Electronic – How much do lithium polymer batteries expand in volume

batterieschargedischargelithium ionlithium-poly

I'm designing a device with a small lithium polymer battery (4x12x30 mm, 120 mA-h). Looks like this:

enter image description here

I've heard that there is a rule of thumb that the space left for the battery in a case should be around 10% larger (I suppose primarily in thickness) than the nominal dimensions to allow for expansion. Extra 10% seems quite large.

  1. Where does this rule of thumb come from? Is there any official recommendation for how large a compartment to put lithium polymer cells in?

  2. How much do these batteries expand and shrink in normal use? For example during charging/discharging cycles, temperature cycles over normal temperature range (-20C to 60C), etc.

  3. What happens if the battery is in a rigid compartment in the case of malfunction? It's pretty common to have batteries "puff out" if they get shorted internally, but what happens if the battery is in a compartment that prevents expansion? (Assume the compartment is strong enough to withstand the pressure build up) Does the pressure/walls make the short worse, or better?

Best Answer

This paper measured a cell, they reported a max of 0.5% expansion over a charge cycle:

enter image description here Source: Expansion of Lithium Ion Pouch Cell Batteries: Observations from Neutron Imaging Figure 7

Over the lifetime of the battery, it swelled more than 1.5%

enter image description here
Source: Expansion of Lithium Ion Pouch Cell Batteries: Observations from Neutron Imaging Figure 9

One could use these numbers for a baseline, but it's not that hard to make these measurements to a reasonable degree. Since batteries are made with different anode\cathode and electrolyte combinations that vary from manufacturer to manufacturer, it would be wise to consult the manufacturer on swelling or measure it.

If you really want to find out how much your battery is swelling, get a micrometer and measure it discharged, then measure it fully charged and see what the difference is. Measure the cell under maximum discharge, because the cells swell more with thermal expansion. Give your self additional margin to account for differences in batteries and manufacturing tolerances.

Cells also bulge more in the middle if they are heated then the outside. So make sure you measure the middle of the cell.

enter image description here
Source: https://www.researchgate.net/publication/283720424_A_novel_thermal_swelling_model_for_a_rechargeable_lithium-ion_battery_cell