Compression of pouch cells, info/data

Batteries, Chargers, and Battery Management Systems.
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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by NullValue » May 13 2021 1:18pm

flippy wrote:
May 13 2021 7:28am
garolittle wrote:
May 13 2021 7:12am
Advantages of pouch cells … Easier assembly. Disadvantage… Lower energy density than 18650 (usually).
and they blow up.

if that is an advantage or not depends on who you ask here, some people actually think that puffing pouches is actually a good thing.... :confused:
I've heard that having the cells designed to be under compression, actually under compression mitigates that to a certain extent.

I have no idea.

But pouch cells are not all the same, and I've seen these dead shorted and they puffed, but did not catch fire.

We shall see.

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by flippy » May 13 2021 1:41pm

NullValue wrote:
May 13 2021 1:18pm
I've heard that having the cells designed to be under compression, actually under compression mitigates that to a certain extent.
yes, pouches are normally under compression to prevent delamination, something that simply is not relevant in 18650 cells as they have their own compression case.
NullValue wrote:
May 13 2021 1:18pm
But pouch cells are not all the same, and I've seen these dead shorted and they puffed, but did not catch fire.
i have seen several car crashes where the car did not catch fire.... :roll:


ps: the reason why phones and laptop do not have compression casings is simple: they are under VERY low loads. automotive ones are under extreme loads but weight is not a big concern (safety is) so they throw in half a ton of steel against the problem. RC gear like drones have stupid weight restrictions so they simply take the risk with plenty of drones ending up as flying balls of fire or batteries tossed in a ditch because they started puffing after a 20C+ discharge run....
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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by garolittle » May 13 2021 2:34pm

flippy wrote:
May 13 2021 1:41pm


ps: the reason why phones and laptop do not have compression casings is simple: they are under VERY low loads. automotive ones are under extreme loads but weight is not a big concern (safety is) so they throw in half a ton of steel against the problem. RC gear like drones have stupid weight restrictions so they simply take the risk with plenty of drones ending up as flying balls of fire or batteries tossed in a ditch because they started puffing after a 20C+ discharge run....
Well said! :-)

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by TrotterBob » May 13 2021 7:17pm

Well manufactured prismatic (pouch) cells should not puff unless they are abused IMHO. I think Lifepo4 cells puff because people stupidly leave them fully charged all the time or abuse them in other ways.

Been in the Rc aircraft hobby for over 10 years and lipo packs dont puff unless they are pushed too hard or kept at full charge. Unless they're one of the cheaper brands that are known for puffing like the turnigy nano techs used to do. Not sure if they've addressed that issue but that was one of the packs to be avoided a few years back.

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by john61ct » May 14 2021 12:49am

Yes compression case is required for LFP pouch or prismatic.

The metal-cased ones need careful dielectric insulating between each cell as well as the floor and sides of the box

with a material that does not compress, or it defeats the purpose.

And a bit of puffing is inevitable for any bank used hard, does not necessarily mean "abused", it's not preventing it with compression that is abusive.

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by NullValue » May 15 2021 4:26am

john61ct wrote:
May 14 2021 12:49am
Yes compression case is required for LFP pouch or prismatic.
The modules currently shipping include the inter cell frames - but not the big heavy casing for the whole pack.

I'll use the frames with studs and nuts with end plates for compression, then build a case to provide rigidity, space for the bms etc.

Are there any published or derived specs on the compression pressure - too much is probably as bad as none.

back on topic...I've come up with a copper welding method that can work for me, though it's probably more fiddling than most people want to do.

I fold the parallel nickel strip where it passes over the cell, so it creates a tab sticking up.
My copper series connector is .1 mm copper folded lengthwise, dropped over the nickel tab, then spot welded from opposite sides.

I do have to make an adjustment on the welder when I go from the nickel strip to the copper nickel sandwich, but I do get strong welds when I do so.

If I make any big packs, I'll need a second welder to avoid that hassle - and new leads - the ones included are short af as well as not ones where the electrodes can be replaced - probably more insulation than copper too.

If I make jigs to do the folds uniformly, it shouldn't add all that much time to assemble.

I can also probably get the added height down - right now, doing it freehand adds about 1/2" top and bottom.

My folded and welded copper bus is effectively 25mm - which is overkill.

Current on series path will only traverse about .2 mm of nickel per cell.

I still have to come up with a easy method to support the pack.

I could probably get by with just an insulation mat, but I don't like the idea of the pack bouncing along supported by the series rails.

I can probably do fins on the bottom of the case so the battery weight rides on those.

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by Frank » May 15 2021 6:25am

Any chance you could post a picture of this? I'm having trouble visualizing it. (thanks)
Frank

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by spinningmagnets » May 15 2021 7:57am

EDIT:
3-8 psi
This is the correct number.

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Are there any published or derived specs on the compression pressure - too much is probably as bad as none
I communicated to a battery engineer from Zero motorcycles and and XXX-PSI is a helpful goal. They used flat pouch Farasis cells, and then potted them after testing. He mentioned anything is better than nothing, and not much of a benefit above that.

It's my understanding that prismatic means a pouch cell that is folded, but I don't have a reliable reference for that from the web. I will look for a good pic to show.

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by garolittle » May 15 2021 12:32pm

spinningmagnets wrote:
May 15 2021 7:57am
.

It's my understanding that prismatic means a pouch cell that is folded, but I don't have a reliable reference for that from the web. I
Interesting. I’ve always wondered about that. I want to research this too. It seems like the cylindrical cells would naturally have more energy density but I guess it depends on the cells.

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by flippy » May 15 2021 12:42pm

garolittle wrote:
May 15 2021 12:32pm
Interesting. I’ve always wondered about that. I want to research this too. It seems like the cylindrical cells would naturally have more energy density but I guess it depends on the cells.
the energy denisty comes from the chemistry used. prismatics as used in phones and laptops are made for capacity, not discharge power. even if you had the high current/lower capacity cemistry in a prismatic you need a compression case in order to prevent the angry pixies from "huffing and puffing" the cell apart. 18650 already have a compression case in their construction. technically a prrismatic is just a house fire in a plastic bag just waiting for someone to poke a hole in it....

in theory prismastics have higher Wh/L but that is due to their shape. cyclindericals have more "wasted" space in a volume. but they dont need compression so that offsets the acutal end result of how much you can fit in a specific volume.
generally 18650 is nearly impossible to beat in real life applications. the newer and bigger sizes are still too expensive and unreliable to be serious competitors.
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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by Frank » May 15 2021 12:56pm

spinningmagnets wrote:
May 15 2021 7:57am
I communicated to a battery engineer from Zero motorcycles and and 100-psi is a helpful goal. They used flat pouch Farasis cells, and then potted them after testing. He mentioned anything is better than nothing, and not much of a benefit above that.
That sounds insane! It implies that a pack with cells measuring 4" X 5" requires compression of 2,000 lbs. I doubt anyone builds packs anywhere near that. What do the automotive OEM's use I wonder?
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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by flippy » May 15 2021 1:08pm

Frank wrote:
May 15 2021 12:56pm
That sounds insane! It implies that a pack with cells measuring 4" X 5" requires compression of 2,000 lbs. I doubt anyone builds packs anywhere near that. What do the automotive OEM's use I wonder?
3/8 allthread does 600lbs, so 4 of them will handle that with plenty of room to spare.
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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by Frank » May 15 2021 1:45pm

I've never heard of anyone cranking up pressure like that on pouch cells.

My drag race (Lonestar) cells are lightly compressed in a polycarbonate enclosure. It's stiff enough to inhibit swelling but nowhere near that kind of pressure.

I wonder if Zero actually does that?
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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by garolittle » May 15 2021 2:31pm

flippy wrote:
May 15 2021 12:42pm
…..
generally 18650 is nearly impossible to beat in real life applications. the newer and bigger sizes are still too expensive and unreliable to be serious competitors.
excellent. Thank you.

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by spinningmagnets » May 15 2021 2:38pm

The Battery engineer I spoke to also said, "anything is better than nothing" so apply a sturdy housing as protection, with a modest amount of compression on the flat sides and you are miles ahead of anyone who is just using duct tape to wrap a pack, with no compression at all...

Start with 10-psi, and see if that reduces voltage sag in your application.

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by flippy » May 15 2021 3:32pm

Frank wrote:
May 15 2021 1:45pm
I've never heard of anyone cranking up pressure like that on pouch cells.
My drag race (Lonestar) cells are lightly compressed in a polycarbonate enclosure. It's stiff enough to inhibit swelling but nowhere near that kind of pressure.
I wonder if Zero actually does that?
it depends on the size of the pouch and the application. if the bags are small and in a high C situation: you want to really crank down on those bags to prevent any gases from forming.

people tend to forget that its force over surface, not just a point load.
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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by serious_sam » May 15 2021 6:03pm

spinningmagnets wrote:
May 15 2021 7:57am
I communicated to a battery engineer from Zero motorcycles and and 100-psi is a helpful goal.
I recall reading a cell datasheet that listed the pressure requirements, and it was nothing near that number. I'm fairly certain it was in the single digits (or maybe low double). Unfortunately I can't locate the datasheet...

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by flippy » May 15 2021 6:37pm

pressure depends on the expected application. the pack of a volt has strech bolts to get the proper pressiure. for context: its the same style of bolt used to keep the cylinder head on the engine. offically they are a 1 shot use but nobody actually replaces them.

fun fact: even with several tons of force those cells in the gen 1 packs still burst into flames pretty often.
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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by serious_sam » May 15 2021 7:57pm

flippy wrote:
May 15 2021 6:37pm
pressure depends on the expected application. the pack of a volt has strech bolts to get the proper pressiure. for context: its the same style of bolt used to keep the cylinder head on the engine. offically they are a 1 shot use but nobody actually replaces them.

fun fact: even with several tons of force those cells in the gen 1 packs still burst into flames pretty often.
Operational preload requirement is different to damage mitigation and containment.

To put it into perspective, "normal" cell puffing is limited to the strength of the bonding around the perimeter of the outer layers. Past that pressure, the cell perimeter will peel open and start to expel the internals. So pack preload/ compression need not exceed that intrinsic limit.

Something else to consider: cells will produce small amounts of gas in normal operation, and if they are held in an extremely rigid enclosure, the pouch may over-pressurise and burst. Properly designed packs include a compression preload, but with a slightly elastic packing (usually a specialised PU foam) that allows for minor expansion of the pouch without significant increase in preload.

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by serious_sam » May 16 2021 3:00am

serious_sam wrote:
May 15 2021 7:57pm
specialised PU foam
norseal pf100 foam.pdf
(772.84 KiB) Downloaded 14 times

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by flippy » May 16 2021 3:33am

serious_sam wrote:
May 15 2021 7:57pm
Something else to consider: cells will produce small amounts of gas in normal operation, and if they are held in an extremely rigid enclosure, the pouch may over-pressurise and burst. Properly designed packs include a compression preload, but with a slightly elastic packing (usually a specialised PU foam) that allows for minor expansion of the pouch without significant increase in preload.
sorry, that logic completly falls apart with cylindrical cells. :wink:

the main reason you want flexibility in your pouch pack is simple: shock absorbtion. large compressed packs are quite sensitive to microshocks wich can mess up the contents of the pouch over time. especially in automotive and industrial applications. i have seen plenty of that from early pack designs from smaller companies that make small runs of application specific vechicles.

this is also why you dont want your pack potted (and a myriad of other reasons).
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Compression of pouch cells, info/data

Post by spinningmagnets » May 16 2021 11:24am

I'm starting this thread to create a reference concerning compression of pouch cells, and moving that existing discussion to here. The benefits and drawbacks of various methods, and the results of various levels of compression.

It is generally agreed upon that some compression is "good", but how much?

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by Hummina Shadeeba » May 16 2021 11:30am

You need the resin in a potting to be soft enough to expand a bit as the cylindrical cells slightly expand with heat. Otherwise potting seems the safest way to deal with a battery in my mind. I can throw my potted battery down the street

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by garolittle » May 16 2021 11:35am

Hummina Shadeeba wrote:
May 16 2021 11:30am
You need the resin in a potting to be soft enough to expand a bit as the cylindrical cells slightly expand with heat. Otherwise potting seems the safest way to deal with a battery in my mind. I can throw my potted battery down the street
Thanks for the info. Do you have a link to the exact putting material you use? If you can share it that would be great. Hope all is well. Thanks. Gary

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Re: Spot Welding Copper Strips to 18650 Battery Cells

Post by flippy » May 16 2021 11:40am

Hummina Shadeeba wrote:
May 16 2021 11:30am
You need the resin in a potting to be soft enough to expand a bit as the cylindrical cells slightly expand with heat. Otherwise potting seems the safest way to deal with a battery in my mind. I can throw my potted battery down the street
potting is like a crumple zone in a car. great for hitting a tree, no so great at keeping the thing together for 200k, its either too brittle to survive the shocks for years or too soft to provide the support you need...

trust me, i tried many variations, even using vacuum chambers to fill batteries with a dozen types of resin. none lasted or geve the performance needed. the only thing it worked for was for diving gear.
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