The mechanical design of the cell has to hold all of the elements of the cell together and provide a leak proof container over the lifetime. In the case of venting and thermal runaway this needs to be managed. The mechanics of a cell are quite dependent on cell format (button, cylindrical, pouch, prismatic). The fundamental requirements are:
- maintain sealing of the cell active contents
- locate the positive and negative tabs
- controlled failure during venting
- controlled failure during thermal runaway
The prismatic cell case provides sealing, venting control and some structure, but still requires an external force applied to the largest surfaces to maintain the connection between active layers.
The pouch cell wrap is just a sealing system and all of the mechanical structure needs to be provide by the module or pack. Also, the pressure on the largest surface is required to stop the active layers delaminating over the lifetime. This pressure can range from 0.25 bar up to 5 bar and is dependent on chemistry.
The mechanical design of a battery pack needs to consider every element of the system. You need to look at static stiffness, dynamic stiffness and behaviour of components.
For the design of the 2019 Porsche Taycan the battery housing is a load-bearing component of the body structure:
- truss-design battery frame with multiple subdivisions
- 28 bolts fix the battery to the body structure
- battery contributes ~10% of the vehicle’s overall stiffness
Thermal cycling of components can have a significant impact on the overall mechanical design.
When looking at the mechanics of the pack it is important not to forget the busbars and control cables. These need to be secured and isolated.