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

[wb9k]

I think you're on the right track Dave, though I do think the heat sinks have value even air-cooled and in lower-current applications. Even a ten degree temperature gradient across a pack will yield results that become obvious toward the end of life (EOL). Basically, the cells that have lived at higher temps will die sooner. Usually, this happens to the cells toward the pack interior.

I should be able to get your package out to you tomorrow.

 

[bigmoose]

Now the specification chart that I have seen, shows these cells with 90% capacity retention after 3000 cycles to 100% DOD. That would be about 10 years of riding to 100% DOD every day... That's seems a pretty good life. The chart I saw did not have life versus cell temperature however.

Thanks again for the courtesy, and please ship when you are free. No need to pressure yourself.

 

[wb9k]

Now the specification chart that I have seen, shows these cells with 90% capacity retention after 3000 cycles to 100% DOD. That would be about 10 years of riding to 100% DOD every day... That's seems a pretty good life. The chart I saw did not have life versus cell temperature however.

Thanks again for the courtesy, and please ship when you are free. No need to pressure yourself.

At 1 or 2 C discharge rates and 25 degrees C, that kind of life is believeable. Higher average temperature, like current, means shorter life--for all batteries. The detailed spec for the pouch cell does address temperature and shows accelerated aging as average temperature rises.

 

[bigmoose]

Your of course right. Made me go back and look at the "asterisk" on that chart. It states the chart is for 23 degC and +1C and -2C.

 

[torqueon]

Pardon me for posting, as I have no real credentials. The compression issue keeps me thinking about corsets and the classic parabolic curves etc. for even pressure at all points of the Z axis.

The problem I see with banding and or bolting is that more pressure will occur on the perimeter and the center of the cells will lack pressure.
Unless the end plates have some crown built in. or some heavy plates. We don,t want heavy here do we!! Yes plastic light but does creep over time.
The thought is build in crown for the end plates or create the right curve on outside of plates, to work with bands and bolts.
Many bridges are built on this idea. The math is where I stop, sorry.

 

[999zip999]

Yes a reverse curb spring plate that with a flat plate to compress on ? These is getting more for a car battery with the weight that all this will be adding. I guees the reward is the 10yr. battery.

 

[John in CR]

Since even pressure is required, my immediate thought is to put something inflatable to press evenly on each endplate. How hard would it be to use a couple of car or truck inner tubes to make a well sealed bladder for each end to fit the shape of a cell? What I like is that the pressure is adjustable, measurable, evenly distributed, and can maintain a near constant pressure regardless of SOC. Plus air doesn't weigh much at 5-10psi.

John

A mechanically constrained inflated bladder system sounds pretty good. That's clever, I like it.

If you've got a closed cell dense foam of a material that remains elastic, and you crush it, it behaves similar to a bunch of little bladders all constrained in a sheet of silicone rubber (or whatever material).

Wouldn't lots of little bladders be hard to create even pressure. If I can come up with a couple of air bladders that maintain a good seal, then I just secure the pack in it's battery bay and pump up both bladders to the desired pressure. It should also help secure the pack.

Now to come up with the bladders. I may be able to find a hot water bottle of just the right size, and then it's a matter of sealing and a valve, but something more elastic would be better for even distribution so the end plate can be thin and light. I keep coming back to a truck innertube cut to the right dimensions, but sealing the seams seem iffy.

With stiff end plates maybe just fold up a bike innertube to apply pressure fairly evenly to the end plate.

What about putting the pack in a vacuum bag and pulling a vacuum on the whole thing to let the atmosphere provide the pressure?

For my 2p wide 20s RC lipo pack I used your electrical tape method for compression, which has worked well, but that's narrower and required stiff heavy plywood end plates to get a nice pressure distribution that hasn't seemed to deform.

I like the air bladder approach if I can come up with the bladders, because I don't see springs or straps working well from an even distribution standpoint. Pulling a vacuum seems good, but I'd have no way to monitor the pressure.

John

 

[agniusm]

This is what i was thinking some time ago when playing about with the kit and the cells for compression, strap method:

It would be easy to mill out on CNC router and the material, glass fibre board is cheap, easy to work on, stiff and fire resistant.
With this idea it would be possible to manufacture side covers from same material and there would be enough meat on pressure plate to screw or rivet side covers in (2 sides + bottom)
The problem i had was strapping tool with high price, not viable for most. Another method to use steel ties with bolt for tension to be added easily but they are not that neat:

Sorry for blurry pic.
Here i was trying to replicate heat sink trays:

.5mm aluminum sheet.

 

[megacycle]

Had some 3mm phenol end plates made up around the time of my build.
http://endless-sphere.com/forums/viewtopic.php?f=14&t=38545&start=330
Really hard to bend this stuff, the ends are supported within a dry walling frame, so i could place extra end plate/s to get the desired pressure, now we know the pressure is a lot more and yeh band-it, i would of prefered some kind of camlock onto a steel band so the bands don't need cutting, but the nearest thing would be hose clamps, which stick out to much for my liking.

 

[flexy]

I use expanding polyurethane foam in my job when fitting door and window frames, if they aren't secured first, the pressure of the curing foam is enough to bow them out of shape.

If you had a battery box with thick end plates (6mm aluminium or phenolic) and leave say a 25mm space at one end, after the cells are all fitted, fill the space with foam and let it cure, the pressure would build as the foam sets, then your left with a closed cell foam end spacer that is putting pressure on the cells.

You would have to experiment to get it right, foams with varying rates of expansion are available (including fire resistant), they are also dependent on temperature and humidity as to how quickly they expand and cure.

 

[ambroseliao]

I received the A123 end plates from DH today. These are definitely pro grade and heavy duty! Thanks DH!

There are rubber pads about the size of the actual A123 cell size on the flat sides of the plates. These are not light, but can certainly take a lot of pressure and abuse!

 

[999zip999]

They look very strong looking forward to see your build. It will be getting bigger and heavier, But if this is the secert for long life I'm for it. The problem we need 1,000 cycles to start to see resluts.

 

[nicobie]

I like John's idea using a vacuum press to assemble the pack. Next time I make up a pack I'll try it. I have a vacuum press system that I use for gluing up veneer panels. It's amazing how much pressure it can apply.
when using it to glue up a one square foot section it is the same as putting a 1500 lb weight on the top of it.

http://www.joewoodworker.com/veneering/vacuumpressurechart.pdf

 

[999zip999]

I have a vacum on my vacum caster with a easy access pipe. 29.3 where ever's they messure vacum with. Well boil water at room temp. I guess like the vacum of outer space. So Nicobie after the press how you going to strap it together ?

 

[megacycle]

I use expanding polyurethane foam in my job when fitting door and window frames, if they aren't secured first, the pressure of the curing foam is enough to bow them out of shape.

If you had a battery box with thick end plates (6mm aluminium or phenolic) and leave say a 25mm space at one end, after the cells are all fitted, fill the space with foam and let it cure, the pressure would build as the foam sets, then your left with a closed cell foam end spacer that is putting pressure on the cells.

You would have to experiment to get it right, foams with varying rates of expansion are available (including fire resistant), they are also dependent on temperature and humidity as to how quickly they expand and cure.

Brilliant idea flexy, i use expandit a lot, know exactly what you mean but i did'nt know about the varying rates of expansion excellent.
I could line the inside of my dry walling shells and shrink wrap the pack,bandit, inject from the top and place a phenol plate in the other axis internally with the tabs poking through, the expandit or similar besides then being confined and pressurize, cures rigid but i think would absorb vibration well, so basically this could be a panacea, or Cure All LOL, for my pack issues cheerz

 

[Ypedal]

I experimented with canned expanding foam a few years back, made more of a mess than it was worth, makes servicing ( heaven forbid ) a nightmare. .. and on PSI cells with threaded lugs it pushed the bus bars and caused connection problems between some terminals.... more cons and pros in my small testing sample.

 

[Harold in CR]

Would it be feasible to inject that foam into a sealed 1 quart size freezer bag inside another, so, after it sets and you MAY need to access the pack, later on, it would be possible to slip that plastic bag out of the battery ?

That foam sticks to anything, really well.

 

[megacycle]

I experimented with canned expanding foam a few years back, made more of a mess than it was worth, makes servicing ( heaven forbid ) a nightmare. .. and on PSI cells with threaded lugs it pushed the bus bars and caused connection problems between some terminals.... more cons and pros in my small testing sample.

Yeh it could get out of hand, if i don't keep an eye on it, i know exactly what you mean.
The adhesion quality is so great too, i can't get if off my skin for days and with gloves, forget about reusing them.
Another way, along the idea of Johns, would inject into a bladder/balloon and confine it between two plates.
If it popped it could be a drama LOL.

 

[megacycle]

Would it be feasible to inject that foam into a sealed 1 quart size freezer bag inside another, so, after it sets and you MAY need to access the pack, later on, it would be possible to slip that plastic bag out of the battery ?

That foam sticks to anything, really well.

Would it be capable to contain this level of expansion, i don't know if it would cure, same with the balloon, is it urethane?
Does it need air to cure too?

 

[megacycle]

Aaarrghh solvents melt latex, so no good with .. won't go there children could be listening.

 

[nicobie]

I have a vacum on my vacum caster with a easy access pipe. 29.3 where ever's they messure vacum with. Well boil water at room temp. I guess like the vacum of outer space. So Nicobie after the press how you going to strap it together ?

I can easily make a vacuum bag of the right size that stays with the pack forever. I have already made half a dozen of them for bigger stuff.

Piece of cake.

http://www.djmarks.com/stories/djm/forming_curves_in_a_vacuum_press_60152.asp

 

[megacycle]

The vac packing looks good, and you should know the applied pressure by your pump guage 8)
What's done then, is it boxed and banded on completion or can a formed bag hold it?
Never dealt with this process interesting.

 

[kfong]

Vacuum bagging and a composite shell would be easy to implement. You could use precut end plates then cover unit in plastic to keep out resins. Wrap with carbon fiber to create a fireproof battery. Once cured, pressure would be locked in place. How much of a vacuum are we shooting for?

 

[megacycle]

Vacuum bagging and a composite shell would be easy to implement. You could use precut end plates then cover unit in plastic to keep out resins. Wrap with carbon fiber to create a fireproof battery. Once cured, pressure would be locked in place. How much of a vacuum are we shooting for?

The only problem i can see with it, for some, is it's kinda permanent, if you wanted to work on cells, you'd have to cut it away.

 

[999zip999]

My friend made a vacum thing to make parts for a mushtang fenders and stuff but he added heat I think for curing. I think just for making a tight model and for time curing so some epoxy resin. ???