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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?

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    \$\begingroup\$ In the papers I read, it was the anode or cathode that swelled 10%, so that's where I think 10% comes from. \$\endgroup\$ – laptop2d Feb 2 at 17:46
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    \$\begingroup\$ @mkeith I think the most important thing is to do the swell testing yourself, or get it from the manufacturer. It's not hard to do, just need a set of micrometers and a way to charge and discharge the cell \$\endgroup\$ – laptop2d Feb 2 at 21:18
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    \$\begingroup\$ As far as puffed up batteries, the battery must be designed to either expand or vent. Bag type batteries like the one in your photo expand, hard shells like 18650s vent. A failure point must be provided in order to prevent catastrophic failure, so if you design something with a bag type battery, if possible, put failure points in the casing to ensure it will pop/split open if the battery bag expands, and allow no nearby shapes that will puncture the expanding battery. Only if it is not possible to have the case expand, a failure point must be provided to ensure the case will vent. \$\endgroup\$ – K H Feb 2 at 21:35
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    \$\begingroup\$ Most of this is basically preventing a fire from becoming an explosion. If a battery type is used that must vent, IE 18650, then the casing must be able to vent or expand as well. A weak point in plastic, a rubber/soft glue plug or snap apart tensioned hooks are all acceptable. Anecdotally, this year I ended up using my Samsung Galaxy S6 until the battery could hold only enough charge to power the phone for about 2 minutes. I use Lifeproof cases, which are waterproof, so totally sealed and the front and back clip and hold together with quite significant force. \$\endgroup\$ – K H Feb 2 at 21:46
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    \$\begingroup\$ When I finally got around to replacing it and opened up the outer case, without the strong pressure of it holding the phone together, the front and back covers were both popped right off the frame by the expanding battery(designed failure point), so I can provide a pic or two for your question if you like. Note that despite the compression, the waterproof case does have adequate failure points, as if the battery did vent (expand so much as to pop), the usb port cover, ear phone jack cover and assorted thin membranes would open/fail, preventing an explosion. \$\endgroup\$ – K H Feb 2 at 21:46
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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

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Although I cannot compete with the wealth of detail given in previous answers, I think it might be helpful to give you an example why this much extra space is used for a good reason.

A Lipo does not only swell during normal operation with temperature and charging/discharging, but also when it ages. Have a look for electrolyte decomposition to find more about this phenomenon, but ultimately it breaks down to the creation of gases (mostly oxygen) inside the LiPo.
Just to give you an idea about what you can expect: A frequently used 3.5 year old battery I measured during writing this expanded from 25mm (according to the reseller, couldn't find a datasheet) to almost 32mm, which is more than 25%! I guess the datasheet values are not as optimistic as the reseller's ones, but still this is a substancial increase, which should be taken into account while designing your product.

If the battery has no room to swell, it will become a possible risk of fire and - in the worst case - even explosion. See the comments on below your question, this has been described by K H in detail.

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  • \$\begingroup\$ Hmm, what type of battery was it? 25mm is a lot for a pouch cell, do you mean 2.5mm? Does it seem puffed? I think the typical tolerance on cell thickness is almost 0.5 mm. \$\endgroup\$ – Alex I Feb 7 at 9:45
  • \$\begingroup\$ I was talking about a 4S 2200mAh LiPo battery used for RC cars and multicopters, the big variant of your small single cell (the 25mm are intended). It is not puffed yet, but I would not recommend using it anymore. I dug a bit deeper and found an old 1S battery by now, which is quite similar to yours. It looks swollen as well, but not as horrible as the big one. Unfortunately I can't tell you numerically, since I don't know how thick it was originally. \$\endgroup\$ – K. Krull Feb 7 at 12:45

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