Spot-Welding a123 Prisimatic tabs

[oatnet]

I've built a few packs from the prisimatic a123 cells by folding the tabs together 3 times around a balance wire, crimping it flat, and sealing it with a dab of solder at the end.

With most mechanical joins, the contact area is only on the side where the tabs overlap, and if the fastener holding the tabs together comes loose, the tabs separate/arc. With my technique, BOTH sides of each tab become contact areas, and it takes a great deal of force to unroll the folds. The folds are further secured by a layer of heat-shrink (that oozes adhesive all around the join), a plastic clip, and a layer of fiber tape. I got some skeptical opinions at the time, but I've pulled 100 amps through this termination without any sign of heating. However, I also recognize that at some point, despite being sealed in the amber of heatshrink adhesive, the tabs could oxidize, and add resistance, so bonding the metal is also desireable.

I saw some footage of an a123 factory, which shows them laser-welding the pouches, and it stuck in my brain. I don't have a laser welder, and while I could probably tig-weld the tabs together, they would get very hot which would trash the cells. Spot-welding is out, because the battery tabs are thin and made of low resistance metal that won't heat up when the welder's current goes through them.

Then I had the idea to put a thin strip of steel on either side of the weld, making a sandwich with thin steel strips on the outside, and the battery tabs on the inside. When I zapped the sandwich with the spot welder, the steel on either side had enough resistance to get hot, which melted the battery tabs, and welded them together.

I use 3 vice-grips to hold the sandwich together, exposing only the section of steel strip I am about to weld. The vice-grips help dissipate the heat, and I have a wet paper towel resting on top of the tabs. I hear water sizzle when the weld happens, but by the time I shake off a glove and grab the tab it is ice-cold again, so I don't think the cells are getting heat at all.

This is Harbor Freights $160 "115v spot-welder" I could have purchased the same product on ebay for $35 less, but I wanted to be able to return it. Anyhow, on to the pics...

As with any weld, surface preparation is essential. I did my initial testing on tabs from dead cells; they were wrinkled and covered with

junk, and the welds did not hold well. I clean the battery tabs with a paper towel and alcohol, polishing them until they shine like mirrors.

I thoroughly wire-brushed both sides the sheet steel before I cut it into strips. Use weldable steel not galvanized steel, which is plated with zinc. Zinc fumes are hazardous, and the plating does wierd things to the weld. I got this 22ga sheet from Ace Hardware for $5, only used 1/8th of it.

I used shears for a while but my cuts were not consistant, and it was a pain. I got this paper cutter at a swap meet for $5, it did a great job of cutting strips. Although it could only cut half way through...

00_cutting strips IMG_5994.jpg (83.75 KiB) Viewed 2837 times

... it was simple to flip the sheet over, line up the cut edge with the table, and finish the cut.

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Here is a stack of strips, long as the tab is wide, and just big enough to accomodate the weld head.

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Preparation - here I am balance charging a stack of 8 cells to make a 24v pack. I charge them up, let them self discharge while clamped together, and repeat a few times to give them the best possible initial balance. I also prep-cook the operation by cutting a stack of balance wires, heat shrink tubing, plastic clips, and strips of fiber-backed tape, so that I can just grab them during the build.

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[oatnet]

Here is the actual Weld process. Note that picture #5, showing the inside of a weld, needs to be clicked on to be viewed in a new page. Most of the pics need to be clicked on just to open full size.

Note the weld tips on the right side of the picture!

Tape on tab condoms (unshrunk heatshrink) to avoid a short. Tape a pair of cells together so that the exposed tabs line up, press the tabs together at the base. Put a pair of steel strips on either sides of the tabs, as close to the end as you can get it. Clamp it with vicegrips so that just the area to be welded is exposed. This is tricky, I use a small vice-grip on top of the weld area to jockey them into position, before I put this large vice grip on to hold everything together, then I take off the small vice-grip to expose the weld area.

Clamp with the welder tips, and zap it for just an instant, the steel will glow, and melted metal will gush out of the weld.

1_tabs and strips clamped IMG_0206.JPG (41.78 KiB) Viewed 2835 times

Here is a first weld. It is fragile at this point, and the steel strips can rotate around the weld spot and break it. I carefully put the small vice-grips right on top of the weld (not too tight, or you crush and break the weld), them move the larger vice-grips over enough to expose the next weld area. Repeat until you have 5-6 welds on the tab, make each weld outside the metal blued by the last weld.

The tabs must be clamped everywhere but the spot you are about to weld, or the steel strips will creep apart and tear the other welds. 3 vice grips are sufficient.

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The rightmost weld was done for too long, 3rd to the left is perfect, 4th/5th/6th wasn't zapped long enough.

If you don't hold the current long enough, the steel strips don't get involved with the weld, and fall off. The weld is much more stable if the steel strips are melted together.

If you hold the current too long, the metal of the tabs evaporates out of the weld, and the rest of the tab gets brittle and breaks on the first fold.

You only get one shot at each weld. If you try to weld it again, the tab gets too brittle to fold. If your weld was too short, the other welds will pick up the slack, so move on and do a better job on the next one.

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Here is what they look like up close. Note the oozed metal, I clip this off before folding.

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5_Opened Weld IMG_0264.JPG

On the left is one steel strip pried off so you can see the involvement of the metal below.

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The balance wire is laid in before the first fold is completed. It will be secured by clamping force and final spot welds, and if it is pulled hard enough to tear it out, I'd rather lose the balance wire than rip the tabs off. :D

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I use wide vice grips to start the first fold evenly, over the balance wire.

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Then I fold it two more times...

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... and crimp it flat as I can with giant pliers. Then I spot-weld the whole package 4 more times along the strip (need a picture of this), which melts everything inside. These welds generate a lot of heat, so I pause between each weld and cool the tab down.

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[oatnet]

I use heatshrink (2") that is stiff with adhesive. WHen melted, it flows an amber-like substange over the tab area and into the rolled ends of the tab, making a solid stiff unit. I've pulled this off of dead cells, it was everywhere and it was tough.

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Then I slide a plastic clip over the end. This applies tension holds the base of the tabs together, and supplies a hard surface over the shrinkwrap.

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This is what the clip looks like in profile.

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I got the clips from this. I cut each spine into 4-5 clips as wide as the tabs.

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Fiber tape maintains the clip's tension and holds it together even if the clip is broken. I make the tape tight at the base of the tab, which makes the ends of the tape cross over each other when folded.

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Here is a 24v16ah pack assembled.

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I separate the cells with (3) sheets of newspaper. It looks sort of ratty, but they act as an insulator and a wear barrier, one sheet for each cell and one to wear between them. I cut a chunk out of the top where the cells are welded together, and the remaining bit fits between the taped clips to keep them separated.

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Here is a pack with the newspaper installed.

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Three 24v cells sitting in the Pelican case that will be mounted to the forks of an upcoming project.

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The case, 24s16ah a123 pack inside, and 8 clamps that will mount it to RockShox Boxxer Forks, weighs 27.4 lbs, not bad for 1.42kw. Each clamp will support only 3.42 pounds - including its own weight.

k_ case 27.4 on scale IMG_0270.JPG (70.21 KiB) Viewed 2832 times

[Doctorbass]

Nice work!

Doc

[BLUESTREAK]

NICE WORK OATNET. would it help if instead of mildsteel strips you used stainless steel strips?. I am partial to stainless steel for everthing.

[regmeister]

deleted

[oatnet]

Thanks Doc!

Thanks Bluestreak! You can spot weld some varieties of stainless, some other you can't. Since this is sacrificial metal, not structural, mild steel was enough for me, cheap, and readily available.

-JD

[neptronix]

Great info, oatnet. Thanks for the post.

Mods - this should be stickied.

[EBJ]

looks good. I like the clamping method.

Has anyone actually tried tig welding the tabs?

I would be willing to bet you could weld with less heat tig welding than you can with spot welding.

[oatnet]

Thanks for the kind words, guys!

I would love to tig them but I'd think the process would dump a TON of heat into the cell and I'd expect damage. Ever touch a small piece of 1/8" after doing a 1" seam? HOT! I did a lot of 1/8"
aluminum in class, but nothing thin as the foils these tabs are made of, so they could require less heat. Maybe you could penetrate a puddle before the tab heats up, and "spot weld" it that way, but I am skeptical.

The TV show I mentioned, about a123 manufacturing the prismatics and packifying them, showed cells being joined and the process described it as laser-welding. IIRC, the cell zipped past a device and then two pieces of severed tab fell away... I guess that happens FAST, and doesn't dump heat. Surely amongst us battery geeks, we have some laser geeks who have the hardware and skills and want to give it a shot!

Now I need to work a new termination method for the packs. I used to crimp and solder an 8ga wire on there, but after dissecting solder joins made by myself and others, solder doesn't seem to stay bonded long-term. It gets up into the ends of my crimped/rolled tabs and keep them in place, but I can't count on it for actual conduction.

I am thinking (4) 14ga wires, each terminated in a 45a Anderson, for a total 180a rated throughput, and well able to handle 240a (15c). That is futureproofing, the max I would choose to run this chemistry at, but for right now it will be driving my Methods Controller @100a peaks. Anyhow, I thought I'd crimp in the wires with slivers of steel and some sacrificial aluminum, zap it and see if I can melt it solid. Hmm, wonder if solder would bond to the tabs while they are molten, if I used that instead of aluminum.

-JD

[oatnet]

Wow oatnet, that's great info.

I was thinking fish paper might work well in the place of your newspaper. I had a link to a really inexpensive source for it, but unfortunately can't find the link.

http://search.digikey.com/scripts/DkSea ... e=560GC-ND
http://www.polymerplastics.com/composite_vulcan2.shtml

probably a bit too expensive for the application though.

I just realized I missed this post - thanks for the comments, and I'll look into those alternatives!

-JD

[texaspyro]

Even posterboard would be orders of magnitude better than newspaper... fish paper or other phenolic based material would be even better.

[999zip999]

Hay Oatnet, I can use stuff as a heat sink between the tabs and cell. You could make a jig as to take the heat. I don't know the best thing. But it can be a toothless aligratetor clip or something parelle jaws ? It's beatiful. Good work.

[EBJ]

Thanks for the kind words, guys!

I would love to tig them but I'd think the process would dump a TON of heat into the cell and I'd expect damage. Ever touch a small piece of 1/8" after doing a 1" seam? HOT! I did a lot of 1/8"
aluminum in class, but nothing thin as the foils these tabs are made of, so they could require less heat.

-JD

Yeah, 1/8" thick aluminum requires a lot of amps. But that thinner stuff would require a lot less. (general rule in my classes were 1 amp per .001" thickness. +15% for aluminum)
I imagine a weld w/ no filler right on the ends would work on those tabs, or you should be able to do just as you mentioned... just use the tig to do some no-filler tac welds. Agreed the heat-sink would be a good jig.

[999zip999]

I think a heat snink is very good plus it will hold the tabs together.

[nechaus]

very good, i think i will use this method when i get my a123's

I have seen a guy mig weld around a coke can, pretty remarkable

[oatnet]

Been seek for a few days, bad for getting projects done but but gives me a chance to get around to things like trimming the pics for this post on how I addressed terminating the cells.

While spot welding is a great way to join the tabs on a123 prisimatics, it was not effective for connecting power leads to the pack. Instead I put a pair of copper straps on either side of the a123 tab, soldered wires to them, and screwed the straps together.

I cut the copper straps to length, and drilled (2) holes to solder the power wires, and (3) holes for the screws. The ones on the bottom have been tinned to avoid oxidation.

00_IMG_1504.jpg (51.2 KiB) Viewed 1478 times

To create a mechanical join, I put one wire on each side of the strap, and feed half of its diameter through the hole. The half that remains behind, gets soldered to the half of the other wire coming through the hole. With solder through the wires and through the holes, it is pretty solid.

This gives me the low resistance of 8ga throughput, with the the thin profile and flexibility of (4) 14ga wires. The rule of thumb is that a pair of wires is the equivalent to a wire 3ga lower. Each strap has (2) 14ga wires soldered to it, which are equivalent to 11ga. I put a strap on either side of each battery tab, so the (2) 11ga are equivalent to 8ga.

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Looks like I only took pics at this stage of the first one I did, which was my crappiest of some crappy jobs, but you get the idea.

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Here is a completed tab. Wierd how the tinning looks so ripply in the pics.

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Left, a stack of finished straps.

Center, a strap as it will be used on a single a123 cell tab. The pair of red wires gets cut in the middle, to give 2 wires on each strap.
Right, a pair of straps bolted together, ready for an a123 cell tab.

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8 sets of power takeoffs built up for (4) packs. On the lower right, you can see a disassembled power takeoff.

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The 'sandwich' is stacked up in this picture:

(3) stainless Bolts
nickel strip
(future, folded-over a123 cell tab-end)
tinned copper strap
(future, main a123 cell tab)tinned copper strap
Nyloc nut

The nickel strip is from my spot welding stash, it isn't very thick, but it helps keep the folded a123 tab-end in place. The holes in the straps get threaded by the bolts on first insertion, which takes a little effort. The nyloc nuts allow for a lot of torque, and the abundant adhesive from shrinkwrap helps keep them from loosening.

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Clamp the straps around the a123 tab's base; leave clearance such that the tab does not short out against the body. A misappropriated wood auguer easily perforates the cell tab for the bolt. I tried to do both the main tab and the fold at the same time, but too many parts were in motion for it to work out.

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with all three bolts firmly in place, I fold the tab halfway over the straps.

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Back out the two outside bolts as much as possible, while still holding the sandwich in place. Remove the center bolt, fold the cell tab over the rest of the way, and auger it through center hole. Because this techninique dictated the direction of the fold, the nickel strip ended up on the bolt side as pictured.

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Once the center bolt and nickel strip are on, the other two are easy.

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A finished join.

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On the left, a lug has been added for the balance wire. Note the nickel strip under the nuts, holding the folded cell tab against the tinned copper strap. Say that three times fast for a gold star.

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2 pieces of heavy-adhesive heatshrink will seal the whole thing in amber.

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A wide piece fits over the whole sandwich. It is tapered to make a completely encapuslate the right side, but fit under the power wires on the left.

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Another view of the wide heatshrink before shrunk.

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Shrunk, ready for the next piece.

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The second piece slides down the red power wires on the right, encapsulating the copper straps all the way up to the cell tab. Cut this piece at an angle, so the point covers the seam of the first piece of heatshrink. The balance wire emerges to the left.

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Another, darker view, but you can see how the point in the second piece of heatshrink covers the seam in the first piece of heatshrink.

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[docnjoj]

Thanks oatnet. I was thinking about something similar using copper strips but now I don't have to think as hard! This seems like a pretty foolproof way of connecting pouch a123 cells. What sizes are the nuts and bolts?
otherDoc

[oatnet]

Thanks oatnet. I was thinking about something similar using copper strips but now I don't have to think as hard! This seems like a pretty foolproof way of connecting pouch a123 cells. What sizes are the nuts and bolts?
otherDoc

I think the bolts were 6/32 x 3/8", I realized I needed more length and went to a larger size so I don't remember clearly. I'm sure a lot of folks have thought of doing something similar, I hope this gives them ideas on what they would like to do (or not do! ). Mostly I hope I've shown that these cells are pretty easy to work with.

-JD

[dnmun]

your stuff is inspirational. you do such good and thoughtful layout and then you have the good sense to do it right the first time.

you are right, this is how 'engineers' learn to do stuff. the right way.

better than learning from mistakes, like the current blowout on the total platform in the north sea. more bad engineering there.

[oatnet]

your stuff is inspirational. you do such good and thoughtful layout and then you have the good sense to do it right the first time.

Thanks dnmun, that is nice of you to say!

-JD

[oatnet]

I bought (110) cells from Victpower, thanks to Doc Bass for pointing that deal out to me. I tested them all for volltage, most were around 3.27v, 2 were 3.24v. I noticed (1) with a misaligned tab, it jumped out at me, if there are others I didn't notice them on this quick check.

I also have a few cells from the cell_man era, in the 16ah and 20ah flavors, so I thought I would compare and contrast. I noticed right away that the foil used on the new USA cells feels thicker and more solid, more puncture and flex resistant. Both 20ah cells are exactly the same dimensions, and tabs line up in the same spot and are the same dimensions. The new USA cells weigh 6.7% more, which I would attribute to the thicker foil.

I think it would be really hard to counterfit these cells, and have the same dimensions, really similar weight, and have identical tab placement and construction. They look clean and new and unused.

(8) boxes of cells!

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On the left, a new USA 20ah cell from Victpower. Center, a cell_man 16ah cell. Right, cell_man 20ah cell.

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One side of 20ah victpower vs cell_man

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the other side of 20ah victpower vs cell_man

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comparing the tops

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The edges

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The USA 20ah cell weighs 494g, 6.% more than the original unlabeled cell_man 20ah cell.

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The cell_man cell weighs 466g

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The 16ah cell weighs 399g.

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[docnjoj]

Thanks oatnet for that timely information. My order of 14 is on it's way fromVictpower. They seem like a class operation so far.
otherDoc

[oatnet]

Someone asked for this a while ago, I got around to cutting open a dead 16ah a123 prisimatic, that had previously been used for my spot-welding testing. There was a chemical smell, but no juice or powder flying around. Like any prisimatic, the layers of subcells are stacked on each other like a book. Enjoy!

-JD

0 new cell IMG_2236.JPG (238.27 KiB) Viewed 1130 times

1 unfolded IMG_2237.JPG (235.39 KiB) Viewed 1130 times

3 tab IMG_2244.JPG (205.84 KiB) Viewed 1130 times

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[oatnet]

In this thread dnmun speculated on spot-welding cotter pins to join a123 packs. Today I tried it out, and I like it. It does not shift around like the two steel plates used to so it is easier to setup. I don't have to adjust the two pairs of vice grips for each weld, just clamp it on one side with vice grips and zap zap zap down the row, it is much faster. This is a simple, effective method for welding cells together in high-quality join, without heat-damaging them.

I used stainless steel 1/8" x 2" cotter pins, .65 each from my local Ace hardware. The rest of the technique is the same stuff I have been posting here for two years.

Position the pin. The tough part is getting the round end of the pin over the tabs, it is quite tight there.

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First weld completed, spot welder still in place, and the vice grips where they stayed for the whole operation.

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A completed weld - note how the entire edge of the tabs are melted, welded together.

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The 18ga balance wire doesn't carry much load, so it just rests the middle of the roll. I make it a little long, and bend the end up into the next fold, so it doesn't pull out.

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like this, on the next fold.

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Last fold complete, and the whole mess crimped with electrician's pliers. The round cotter pin keeps it from crimping as flat as it did with my folding method, but between the weld, and both sides of both tabs being in full contact with each other, it just isn't an issue.

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A layer of adhesive heat shrink to seal it in amber

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Heat shrink folded over at the top

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Sliide on one of the plastic report clips I use (bought more at staples today!)

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A layer of fiberglass-reinforced tape to keep the plastic clip from stretching out.

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The whole assembly from the side.

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