LFP Cell Clamping Pressure

The LFP cell clamping pressure is important as it keeps the active layers in contact. However, we have to remember that LFP cells expand over their lifetime by 10 to 20% in thickness and during charge / discharge they expand / contract respectively. Hence there is an irreversible and reversible aspect to cell expansion.

Here we want to look at the requirements for Lithium Iron Phosphate chemistry. In the specifications for the EVE LF50F prismatic LFP cell they state that for lifetime testing the cell should be held in a jig.

The specification sheet states that the cell should be clamped between steel or aluminium plates that are >8mm thick and are held together with 4 off M6 bolts.

The force at the start of the test should be 200+/-20kgf (~1.96+/-0.2kN).

This cell has the dimensions 148.3 x 127 x 26.7mm and hence the starting pressure on the face is:

= 1960 / (0.127 x 0.1483) = 104,066Nm^-2 ~ 1 bar

Assuming the force is shared equally between the 4 bolts we can estimate the tightening torque that should be applied to each.

Torque (Nm) = k (coefficient of friction) x Axial Force (N) x Bolt Diameter (m) [1]

in this case, bolt torque (Nm) = 0.2 x 490N x 0.006m = 0.588Nm

Over the lifetime of the cell it is expected to expand by 10 to 20% and hence in this case that is 2.67mm to 5.34mm and hence we would need a foam that can absorb that amount of travel and remain linear in terms of the pressure it can apply.

A compression pad sits between the cells and manages the breathing and lifetime expansion of the cell. The challenge is specifying this foam in terms of rate, thickness and durability.

Lets look at a polyurethane micro-cellular foam [2]. At the starting pressure for the cell of 1 bar (104kPa) the strain would be ~65% and hence the foam thickness would have been compressed to 3mm x 0.65 = 1.95mm

This doesn’t leave us with much room for expansion over the lifetime of the cell.

This post has been built based on the support of and sponsorship from: Eatron Technologies and About:Energy

Another option is to start the system with a lower pre-set. If we started with a condition where the foam was just in contact then at the mid-lifetime condition the cell would have expanded ~1.35mm and hence for a 3mm thick foam pad this would be a strain of 45%. The result based on the above graph would be a pressure ~45kPa on the face of the cell. Below that stated as the optimal for lifetime, but what does that mean?

The other option is to run sensitivity studies on cell face pressure vs. time vs. life in context of the pack being designed. The irreversible EOL cell swelling force is not just a function of the thermal and electrical conditions but also the mechanical constraint and boundary conditions of the cell. Cell suppliers may only provide one or two test conditions with a certain end plate stiffness (due to time it takes to show life degradation plots) and this may lead you down the wrong path – another reason why cell characterisation in pack context is so important for cell to pack designs. Optimising life needs to be balanced against other objectives of the pack like volumetric energy density or cost.

A compressive interstitial foam seems the most optimal for life but I’ve found this does constrain options that might be better depending on your pack requirements. For example, if removing foams ensures better thermal transfer and minimising dT across the pack, it may in fact work out in your favour to increase cell pressure toward EOL

Nick Milanovic

References

1. Bolt Torque Calculator, The Engineering Toolbox
2. Norseal® PF100 Series, Micro-Cellular Polyurethane Foams, Saint-Gobain

See also: Cell Electrode Pressure and Expansion