LiIon as source for a spot welder? (1000A for a few miliseconds)

Ha. Ok. Don’t coin any new phrases like ‘curiosity maimed the engineer’.

hehe I’ve been electrocuted more than once :slight_smile: Still can’t get enough of it.

I find this idea very intriguing - the problem is that I would use this welder a lot, and then I feel I would have a 10pack of VTC5A cells sitting in my room, being short circuited for hundreds if not thousands of times over a year or two. And only one internal short circuit could be potentially catastrophic, like having a ticking bomb.

Still I’m intrigued to somehow test this premise. I could build it and then torture test it, set it in an infinite loop with pulsed short circuits to test if something bad would happen. I live in a flat so I don’t have a safe way of doing it.

Just buy some D-cell-size Maxwell supercaps. They’re damn impressive. And they won’t explode and kill you.

You don’t need to get all fancy with the control side, either. Just a momentary foot switch that applies power to a FET’s gate and there’s your spot welds.

yeah I will feel better with caps and not shorting VTC’s. When I was a kid I had tons of fun with some 0.2F caps, Now I can afford much more capacity :slight_smile:

I already ordered FET’s, I plan to control them with attiny85, but I think I need a gate driver to avoid ringing/spikes. I don’t yet know how to make gate driver, or which integrated gate driver to use.

This guy did all the math:

http://www.turtlesarehere.com/html/cd_welder.html

This is not a test result for an actual battery, but a test specification with criteria for “test passed”.

Note that “does not vent” or “does not spew electrolyte” or “cell performance still meets specifications after test” are NOT listed as pass criteria.
That is, a cell passes this test even if it vents and spews out electrolyte and is dead afterwards. It must just not ignite and not explode or pop. Controlled venting and terminal damage is allowed.

A high-current cell without PTC (like the VTC5A) will usually vent on an extended low-ohmic short (if the shorting wire can take the current and doesn’t melt after a second). At the very least, the cell is damaged afterwards.

Ohm’s law: 200A current requires max. 20mOhms total resistance for a fully charged cell. That is, internal resistance plus contact and wiring resistance plus welding point resistance. So it could be possible to get the required current out of a VTC5A for a fraction of a second, but maybe not very wise to do this repeatedly. And if the welder fails to end the current pulse (FET shorted or the like), you can only hope that some other component will melt faster than it takes your cell array to start venting.

Edit:

Actually, this is a requirement specification for an “over current protective circuit” (7.2.4.3), not a performance specification of the cell.

[quote=vex_zg]

[quote=Joshk]

I can completely relate on both counts separately, and on the statement as a whole.

Stick a VTC5 in a dead short contraption until failure to get a baseline for worst case scenario, then build the thing if you aren’t scared off by the results of the first test.

Oh an Youtube results (of at least the first test) please.

if you really need to use “batteries” for a spot welder, then only a automotive lead-acid battery than safely provide that momentary high current needed. A larger good quality cranking battery can provide 1000+ amps of current, as they are designed to do. One problem is weight & size.

Car battery? Why not Boat battery (Marine Sealed AGM)? I don’t think you want hydrogen and sparks inside your house at same time.

I would feel defeated if on top of all the lithium I have at home I bring that oversize and overweight behemoth home.

Any advice on which caps to use?

Maxwell D-size 2.7V/350 farad. Six in series will start my car.

Any of maxwells caps are impressive.

As for the Li-ion, if you are really determined to use cylindrical cells you could use some of A123 systems’ 26650s. They are rated for 70 amps continuous, and just get a massive sub ohm power resistor to keep the current under the 100 amp pulse maximum.

Remember that supercaps aren’t your grandad’s caps. They will behave as a dead short for a shockingly long while when you start to charge them so plan your charging circuit appropriately.

“Callies Kustoms ” ” did that test, with Panasonic cells it is on their website.

i don’t think he will (or should be) be “spot-welding” in a house in the first place. Common sense should be in play here, also its unlikely that one is going to make the spot welder “on” the battery, cables would be used with the battery many feet away. Welding along side ANY battery with a plastic shell sealed or not would be just stupid.

Why not just use the transformer out of a microwave oven like the rest of us nutjobs that build a spot welder? Granted I only use mine to weld (mostly) vehicle body panels but some tinkering can get you to the amps you need for battery tabs.

It makes no real sense to use batteries, but I wonder about all the fear…?
The approach seems to be ok(short burst+high current+a lot parallel cell) so everything in spec and safe.

Like others said the transformer from an old microwave would cost next to nothing and works, so this seems to be the best lowcost variant.

just to be sure we are on the same page what spot welding is: it is a process where typically you let big current in range of 1000A run through two thin sheets of metal to weld them together. The pulses are typically between 2ms and 20ms. There can be some smaller sparks but the spot welding itself does not generate lethal voltages or sparks.

this it how it looks typically

My other option is to buy a spot welder from china (cca 200:money_mouth_face: but I have safety concerns there because that thing connects to the live grid and with those large currents things can blow it inside of it, so I’d rather deal with non lethal voltages in range up to 16V.

The cable from the welding energy source (18650 VTC5A in the initial proposal) would be perhaps 50cm long 6 to 10AWG cable. So not many feet away, just the minimum needed to reach the object of the welding.

you do not need 1000 amps for battery tabs. i think even low quality spot welder from china will be safer than shorting out li ion cells,
worst thing welder can do, is pop the fuse, li ion otoh will do a lot worst, you can get seriously hurt if you breathe that smoke.

now i do recommend you have class D (yellow) fire extinguisher, it is the only type that deals with metal fires, no powder, water, or co2 will do anything to burning lithium.

actually there are few good diy spot welder plans on instructables.

unless you need it to be mobile, not tied up to ac lines, than use lead acid, they do not blow up with no warning, they heat up swell before so you’ll notice if there is an issue.
also i would not recommend marine batteries, they are deep cycle, and have solid plates, that means you can discharge deep, but screen plates or starting batteries, will give put out more current, that is when you compare 2 same size\capacity batteries.

You have not seen the people who have been harmed by li ion battery explosions or people who die in fires, have you?

There is a difference between challenging dogma and attempting to create unstable explosives and hoping they do not explode

machinist/welding is what i do for a job/profession. i should not need to say more.

allright all, let’s relax a bit, I have a family, experience, education, I’d like to blow up things just for fun but I will not put people or property in danger.

I initiated this to get some feedback. Feedback received. I will not use liIon cells as the power source. I might torture test a VTC5A in a safe manner to document what happens.

That all said - dogmas and laws of physics and everything else should be always open for challenging. That’s the only way forward. Don’t get irritated if your opinions are challenged, it can provide learning possibilities.

Anyway now I need:

1. a capacitor with low ESR (rockford fosgate seems ok, 1F 1.2mOhm ESR)
this guy tested a few of them

2. MOSFETS (ordered a bunch of these)

http://www.newark.com/international-rectifier/irfp3206pbf/n-channel-mosfet-60v-200a-to-247ac/dp/08N6350?ost=IRFP3206PbF&selectedCategoryId=&categoryName=All+Categories&categoryNameResp=All%2BCategories

3. MOSFET gate driver. I have no clue yet how to do this but I don’t think I can drive 10 200A fets directly from Attiny85. Something bad will happen.

4. protective glasses & gloves