Spot-Welding a123 Prisimatic tabs

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...

Nice work!

Doc

NICE WORK OATNET. would it help if instead of mildsteel strips you used stainless steel strips?. I am partial to stainless steel for everthing. 8) 8)

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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

Great info, oatnet. Thanks for the post.

Mods - this should be stickied.

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.

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! :lol:

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

regmeister said:

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/DkSearch/dksus.dll?Detail&name=560GC-ND
http://www.polymerplastics.com/composite_vulcan2.shtml

probably a bit too expensive for the application though.

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

-JD

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

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.

oatnet said:

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

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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.

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

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

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.



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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

docnjoj said:

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

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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

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.

dnmun said:

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.

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Thanks dnmun, that is nice of you to say!

-JD

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.





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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

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











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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.

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