A123 20Ah Pressure Thoughts--and bits for experiments.

[dnmun]

now i wanna find compliant pads. i thought it was interesting how much the rate of charge affects capacity and aging.

they use the 3.60V as the maximum charged voltage but for all of us there has been an understanding from other sources like battery university that 3.65v was fully charged and then we learned from you last year about the lithium plating out above 3.60V.

do you know it the lower grade pouch cells such as the one li ping uses are subject to the same effects?

i would expect so but he would use a very high charger voltage for his packs to ensure that the packs would balance faster and when they became unbalanced.

ditto for the Vpower 18650 cells.

 

[megacycle]

I didn't expect it either

Gold mate, how long have we being looking for this info.
No guessing anymore.

now i wanna find compliant pads.
i thought it was interesting how much the rate of charge affects capacity and aging

Poron 4701 urethane foam @ 1.19mm was the test material used.
Intresting they are using kWh throughput as the measure, totally understandable.
Looks like down to 90% capacity/ approx 2500 cycle/ approx 3C.
Seems to fit with 'current' knowledge, above 3C mark longevity is taking a dive.
(going on 300kWh mark and i'm estimating 60Wh charge/discharge cell)

 

[wb9k]

now i wanna find compliant pads. i thought it was interesting how much the rate of charge affects capacity and aging.

they use the 3.60V as the maximum charged voltage but for all of us there has been an understanding from other sources like battery university that 3.65v was fully charged and then we learned from you last year about the lithium plating out above 3.60V.

do you know it the lower grade pouch cells such as the one li ping uses are subject to the same effects?

i would expect so but he would use a very high charger voltage for his packs to ensure that the packs would balance faster and when they became unbalanced.

ditto for the Vpower 18650 cells.

As far as I know, this is all endemic to the chemistry, so grading should not affect the voltage threshold numbers. Same for cylindrical cells...packaging is irrelevant. Using a very high charger voltage to speed balancing is not a recommended practice. Little by little, this eats up calendar life by plating Li onto the cathode. Li that has plated onto the cathode is afterward permanently unavailable for energy transfer in the cell. Likewise, the electrode area plated with Li can no longer accept Li during charging. Thus, capacity loss (from loss of the "cycleable" Li) and impedance rise (from loss of usable surface area on the cathodes) both result.

 

[liveforphysics]

other sources like battery university that 3.65v was fully charged and then we learned from you last year about the lithium plating out above 3.60V.

If it's not info about Lead acid or NiCd or NiMH, I would not recommend trusting it from 'battery university'. Historically it's been impressively inaccurate for most all matters regarding LIB's.

That said, at all levels of industry folks are making new discoveries everyday, and nobody has LIB's understood that well yet. Unfortunately this doesn't stop hundreds of white papers and presentation videos from being produced with people claiming they have everything figured out already.

 

[wb9k]

For the record, I don't think I have it all figured out by a longshot. Tomorrow sure would be boring if I did. However, I've had several cell scientists and engineers tell me that A123 cells are absolutely plating Li if the terminal voltage is held (C.V.'d) above 3.60V after charge current drops off. They "are rated" to go as high as 3.8V for up to 10 seconds at a time, at high charge rates---as long as they settle below 3.6 you're considered to be OK.

 

[whatever]

with regard to using atmospheric air pressure for compression, if a plastic bag could be sealed along the top of the cell
( in pink) it would leave the tabs exposed for electrical connections, and should give 4-12psi once the air is sucked out.
Would remove the need for steel straps etc. The place to suck out the air should be off the face of the cell ( black dot in pic)

 

[dnmun]

you may wanna reconsider the logic. the pouch is already currently exposed to atmospheric pressure on all faces.

 

[liveforphysics]

with regard to using atmospheric air pressure for compression, if a plastic bag could be sealed along the top of the cell
( in pink) it would leave the tabs exposed for electrical connections, and should give 4-12psi once the air is sucked out.
Would remove the need for steel straps etc. The place to suck out the air should be off the face of the cell ( black dot in pic)

Just before final sealing after formation, each cell is vacuum sealed inside it's foil pouch.

 

[whatever]

Vacuum bagging is common method for compressing items, the logic should be ok.

The cells internal pressure I would think may vary due to gas formation internally, even a tiny amount of H2 gas formation would change internal pressure , even if vacuumed sealed initially. That being said it might be one reason for soft and hard cells.

 

[999zip999]

I have a vacuum for casting it draws down to 29 something. It even boils water at room temperature. What I didn't like was putting the pressure around the druttleght build with aluminum-plastic blocks for tabs and with long bolted rod bolts . I can make a plexiglass clear box for that area. Then the wire exit ? How much persure at 29 of vacum could there be ? Pic later.
Or what kind of vacuum material is out there ?
Maybe just keep to the mega-wrap method. With 580 low cycles so far. Bulk charging to 3.45v-3.5v as two 12s packs. But needs rebuild. So thinking of improvements.
Sorry phone texting in a car.

 

[friendly1uk]

with regard to using atmospheric air pressure for compression, if a plastic bag could be sealed along the top of the cell
( in pink) it would leave the tabs exposed for electrical connections, and should give 4-12psi once the air is sucked out.
Would remove the need for steel straps etc. The place to suck out the air should be off the face of the cell ( black dot in pic)

Just before final sealing after formation, each cell is vacuum sealed inside it's foil pouch.

If that is to add atmospheric compression, then vac bagging the lot would be cancelling out work already done. Pound for pound.

 

[999zip999]

Canceling the vacum in the cell ?

 

[dnmun]

try using a regular vacuum chamber. put the pouch inside the chamber and then pump it down with a good vacuum pump like a turbomolecular pump we would use in our lab. see how low a vacuum you can pump on the volume surrounding the pouch and then discharge the pouch rapidly, maybe up to 20C if you can.

we would all like to watch so if you can do the video stuff then record it and post up so we can see how the pouches would work in a real vacuum like outer space.

 

[wb9k]

It would tend to pull outwardly on the cell pouch, making it harder to suppress gas formation.

I thought we had this discussion here already, maybe I'm mistaken. There are many reasons the approach is undesirable, and at the end of the day, it flat out just doesn't work.

 

[dnmun]

it would be neat to watch imo. but i doubt if these people have access to a real vacuum chamber anyway.

 

[wb9k]

it would be neat to watch imo. but i doubt if these people have access to a real vacuum chamber anyway.

It would be neat...and I bet you could easily show an elevated tendency for the cell to swell under vacuum. The MA guys used to use vacuum chambers to test their calculations on cylindrical vent mechanisms, I believe. I don't have immediate access to one myself, though I could probably find one if I dug around Livonia enough. Having the time is another matter.

 

[dnmun]

sorry, just being silly i guess. just do not see how they don't see the logic of it at all. i am sure the solvents would be vapor long before it got low enuff anyway.

 

[999zip999]

Ok ok how about a vacum sleeve for compressing around the cells. If needed I would use a mechanical clamping. Not just Megawrap. But do have 1/2 hp vacum or so. But don't have time to pay with your vacum idea's. Megawrap.

 

[wb9k]

You need super-rigid end plates to keep cells from warping and bending under the pressure. As others have pointed out, the bands act as a spring. You need this, along with compressible media interspersed throughout the module, to apply pressure evenly across the pack. Is your plastic wrap going to spring back after stretching force is applied to it by the cells? How would you maintain a square geometry? Seems like this approach will make the lateral surface area of the cells to accommodate (and thus, pack size) that much larger...a hard sell I would think, even if the approach could be made to work.

 

[liveforphysics]

The vacuum chamber test is a standard UN38.8 shipping test, and believe it or not, even hobby RC cells do fine in it. I've tested perhaps 50 different types of cells in vacuum with no issues.

What will give you issues is anything that causes uneven cell clamp pressures. If you put some big band clamps on your end plates, and you don't have even pressure distribution, or it drifts from being even pressure distribution, the cell doesn't share current evenly through it's active materials if you don't have even pressure causing even distributed cell impedance through the stacks of active material.

Even clamp load at ~3-5psi or don't try as it will do more harm than good.

 

[megacycle]

The Poron 4701-30-25047 found on Alibaba, made by Rogers Corp.
http://m.alibaba.com/product/1022744975/4701-30-25037-04P-PORON-ROGER.html http://m.alibaba.com/product/1022744975/4701-30-25037-04P-PORON-ROGER/specifications.html

 

[999zip999]

I have A123 end plates from w9bk, but to heavy for a bike. Maybe the bands if I had the means or care at 40amp. draw.

 

[wb9k]

The vacuum chamber test is a standard UN38.8 shipping test, and believe it or not, even hobby RC cells do fine in it. I've tested perhaps 50 different types of cells in vacuum with no issues.

What will give you issues is anything that causes uneven cell clamp pressures. If you put some big band clamps on your end plates, and you don't have even pressure distribution, or it drifts from being even pressure distribution, the cell doesn't share current evenly through it's active materials if you don't have even pressure causing even distributed cell impedance through the stacks of active material.

Even clamp load at ~3-5psi or don't try as it will do more harm than good.

What are the test conditions for that test? I would only expect the cell to swell more easily than without the vacuum---I still expect you would have to be loading the cell pretty heavily. A properly sealed cell won't just swell up, sitting there in a vacuum doing nothing.

Keeping the pressure even is indeed important, but I'm not sure I would say NO compression is better than uneven compression, unless maybe the pressure imbalance was really severe.

 

[liveforphysics]

The vacuum chamber test is a standard UN38.8 shipping test, and believe it or not, even hobby RC cells do fine in it. I've tested perhaps 50 different types of cells in vacuum with no issues.

What will give you issues is anything that causes uneven cell clamp pressures. If you put some big band clamps on your end plates, and you don't have even pressure distribution, or it drifts from being even pressure distribution, the cell doesn't share current evenly through it's active materials if you don't have even pressure causing even distributed cell impedance through the stacks of active material.

Even clamp load at ~3-5psi or don't try as it will do more harm than good.

What are the test conditions for that test? I would only expect the cell to swell more easily than without the vacuum---I still expect you would have to be loading the cell pretty heavily. A properly sealed cell won't just swell up, sitting there in a vacuum doing nothing.

Keeping the pressure even is indeed important, but I'm not sure I would say NO compression is better than uneven compression, unless maybe the pressure imbalance was really severe.

The shipping regulation is something like 24hrs in a near vacuum. I do 96hrs in as low of vacuum as my pump can go. The UN test is based on percentage mass lost during the test, my personal pass criteria is based on mass loss as well as cell physical dimensions changing (thickness for a pouch measured between flat plates).

It surprised me as well, but every pouch I've tested does fine, even the RC stuff I tested for fun. Keep in mind, I'm testing new cells only not some old puffed stuff, we know if you create enough gas inside the pouch you rupture even at 1atm let alone in vacuum.

 

[wb9k]

The shipping regulation is something like 24hrs in a near vacuum. I do 96hrs in as low of vacuum as my pump can go. The UN test is based on percentage mass lost during the test, my personal pass criteria is based on mass loss as well as cell physical dimensions changing (thickness for a pouch measured between flat plates).

It surprised me as well, but every pouch I've tested does fine, even the RC stuff I tested for fun. Keep in mind, I'm testing new cells only not some old puffed stuff, we know if you create enough gas inside the pouch you rupture even at 1atm let alone in vacuum.

OK, so no electrical load on the cell while under vacuum. What kind of limits do you consider acceptable for thickness dimension change? A little bit in a brand new cell wouldn't necessarily surprise me, depending on your measurement method and its repeatability, but maybe I'm wrong in my assumptions there.