PSI is not a meaningful spring rate. Springs are rated using force per distance (lb/in or kg/mm) and free length.

Here is a design process I came up with that should work. I am not an ME so if someone wants to double check my work, please do so.

You have to know 3 things going in:

Here is what we think we know:

Spring free length (length where force = 0kg) = 24.4mm

I am using 5/16" threaded rod so the spring ID should be > 8.5mm

Looking at the Lee Spring on-line catalog, one spring that meets these requirement is LC 049E 09 S

Here is a design process I came up with that should work. I am not an ME so if someone wants to double check my work, please do so.

You have to know 3 things going in:

- The compliance distance the spring has to work over. This is how much a cell is going to expand and contract as you go from 0% to 100% state of charge over the operating temperature range multiplied by how many cells you are compressing in a stack.
- The minimum and maximum force at each end of the compliance range.
- Minimum Spring ID (diameter of the threaded rod plus some clearance value)

Here is what we think we know:

- The optimal compressive force on the cell is 12 PSI with an acceptable range of 8 PSI to 16 PSI. For my design I am going reduce the acceptable pressure range to 10 to 14 PSI.
- Cell Dimensions (measured from my cells) = 173mm x 200mm = 0.0346m^2
- Compliance (how much the cells expand from 0% to 100% SOC) = 1mm per cell x 4 cells = 4mm total
- The individual spring force is the total force divided by the number of springs (4)

- 14 PSI = 9742kg/m^2 * 0.0346m^2 / 4 = 85.1kg
- 12 PSI = 8436kg/m^2 * 0.0346m^2 / 4 = 73.0kg
- 10 PSI = 7030kg/m^2 * 0.0336m^2 / 4 = 60.8kg

- Spring Rate = (85.1kg - 60.8kg) / 4mm = 6kg/mm

Spring free length (length where force = 0kg) = 24.4mm

I am using 5/16" threaded rod so the spring ID should be > 8.5mm

Looking at the Lee Spring on-line catalog, one spring that meets these requirement is LC 049E 09 S

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