This topic has been the matter of debate inside A123 for a long time....probably forever. We've reached a point where I think everyone can agree that some compression is absolutely necessary, and it needs to be distributed as evenly as possible. In my mind at least, these two points have been proven to be non-negotiable--both with data from in house and from places like this. The remaining debate is about how much pressure is enough. The answer, I believe, lies at least partly in the application. Lower pressures should be acceptable in lower-demand applications. I would consider an e-bike to be a very light application for these cells. However, in a full-size car, or a big industrial hybrid vehicle (where A123 is the market leader) cells can regularly see spikes of 20C or greater (50C is not unheard of)on a regular basis. Higher compression force will be needed in these applications to keep cells healthy.
Now, I can't put the verbiage of A123 specs on the internet. I do think I can tell you that pressure specified is to be measured after 72 hours of rest (after banding). My guess--and this is just conjecture on my part--is that the upper limit was derived by assuming there would be no relaxation of pressure at all. This may well be what the machine applies. However, the lower limit is much lower than that. The lower limit I recommend is 10 psi, the number I was using when I first got involved in this conversation here. However, there are those who believe as little as half of that is OK, and after more thought, I can't say for sure that they're wrong. It may be that below 10 psi is OK, but I would expect that the evenness of it's application would be more and more critical at those lower levels.
I'm getting ready to scrap a bunch of stuff in my lab this week. I can salvage some endplates and stainless steel banding material and send it to anyone who wants to experiment with them for the price of shipping. Send me a FedEx account number, and I'll ship by tomorrow. I've offered this and other things in PM's, but nobody has responded. Looking around the board, I see there's a long history of scamming that's gone on, and in that context perhaps that's understandable. But I'm for real. Like many of you, I just want to see this stuff succeed.
Anyone interested in some bits for experimenting, PM me. First come, first served. Just endplates and bands right now....no electronics, no cells--yet.
start bigmoose edit: I just wanted to add a reference in this thread to the compression numbers required for A123 pouch cells, so it doesn't get lost in the future.
Here is the thread reference for the numbers: http://endless-sphere.com/forums/viewto ... 09#p773809
I (bigmoose) calculated the stack compressive force from the "stack pressure in psi" given by wb9k. He (wb9k) said the spec calls for 10 to 25 psi (pounds per square inch) compression on the cells.
wb9k was questioned by major:bigmoose wrote:So folks since a cell is 57 square inches in area that says that the A123 20 Ahr pouch cells want: 570 to 1425 lbs of compressive FORCE on the stack!
major wrote:So you are confirming you need to have almost 3/4 ton force on a cell?
Now the data is all in one placewb9k wrote:As a max, yes.
...end bigmoose edit