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
Root Sum Square Stack
Meaning
Statistical tolerance analysis techniques calculate the expected variation of an assembly by combining individual component variations quadratically. Manufacturing engineers apply the root sum square stack to predict the assembly yield of a battery pack. This method assumes that the deviations of individual parts are normally distributed and independent.
It allows for looser component tolerances while maintaining high assembly quality, which directly reduces the cost of precision machining. This method is the preferred option for high-volume automotive packs.
Statistical Basis
Standard probability theory holds that the variance of the sum of independent variables equals the sum of their individual variances. Using the root sum square stack involves squaring the tolerance of each part, summing these squares, and taking the square root of the total. This calculation yields a much smaller assembly tolerance than a simple addition of limits.
Practical Application
Designing battery modules requires balancing the variations of cells and end plates. Applying the root sum square stack allows designers to specify cost-effective tolerances for each component. Sourcing teams can then use cheaper manufacturing processes.
This approach lowers the total production cost without compromising the mechanical fit.
Limit Condition
Correct results depend on the assumption that manufacturing processes are centered and stable. When a supplier’s process experiences a mean shift or has a non-normal distribution, the root sum square stack underestimates the defect rate. Quality teams must monitor process capability index values to ensure the validity of this statistical model.
Process drift requires immediate adjustment.