is compression both through the cell layers and along their plane. BMW proposes that the additional mechanical stress could counter dendrites, metal growths capable of penetrating the solid electrolyte and causing a short circuit. It also aims to reduce relative movement between stacks during cycling.
This still requires external pressure hardware. Turning the cells sideways does not eliminate the job of squeezing them, and BMW supplies no measured improvement in battery life.
Room To Expand
Keeping everything pressed together creates another problem. As adjacent stacks grow, their corners must have somewhere to go. Contact in the wrong place could produce uneven pressure across the assembly.
BMW leaves a small volume open where four stacks meet. That clearance becomes smaller as the surrounding blocks expand and larger as they shrink. The geometry lets neighboring faces continue transmitting force while reserving space for their changing dimensions.
The unequal cell counts serve a related purpose. Expansion through the thickness of a stack can differ from expansion along its layers. Choosing how many cells each block contains, and where it sits, helps accommodate those differences. In one example, a thinner stack helps form the empty space between its neighbors.
Leaving clearances means reserving volume that cannot hold active material, while the surrounding plates also need space. BMW does not quantify the resulting pack energy density, weight or cost, so there is no basis for calculating how much capacity this arrangement might sacrifice or preserve.
A second version extends the idea into three dimensions. A repeating three-by-three-by-three array introduces a third stack orientation, allowing compression through the layers and along both directions within them. The blocks retain their orientations during use.
A Practical Test For Solid-State Batteries
BMW’s existing i7 demonstrator uses Solid Power cells, with expansion, pressure control and temperature among the subjects under investigation. BMW has also brought Samsung SDI into its solid-state collaboration to develop cells for future evaluation vehicles.
Mercedes-Benz has tackled changing cell volume in an EQS test car using a floating cell carrier and pneumatic actuators. The competing approaches underline why changing the chemistry still leaves substantial mechanical work inside the pack.
For BMW’s alternating layout, electrical connections and cooling remain insufficiently explained to judge a complete vehicle installation.
BMW makes a credible case for testing this geometry because it addresses both contact pressure and the movement that disrupts it. Any improvement in capacity retained over repeated cycles has to justify the space and weight of the pressure hardware. For owners keeping an EV for years, that would be a more useful result than a higher energy-density figure measured only at the cell level.
