Help wanted - literature needed!

jonescg

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Call-out to the engineers and manufacturers who work on deep-drawn products (aerospace, cell manufacture, diemakers)...

At work I'm doing research into improved battery design etc. and we're looking for ways to improve the outer shell, or 'can' of the cylindrical shell.

I'm struggling to find good research into the deep-drawing process and what it's limitations/challenges are.

So does anyone with experience in this field have some peer-reviewed literature they can point me towards? I suspect I'm not hitting the right search terms.

Cheers,
Chris
 
That's pretty mature technology, don't you think? What "improvements" do you reckon are still left to make?
 
What sort of improvements are you looking to make ?
What draw ratios are you looking at ( height to diameter)
Materials ?.. Steel or alluminium ? ..Thickness ?
Key to any good deep draw process is material quality and specification, backed up by correct tooing geometry.
Once you start drawing thin materials (<1.0mm) you need some very specialised materials and techniques to ensure product integrity...Often those materials and associated tooling detail is proprietry info that is not made available.
I Spent my life bashing out coke and beer cans (high speed , deep drawn and wall ironed) ...so i will see what i can dig up...but dont hold your breath for a "how to" type document reference.
Give me a bit of direction as to exactly what you need to do .
 
Knowing that the production of cell cans is very mature, I'm looking for any literature into problems associated with the process - weaknesses and defects, and how they were resolved. Also things like the crimp seal at the top - some materials will provide enough tension to hold the cap on (steel), while others will yield (aluminium). Is the yield force 2 times greater than the cell would ever experience in normal use? Or 10 times? What are the radial loads experienced by a deep drawn can under pressure? We are looking into aluminium cans, but also small improvements to the cell internals.

But really, any literature, no matter how old will do.
 
#1 source of can failure would be physical damage in use.
#2 potential source of failure would be material defect,..impurities and inclusions in the base metal, or pin holes from mill roll dirt.
#3 likely be surface defects from dirty/damaged tooling , or micro cold welding of the base metal to the drawing tooling. Surface finish and treatments of both the base metal and form tooling are critical.
Highly unlikely the top seal ( roll formed seam) will fail from pressure alone as most cells have inbuilt pressure relief valves, but obviously extreme heat and pressure will lead to catastrophic failure eventually.
I have no idea what a 18mm dia steel battery case can withstand..top seal or main body walls, but i do know that much larger (66mm dia), thinner ( 0.09mm) aluminium cans ,..are designed to withstand 7 bar internal and in practice hold much more. A smaller diameter can will experience much lower forces on the end cap, but its ultimate failure will depend on the detail of the closing seam and the strength of the end cap which will likely deform and escape from the retaining seam if it is not a "double lock" seam.
All of this should be irrelevant if the pressure relief if designed correctly
I dont quite know what you mean by ..."what are the radial loads experienced by a deep drawn can under pressure?" ....
Pressure is just that, force/unit area, or do you mean ..what pressure can the cell walls withstand ?
Radial "burst " pressure, is a function of the wall material thickness , tensile properties, and quality, and could be estimated using Barlows formula for tube strength ..which on something like a Al 18650 can of say 0.2mm wall, comes out at something north of 250+ psi.( 17 bar)....depending on the actual Aluminium used

Screenshot_12_1024x1024.jpg
 
From what I understand the reason steel is used and not aluminium is because the crimp will slowly yield over time, while the steel won't. So it's easy to conceive a different way of sealing the can which would be stronger, but I'm keen to know what research has been done in this area. That said, I'm pretty sure there's lots of proprietary knowledge which is not being shared. Also, most of the market for deep drawn cells is alkaline primary cells - and Al won't work with these chemistries.

We're also looking at the cooling strategies used, and whether or not changing the way the internals are put together can improve the heat transfer. There's a bit more out there on this and other cooling matters, but less on the can making process.
 
I would Check the evidence of the Aluminium "slowly yeilding over time"..it doesnt sound like a realistic senario.
Al doesnt yeild with time, if overloaded it will yeild pretty fast, but what set of curcumstances would cause a load of that level anyway ? ..high internal pressure ? Caused by what , some terminal event.
But the pressure relief valve is there to prevent any mechanical failure of the can.
Any doubts on the strength of the top seam could easily be addressed by increasing the thickness of the metal on that area (not the rest of the can wall) , altering the radii of the seam profile, and if reall desperate ..modifications to the alloy and heat treatment of the metal.
No, i suspect steel is used simply because it is cheaper !
 
Best guess would be "slowly yielding over time" would refer to aluminium's susceptibility to creep. Although that may be an exaggerated fear considering a cell should not be under continuous high internal pressure.

I guess cells will see internal pressure cycles as they charge/discharge, but this may not involve significant pressures and it's only ever going to be a few thousand cycles maximum.

For every possible concern the ultimate answer would appear to be the aluminium 500ml beer can as Hillhater describes. It's about the ultimate deep drawn, thin-wall, low cost, light weight durable pressure vessel going.

I hate to be pessimistic, but considering the amount of R&D money and effort put into 18650 cells by some very big players over the years that there's unlikely to be any low hanging fruit.
 
The Beer/Beverage can manufacturing tech is highly refined and fine tuned to specific requirements.
Most developments now are focused on cost reduction, primarily by reducing the weight of metal required and faster, more efficient, production speeds. Occasionally a customer will request a slighhtly different size or shape, but nothing revolutionary since the Japanese attempted deep drawn cans using pre-printed and coated materials,
Im sure you could reduce the weight (cost) of metal in battery shells, but i doubt that is a focus of your efforts ?
 
Hi guys,
Sorry I'm being a little vague about what I want to know partly because of company IP and all that, but also, I don't know what I need to know in order to start looking. We're looking into why aluminium isn't being used for manufacturing small cylindrical cells. There is a huge weight saving to be had, as well as vastly improved cooling since the thermal conductivity of Al is about 10 x better than stainless steel. But clearly the cost is a major factor. The few leads we have suggested that the crimp seal currently used in steel cans is too tight a radius for aluminium.
 
Aluminum has been used by A123 cans of various sizes, and K2 and others. In this case the can must be the positive terminal, and the copper foil cathode connects to the nickel or copper end terminal that would have traditionally been the positive terminal.

Stainless tends to have improved pinhole resistance which enables a lighter can in practice due to the ability to be so much thinner. The walls of some modern can cells are thinner than most paper.

If the joints to the can are made so that the terminations to connect to the can externally are right on the other side of the can from the interior current collector weld joint, the can is fortunately virtually removed from the current path so being a high resistance metal like stainless doesn't effect resistance meaningfully.
 
For improved cooling with an Al can rather than steel, it could be worthwhile. The first thing I'd do is look at the limiting characteristics for cooling of the steel can cell. If the main problem is getting the heat through the jelly roll to the can, or if getting it through the can is ok but the radiating surface to get to the atmosphere is insufficient, then changing the can material won't help much. I'd think the thermal resistance of a complete cell has already been studied/modelled quite well (somewhere in the literature...).

Hillhater said:
the Japanese attempted deep drawn cans using pre-printed and coated materials

Interesting concept 8)
 
Punx0r said:
I'd think the thermal resistance of a complete cell has already been studied/modelled quite well (somewhere in the literature...).

Yes, and we have stacks of literature on that. The jelly roll produces heat pretty evenly, so 90% of the heat generated inside the can is fairly uniform. The resistance is partially via the stainless steel can, and partially by the low thermal conductivity of the jelly roll in the radial dimension. Axially it's pretty good, but there's a sizeable airgap between the end of the jellyroll and the end of the can, so you're not really any better off.
 
But the can end is only approx 6% of the can surface area, so not a big player in heat dissipation anyway.
And with the wall thicknesses of cans (< 0.1 mm ?) i doubt the difference in thermal properties between steel and aluminium is going to be a game changer either !
Certainly any cell covering (shrink wrap etc) will seriously hurt cooling, so bare cans are a must, and then any thing you can do to ensure good thermal contact internally .
 
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