Looking for BIG capacitors.

[itsmoked]

I am building a capacitive spot welder. Over the years I have thrown away maybe a hundred ~47,000uF screw post type/panel mount type caps. Now I find that I need 6 to 700,000uF at 16 to 25V worth of caps..

I expect to find these things laying around, but now suddenly, no sign of them.

I have looked at maybe 30 spots on the web. Ebay has them but in ones and twos in Bulgaria, etc.

Anyone have a handle on a spot where I can get some/all of these? I don't want to give Digikey $20 a piece for 10 of them. <groan>

 

[itsmoked]

BrianR is that the stuff in a little gray can? I may have some. Everything I'm reading is saying that there are membranes in the batteries that will be damaged, especially Li-Ions. When the cost $11 a piece I am wary of damaging one in the middle of a stack only to discover it by having a laptop explode on my lap...

 

[Warpspeed]

Somewhere in the dim recesses of my rather murky memory, I remember reading about electrically conductive epoxy. It contained silver powder, (or something similar).

Anyone know about this stuff ? And could it be used to "glue" tabs on batteries without incinerating the internal structure ?

 

[BrianR]

itsmoked, my lot is in a black plastic bottle and is a clear liquid.
I have also used it to solder stainless steel but a lot of heat is needed and a 2% silver solder helps a lot too.

I don't know about the cell structure but am suspicious that what you say is correct and I might have a short life for these cells.

 

[dino40]

itsmoked,
If your still doing the Cap route, Check with anybody that works with old computer main frames they may have a few old caps laying about form the power supplies.

 

[Comcokid]

For a low voltage spot welder, you could try using a car battery. I seem to recall some simple low duty cycle discharge-welders actually had a lead-acid battery as the source with a charger to keep the battery up. These used a mechanical means to drive and pull back an electrode against the two pieces of metal to be spot welded.

 

[fsmyth]

"What's wrong with invention -> motorcycles??!?"
Nothing. The point was that the end use is more fun in
those two categories than the inventive process.
<als>

 

[itsmoked]

Thanks dino40.

Comcokid.... I really like that idea because I know the typical voltage used is about 11-13V and what is a lead acid but a reallly big cap?

I have certainly welded my share of dipsticks, etc, across batteries. I even have two new six volt Trojans sitting in my lab. I also have a bin of IGBTs, the big brick ones yanked from a VFD. Perhaps I could run an IGBT with a triggered signal generator to do this whole thing.

 

[fsmyth]

'smoked, the 6-cell (12v) version will work, but I have
found it easier to control the weld diameter and depth
with a higher (36 to 48) voltage source. Plus, your
choice of electrodes is broader as the voltage rises.
Your IGBT's should be capable of handling 50v pulses
easily.
Note that I have never used this in any production type
setting, merely toyed with it as a sort of experiment.

As far as soldering, I have soldered NiCad, NiMH, SLA,
Li-Ion, and any other small cells used in packs, with
no apparent problems. I do use the "solder the tab on
one end, then to the other cell, and fold" method,
sometimes using wire if small strips were not handy.
I have not found it necessary to use any special or
unusual flux; merely lightly sand or scrape the spot
to be soldered, tin it quickly, and solder the tab or
wire after it has cooled. If any cells have been
damaged by this method, it has not shown up yet (and
some rebuilt packs are old enough that the usual aging
effects are starting to appear).

Not trying to throw sand on your spot-welding effort
(I have used far less in the past to justify experimenting
with a new or different technology/field/method/process),
but if the objective is to construct a battery pack, then
I'd say solder it and be done. If there are any negative
effects of soldering cells, I would certainly appreciate
someone jumping in with an explanation.
<als>

<als>

 

[Madcow]

Itsmoked,
This article used a microwave oven transformer.

http://www.ch601.org/tools/homemade/arc welder/build_your_own_arc_welder.htm

Is the coffee mug sized scr more of an apple sized mug or
a grapefruit size mug?

Hope this helps.

 

[itsmoked]

Thanks for the voltage tip fsmyth. It does appear that the entire spot welding scheme is a battle of resistances. The key is that the 'big resistance' be the location you want welded. Well when the place you want welded is a good conductor to another good conductor you see the problem. It often comes down to the cable, electrode, and termination details. I suspect if you go not "the extra mile" but two miles towards the cabling aspect then the energy/voltage situation gets somewhat less important.

fsmyth; I believe I could safely do my multiple 15 cell nicad packs with solder {evil lead solder }.

My concern is more with the relatively new Liions. I think(could be wrong) that they are not vented because water vapor entering them may react badly. Hence the reason they can explode more easily, (witness recent cellphone injuries), and that specifically they have the membranes in them that are not meant to take any heat.

Have you (fsmyth) or anyone else soldered LIions successfully? Or just nicads and NiMh?


Madcow; Yeah I saw that welding article a while ago. Tiz not a spot welder I like the two 115V cooling fans wired in series. Haha!

I fished out my SCR and found that I inflated the size estimate some what... It is actually the size of a grade AAA Jumbo Egg or possibly a small tangerine.