The secret to compression

curiouscarbon

Science Penguin
I wish I had access to some of the high resolution 3D scanners I had before the pandemic started. Take one of those to the side of an EVE cell and you’ll see that the “flat” sides are really very hilly when you look microscopically.

When discharged, the highest ridges are probably at the edges of the cell, meaning the center is concave-ish. When charged, you’ll have the characteristic swelling of a lithium cell.

clamp a cell between two hard, flat surfaces, ok - *maybe* you can get a reasonable pressure distribution that way. Definitely not as consistent as you could with a compression pad.

but clamp multiple cells together under that same 11 psi and you have a very different pressure profile between cells - one that imparts high pressure (>11 psi) at the cell edges when discharged and high pressure at the center when charged (again, larger than the 11psi design value).

adding a pad between cells with the right composition can ensure that you have that 11psi of pressure across the entire face of the cell.
Gosh darn it now you make me want to evaluate a 280/310Ah cell free standing with interval photogrammetric reconstruction of cell face geometry. Spray paint can create speckle pattern that allow very easy reconstruction with even cell phone camera.

Or print out perlin noise:
1631109934789.png

And affix it with adhesive or double sided tape to cell wall.

Go through a charge + discharge cycle. No compression, no enclosure. Single cell.

Every ~30 Ah (~10% SOC) increment, take photos of cell from different angles (4-12 angles should be enough.)

Process each set of ~10 images into a 3D model of the cell face for each state of charge. This could be very valuable data for community.

Wow I don't have time to do this but it would be fun!

Instead got 100Ah Frey cells and am only constraining the cells, not compressing the aluminum shell face.
 

curiouscarbon

Science Penguin
Ok. Hear me out.

People commonly maintain air pressure in car and other vehicle tires, and those pressures can be up to 50 psi or more.

Why not place a large heavy duty whoopie cushion air bladder on both broad sides of the battery assembly??

Inflate air bladder to 12 psi. Check it now and then. Deflate to do maintenance.

Monitor the air pressure in realtime and graph it with SOC or voltage!

Anyone know of a source of inner tube in the shape of an LF280 cell?

Is there a large drawback to this that I am missing? It seems kind of amazing to me. I'm not using 280/310Ah cells in any projects due to the cell geometry changing issue to engineer around. The engineered foam stuff seems pretty awesome for inbetween mutual cell interfaces. These air inflated things would be only for the two end stops.
 

Short_Shot

Photon Sorcerer
Try as you might, I bet it won't hold air long term.

And again that's literally the whole point of the poron. I'm confused as to why people are so against using it for maintaining pressure.
 

Shawn Hayes

Learning Everyday
Would not changes in air temperature change pressure?
Very possibly. The best way to control that is to remove as much humidity from the air as possible. The higher the humidity the more likely air pressure will change with temperature increase. Use nitrogen if you can? It’s dry. We used both nitrogen and argon (not together) in race tires due to its lack of humidity to help control air pressure as tire temperature increased during a race.
However I would suggest that if you have your batteries in a controlled environment with a reasonably stable temperature, normal room air would be sufficient. You only have to worry about air pressure changing with large temperature swings.
Disclaimer: I’m not saying that this will work for this application, I can only tell you how it worked for us in our application for race car tires. Totally different application here.
 

curiouscarbon

Science Penguin
Would not changes in air temperature change pressure?
Good point. Drawback.

With active thermal regulation of big LiFePO4 cell build, that specific downside might be mitigated..

Thank you for pointing out the temperature vs pressure relationship of air. That had not immediately occurred to me.

But now it has me thinking... what is the pressure needs of the cell at different temperatures and will the air expanding with higher temperatures and contracting with lower temperatures result in a net worsening of the compression force profile over SOC and time of day?

Does anyone know of a USA warehouse source of a single LF280 cell?
 

thegoogler

Solar Enthusiast
Ok. Hear me out.

People commonly maintain air pressure in car and other vehicle tires, and those pressures can be up to 50 psi or more.

Why not place a large heavy duty whoopie cushion air bladder on both broad sides of the battery assembly??

Inflate air bladder to 12 psi. Check it now and then. Deflate to do maintenance.

Monitor the air pressure in realtime and graph it with SOC or voltage!

Anyone know of a source of inner tube in the shape of an LF280 cell?

Is there a large drawback to this that I am missing? It seems kind of amazing to me. I'm not using 280/310Ah cells in any projects due to the cell geometry changing issue to engineer around. The engineered foam stuff seems pretty awesome for inbetween mutual cell interfaces. These air inflated things would be only for the two end stops.
Might work for a stationary setup, assuming it never leaks air and the temperature stays constant. Wouldn't work for mobile installations like an RV, particularly if you go from sea level into the mountains. I can tell you what happens to a sealed bag of popcorn at 10,000'...

1632159543300.png
 

curiouscarbon

Science Penguin
Very possibly. The best way to control that is to remove as much humidity from the air as possible. The higher the humidity the more likely air pressure will change with temperature increase. Use nitrogen if you can? It’s dry. We used both nitrogen and argon (not together) in race tires due to its lack of humidity to help control air pressure as tire temperature increased during a race.
However I would suggest that if you have your batteries in a controlled environment with a reasonably stable temperature, normal room air would be sufficient. You only have to worry about air pressure changing with large temperature swings.
Disclaimer: I’m not saying that this will work for this application, I can only tell you how it worked for us in our application for race car tires. Totally different application here.
Thank you for your insights. Good point about using gaseous mixtures other than ambient air (or desiccated ambient air). Need to review my Boyle's law and properties of various commonly available gaseous mixtures.. vehicle tire pressures are often observed to be lower in morning when morning is cooler than daytime.

Will be sure to not misunderstand your experience as endorsing the efficacy of this idea 🤓

Might work for a stationary setup, assuming it never leaks air and the temperature stays constant. Wouldn't work for mobile installations like an RV, particularly if you go from sea level into the mountains. I can tell you what happens to a sealed bag of popcorn at 10,000'...

View attachment 65499
Good point! Doh! POP! 🎊

Air Bladder Cell Compression Tradeoffs​

Imagined Drawbacks
Imagined Benefits
Leak over time (loss of pressure)
Affordable (air tightness is mostly solved)
Leak can cause physical damage (no cushion, sudden movement)
Adjustable (air pressure can be changed at any time)
Puncture can cause leak (foam plastic wood and metal does not do this)
Provides way of assessing cell swelling in real time (measure pressure)
Altitude change = pressure change (vessel overpressure pop)
Easy to replace if damaged (depressurize end vessels)
Can rupture from overpressure (bag of popcorn)
Provides way of removing cells (depressurize end vessels)
Added complexity (got no time for that)
Possible to regulate pressure (offset leak and altitude concern)

Thank you everyone for the thoughts about this concept of battery compression! I think it's flawed! Haha

In order to cover all the bases, I would need:
  • The bag material to be very impermeable to the gas inside (achievable, materials science has got us)
  • No major leaks (achievable, epoxy and patience)
  • Pressure sensor to monitor cell expansion (can be done other ways)
  • Electric pump ready to refill it, preferably
I'm too lazy to refill it manually. maybe it could even maintain 12psi, with a bleed vent valve electronically controlled alongside a pump electronically controlled.

Okay, so... gonna have to add a benefit (drawback?) of this type of system. It could sound totally rad as it bleeds air off.

edit: added drawback
 
Last edited:

Short_Shot

Photon Sorcerer
Thank you for your insights. Good point about using gaseous mixtures other than ambient air (or desiccated ambient air). Need to review my Boyle's law and properties of various commonly available gaseous mixtures.. vehicle tire pressures are often observed to be lower in morning when morning is cooler than daytime.

Will be sure to not misunderstand your experience as endorsing the efficacy of this idea 🤓


Good point! Doh! POP! 🎊

Air Bladder Cell Compression Tradeoffs​

Imagined Drawbacks
Imagined Benefits
Leak over time (loss of pressure)
Affordable (air tightness is mostly solved)
Leak can cause physical damage (no cushion, sudden movement)
Adjustable (air pressure can be changed at any time)
Puncture can cause leak (foam plastic wood and metal does not do this)
Provides way of assessing cell swelling in real time (measure pressure)
Altitude change = pressure change (vessel overpressure pop)
Easy to replace if damaged (depressurize end vessels)
Can rupture from overpressure (bag of popcorn)
Provides way of removing cells (depressurize end vessels)
Possible to regulate pressure (offset leak and altitude concern)

Thank you everyone for the thoughts about this concept of battery compression! I think it's flawed! Haha

In order to cover all the bases, I would need:
  • The bag material to be very impermeable to the gas inside (achievable, materials science has got us)
  • No major leaks (achievable, epoxy and patience)
  • Pressure sensor to monitor cell expansion (can be done other ways)
  • Electric pump ready to refill it, preferably
I'm too lazy to refill it manually. maybe it could even maintain 12psi, with a bleed vent valve electronically controlled alongside a pump electronically controlled.

Okay, so... gonna have to add a benefit (drawback?) of this type of system. It could sound totally rad as it bleeds air off.
If you have active pressure control then you should shoot for the more optimal pressure of what, 15 psi?

Rather than 12 then letting it increase as it charges.
 

curiouscarbon

Science Penguin
If you have active pressure control then you should shoot for the more optimal pressure of what, 15 psi?

Rather than 12 then letting it increase as it charges.

Good point about actively regulating it to an optimal pressure.

Still learning about "what is the darn force to apply?!" (think the 12 psi in my head is just being constantly referenced from some EVE sheet that might be really out of date or I misunderstood 😁)

Some trucks that cool people drive have active pressure regulation mechanisms on their tires!! Can you imagine that?? So fricking cool. Not just TPMS : Tire Pressure Monitoring System. Which is also really cool by itself.

If a gigantic awesome truck can monitor and refill and equalize the pressure of tires while they spin, it fills me with the inspiration to believe it can be done 🤓

I heard some call it ATIS : Automatic Tire Inflation System

The main incentive for fleets to buy TPMS and ATIS technology is fuel savings by maintaining tire pressures. Extending tire life and reducing breakdowns are secondary paybacks.

In the case of battery cell compression, cycle count increase may be analogous to "fuel savings" (electrolyte savings?). Stretching the metaphor maybe.. "Extending cell life"
 

noenegdod

Solar Addict
If you have active pressure control then you should shoot for the more optimal pressure of what, 15 psi?

Rather than 12 then letting it increase as it charges.
A simple relief valve and a pressure regulator would address pressure control, but pressure swings would still be kind of large unless you spent on high quality high accuracy regulators.

Edit: If you were to use mechanical equipment. If you went with electronic pressure control it could be very accurate.
 

thegoogler

Solar Enthusiast
I'm too lazy to refill it manually. maybe it could even maintain 12psi, with a bleed vent valve electronically controlled alongside a pump electronically controlled.

Okay, so... gonna have to add a benefit (drawback?) of this type of system. It could sound totally rad as it bleeds air off.
edit: added drawback
AirLift WirelessOne does this for vehicles with air suspension. I can't recall if that model automatically inflates/deflates for you to maintain a specific PSI or if there's another model I'm thinking of, but the general principle exists, yes. Downsides I've heard:
  1. If it's pumping humid air into the bags you can get condensation and eventually they will start to retain moisture. So for our purpose there might be some maintenance required to ensure they don't fill up (and if they do get some moisture that it's not enough that a leak in the bag could cause you problems
  2. If your air system isn't tight the compressor runs a lot to maintain the right pressure. They're generally noisy when they run.
  3. Relatively expensive as compared to a bicycle pump for the initial fill (though that's not automatic)
My biggest concern would be strength and maintenance of this type of system. Springs, foam sheets, etc are pretty durable. Inner tubes are generally not as robust, particularly when dealing with rectangular batteries with sharp edges.

That said it's a novel idea, and could be fun to try.
 

curiouscarbon

Science Penguin
A simple relief valve and a pressure regulator would address pressure control, but pressure swings would still be kind of large unless you spent on high quality high accuracy regulators.
Say I had a simple DC solenoid air bleed valve that doesn't leak...

And some cheap air pump? Maybe with another DC solenoid valve?

And this 25psi ported pressure sensor, and it reads 24-bit values out (2^24=16777216 max reading value) compensated over temperature range 0-50degC, total error band near ±1.25% so like ~1.5e-6 PSI resolution or one millionth of a PSI granularity of observation..

Do you suppose this is a decent starting point?

Combined with a microcontroller that activates the bleed valve when reading goes over set threshold, and activate pump when under set threshold?
16476-SparkFun_Qwiic_MicroPressure_Sensor-01.jpg



Kinda spitballing implementation ideas, I have some of those ported sensors and silicone tubing around. Seems fun to hack on this.
 

curiouscarbon

Science Penguin
AirLift WirelessOne does this for vehicles with air suspension. I can't recall if that model automatically inflates/deflates for you to maintain a specific PSI or if there's another model I'm thinking of, but the general principle exists, yes. Downsides I've heard:
  1. If it's pumping humid air into the bags you can get condensation and eventually they will start to retain moisture. So for our purpose there might be some maintenance required to ensure they don't fill up (and if they do get some moisture that it's not enough that a leak in the bag could cause you problems
  2. If your air system isn't tight the compressor runs a lot to maintain the right pressure. They're generally noisy when they run.
  3. Relatively expensive as compared to a bicycle pump for the initial fill (though that's not automatic)
My biggest concern would be strength and maintenance of this type of system. Springs, foam sheets, etc are pretty durable. Inner tubes are generally not as robust, particularly when dealing with rectangular batteries with sharp edges.

That said it's a novel idea, and could be fun to try.
Great breakdown of tradeoffs, thank you for that insight

I think foam and springs are the way to go for me.

Inner tubes are less robust and the issues with gasous mixtures and expansion and moisture and maintentance and noise and energy consumption are pretty absent with foam and springs.
 

Professor Farnsworth

𝕃𝕠𝕘𝕚𝕔𝕒𝕝 ℂ𝕠𝕟𝕤𝕖𝕢𝕦𝕖𝕟𝕔𝕖
Seems like you would want a different pressure depending on the SOC, maybe computer controlled from 3psi to 12 psi

What PSI does the vent open at, 12 psi could be a safe limit
 

curiouscarbon

Science Penguin
What PSI does the vent open at, 12 psi could be a safe limit
Much higher than what is internally incurred when 12psi is applied across the entirety of both broad faces of a LF280/310 cell, from what I can gather.

Seen photos of cells that have bloated but not vented, and I can only imagine that the forces acting from inside the cell pushing outwards exceeded 12psi by at least 2-3x factor.
 
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