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

[agniusm]

I know we all have resorted to a nice layer of Gorilla Tape but has anyone used heat shrinking tape to get compression? It wouldn't be that much compression.It would be no different than hobby king cells. Im considering it for my nissan leaf cells since i can't find heat shrink big enough to fit the cells.

Im personally not a fan of bulky end plates.

I got some 2 large sizes from Paul, aka cellman:

The topic is OK. But I miss a discussion about measuring forces applied and compression made out of thin air will do no good. If the man says 15psi, thats what I want to get. There was a lot of speculation going on in regards to these cells, and how one must handle them or build them into batteries which turned out, well, thin air. I think putting 300+kg weight on top of the battery and then strapping it is out of the question which leaves some kind of sensors. Perhaps someone can comment on them before we go into inventing something which will do no good?

 

[flexy]

The topic is OK. But I miss a discussion about measuring forces applied and compression made out of thin air will do no good. If the man says 15psi, thats what I want to get. There was a lot of speculation going on in regards to these cells, and how one must handle them or build them into batteries which turned out, well, thin air. I think putting 300+kg weight on top of the battery and then strapping it is out of the question which leaves some kind of sensors. Perhaps someone can comment on them before we go into inventing something which will do no good?

If by "thin air' you mean using a pressurised air bladder or vacuum bag then the advantage is that you can accurately measure the amount of pressure or vacuum you are applying on the cells.

Even with a band-it strapping tool it is difficult to get the tension exact. If you clamp the cell stack under a measured force then strap them, the straps will add some pressure of their own or be slightly loose and relax when the clamp is removed.

I noticed with several of my cells there were bumps and creases under the cell casing some looked like loose bits of tape, maybe these are creating the problems under pressure by causing high pressure points inside and internal shorts.

 

[cassschr1]

I'll throw this out for disscussion. How about sandwich the pack between 1/4" alum plates with four threaded rods, top to bottom.I have a project thats not finished yet that could incorporate this method. Just don't know how tight to tighten the nuts. Its a plate of alum flush with bottom of the fork with a 1/2"( about 6" long) bolt coming up into the steering tube . The nut is a tapered cylinder of alum with 1/2" threads through the middle. My fork was tapered inside the steer tube so this works well.I know apicture is worth a thousand words but I don't know how to post them. Cells would be laying flat on alum plate right in front of the steer tube like oatnets front fork builds.

 

[xenodius]

I'll throw this out for disscussion. How about sandwich the pack between 1/4" alum plates with four threaded rods, top to bottom.I have a project thats not finished yet that could incorporate this method. Just don't know how tight to tighten the nuts. Its a plate of alum flush with bottom of the fork with a 1/2"( about 6" long) bolt coming up into the steering tube . The nut is a tapered cylinder of alum with 1/2" threads through the middle. My fork was tapered inside the steer tube so this works well.I know apicture is worth a thousand words but I don't know how to post them. Cells would be laying flat on alum plate right in front of the steer tube like oatnets front fork builds.

I believe this was mentioned. A torque wrench would be perfect for proper tightening... add some loctite.

 

[liveforphysics]

I'll throw this out for disscussion. How about sandwich the pack between 1/4" alum plates with four threaded rods, top to bottom.I have a project thats not finished yet that could incorporate this method. Just don't know how tight to tighten the nuts. Its a plate of alum flush with bottom of the fork with a 1/2"( about 6" long) bolt coming up into the steering tube . The nut is a tapered cylinder of alum with 1/2" threads through the middle. My fork was tapered inside the steer tube so this works well.I know apicture is worth a thousand words but I don't know how to post them. Cells would be laying flat on alum plate right in front of the steer tube like oatnets front fork builds.

I believe this was mentioned. A torque wrench would be perfect for proper tightening... add some loctite.

Don't forget, if you make a rigid clamping system, the cells need to expand and contract with temp changes as well as changes in SOC. They swell when charged, shrink when discharged, swell when warm and shrink when cold. If your clamping system doesn't enable maintaining pressure as they swell/shrink, it's not the ideal solution.


John- When I said a bunch of little air bladders captured inside silicone, I meant stuff like this:

http://www.westernrubber.com/materials/weslastomer-sponge/silicone-sponge/

 

[wb9k]

Thin foam pads between cells should be adequate to allow for expansion. It would be interesting to see how much stack pressure increases with temperature and SOC. I doubt that it's SOC per se that causes heating. At low SOC, discharge impedance rises quickly, so there is more thermal rise in the cell assuming a constant load is presented to the pack by the controller. Similarly, at high SOC, charge impdeance grows. In actual practice, the additional impedance is not generally an issue if we're charging intelligently. As HVC approaches, current draw by the cell will decrease simply because the dV compared to the charging supply is lower, so heating shouldn't become an issue.

 

[cassschr1]

Ok, they need to breathe. How many thousands per cell you think would be resonable?

 

[wb9k]

I'm thinking if you can realiably hit 10 psi at room temp, that may well be good enough. The spec allows up to 25 psi at room temp. A good test would be to compress a single cell with a pressure transducer starting at 10 psi, room temp. Then run a 2 or 3C full discharge cycle and see how high the pressure goes with only all else static. Maybe raise the ambient temp to 40 or 50 C make it a little more worst-case.

 

[megacycle]

Because it's not an exact science at the moment, I think without exacting figures the subjects overly pedantic.
Compress to a middling value and leave expansion and contraction to itself.

 

[cassschr1]

The black rubber they use for steel rule and die cutting is 1/4 " thick and pretty stiff. One pad per 12 cells.

 

[wb9k]

Mega, I have a feeling that is perfectly adequate, but it would be nice to have some numbers...really might make a difference to a builder depending on their overall approach. If nothing else, put your mind at ease if it turns out there's plenty of margin.

 

[Harold in CR]

Rubber will hold it's flexibility under pressure a LOT more than cushion type foam. Poured 2 part foam is rigid.

Just sayin.

 

[cassschr1]

Thanks Harold, at least I'm on the right track.

 

[bigmoose]

Think Bands are springs...

 

[John in CR]

Is this such an issue with all pouch cells, or are the A123's especially susceptible for some reason? Is this a likely cause of many of the RC Lipo issues, and then all it takes is 1 improper/unsupervised charge cycle? The more cell failures I see, the more I'm pushed toward cheap cylindrical cells, since I can get them in nice tab welded blocks of 1s in any size and shape 18650's fit nicely, so they're even easier to assemble into a solid battery than these lower energy density cells. Purchases now give me 2-3 years more for the battery guys to get their act more together to give us higher power decade lasting packs.

As has gone on for 5 years, I'm torn in all directions regarding batteries.

 

[999zip999]

JohnCR I thought you would love these at 20c or something for one of your rat rods. Don't know what 7-8ah means ?

 

[major]

JohnCR I thought you would love these at 20c or something for one of your rat rods. Don't know what 7-8ah means ?View attachment 1

Those are nice cells. I have a few. Here's an attempt at about 47C for 35 seconds. My current transformer maxed out at 375A (10V for its output), so the initial peak current is a bit higher. Also note that cell voltage drops to just about one half which indicates this is at peak power out of this cell. Nice long pulse. The little sucker was pretty warm. About 160ºF. And it seemed consistent across the cell body. Terminals were actually cooler, about 100º.

 

[999zip999]

Where did you get them and how much ?

 

[xenodius]

JohnCR I thought you would love these at 20c or something for one of y ourI don't haverat rods. Don't knohink 7- 8ah means ?View attachment 1

Those are nice cells. I have a few. Here's an attempt at about 47C for 35 seconds. My current transformer maxed out at 375A (10V for its output), so the initial peak current is a bit higher. Also note that cell voltage drops to just about one half which indicates this is at peak power out of this cell. Nice long pulse. The little sucker was pretty warm. About 160ºF. And it seemed consistent across the cell body. Terminals were actually cooler, about 100º.

Well that's a great discharge curve, but again, a little ot. Looks like those cells are about $25/ea from Shenzen Questionable. I think better energy density is achievable with pouch cells, but probably not power density. Their spec says .5c charge, wonder what it really is officially?

LFP, totally true- but I don't really know from experience.. exactly how much expansion are we talking? i don't have personal experience. i would be surprised if it increased psi from 10 to 25, but what do I know? is the silicone stiff enough? Forgive my phone-posting abilities.

 

[major]

Where did you get them and how much ?

Sometimes people give me cells or batteries to test and play with, as was the case here.

 

[drutledge]

I'm coming late to this discussion and have only skimmed the thread but for what it's worth I think that in most e-bike situations, compression is nice to have but not a necessity. I have never noticed my cells do anything as far as expand or soften at normal e-bike C rates. The only time I had a cell not just lie there was when I accidentally shorted it with another cell. That got me two very soft pouches. Perhaps under high C rates they will soften up and expand and then you obviously want to keep everything in place.

That being said, here's what I did, also available in the A123 build thread: http://endless-sphere.com/forums/viewtopic.php?f=14&t=38761&start=750
I have the cells sandwiched between plates of Garolite (think fiberglass-like). There is a thin sheet of foam between the pack of cells and the 'case'. If you look closely, you can see that on the part that has two layers of red Garolite, I have three strips of spring steel in-between. This, I hope, equalizes the pressure on the cells. The bolts put pressure on the outside edges and the opposite bent spring steel presses toward the center of the cells. I made this just after the Mid-western buy of A123 cells and it's been going strong ever since. Daily ridden in a cargo bike with a 2-3 C peak discharge rate. Lots of bumps and potholes.

 

[999zip999]

Drutledge the Man. Thanks again for your pack build as I followed your steps for my two 12s A123 20ah packs. Your pics and post are on page 2 of that A123 build thread you posted above. Yes you have a big hauler that can handle some size. So triing to keep as small and light. I have just rapped mine with ribbed tape and now waiting for some end plates. But have to have in hand first. I had one cell that was charging faster than the rest all the time so thinking it was from high I.R. replaced. All is good. They don't like bulk charge without bms as I balance charge everytime. Do yours float a little if not balance charged ? I was just testing to see how they acted.
Most all the cells in the midwest group buy seem good. Was it just are timing of the buy ?

 

[drutledge]

I rarely ever balance. I keep two Celllog 8s on the pack at all times and that will throw off two of the cells over time but I seem to keep a steady <20mv difference at worst. In the middle of the SOC I'm usually at <10mv difference. I only charge to 85% and rarely go lower than 30% so they are pretty babied I guess.

 

[999zip999]

I stopped leaving anything hook up to my pack. Not in use everything is unplugged just in case I'm off to Mex. or something.

 

[agniusm]

i am to get the pricing for pressure plates made out of fiberglass composite. Here are the sketches:

I would like this to work with my kit so everything slides in together and protects from accidental shorts as well as gives rigidity:

I must buy the whole sheet, 2m^2 and that cost 360USD if any one would be interested for a set?