Battery Module Housing Mechanical Tolerances and Stack Calculations
Calculated worst-case and RSS mechanical stack tolerances govern inter-cell compression, housing deflection, bolt torque retention, and production assembly yield
Worst Case Stack Calculation
Meaning
Dimensional analysis methods that assume all components are simultaneously at their extreme limit of tolerance provide a conservative estimate of assembly variation. Designers use a worst case stack calculation to guarantee that every single assembly will fit together, regardless of individual part variation. This approach sums the absolute value of all tolerances in a given chain.
It is used when the cost of an assembly failure is high, such as in aerospace systems or medical devices, where even a single failure could be catastrophic. This technique represents the maximum possible dimensional deviation.
Mathematical Model
Arithmetic addition of the maximum limits of each component defines the outer boundary of variation. Performing a worst case stack calculation involves adding together the plus-minus ranges of every dimension in a stack-up path. This method does not consider the statistical improbability of all parts being at their limit at once.
It yields the largest possible clearance requirement.
Pack Assembly
Managing the gaps between cell terminals and busbars is critical for automatic laser welding. Applying a worst case stack calculation ensures that the welding head will always locate the joint within its focus depth. This prevents bad welds and protects the electrical circuit.
It is the safest choice for safety-critical electrical connections.
Sourcing Cost
Demanding that every part combination fits under the most extreme conditions can lead to unnecessarily tight part tolerances. This conservative worst case stack calculation often drives up manufacturing costs by requiring high-precision processes or expensive sorting steps. Sourcing teams must negotiate a balance between design safety and part cost.
Choosing statistical methods can sometimes release these tight requirements.