Lithium Powered Spot Welder

In this ible, I will be showing how to build a small and portable (and short use) spot welder made WITH lithium cells.

If you are not comfortable with lithium cells (know about different types, how the ratings work etc.), please do read my other ible on salvaging lithium cells first, or stick to lead acid, or ni-cd batteries to avoid starting a fire.

The other ibles I am using in here, other than the cell salvaging ible, are my DIY 1$ cable lugs, and my 3$ DIY electrode holders.

Step 1: Material

You will need

1. Your battery pack. If your are not going the li-ion route, grab an SLA battery, but from here on I assume you are also assembling a lithium pack
2. Your electrode cable, or make your own as mentioned before
3. some lugs if you don't make your own, or these fancier connectors
4. some 1 awg cable
5. solder
6. soldering torch

Step 2: Prepare Your Main Rail

To make my "rail", I just stripped the middle part of a length of 1 AWG cable. I left some sheeting at the front and back to prevent unravelling, and to make it easier to handle.

Step 3: Parallel Your Cells

With your cells' poles all facing in the same direction in the assembly you want, wrap the leads you had added to the cells on the 1AWG cable you prepped. Wrap them alternating up and down.

Step 4: Solder the Leads to the Rail

Solder the leads to the rail, alternating in a diagonal pattern. So front right, followed by back left, followed by front left, followed by back right, etc. Just try not to hover in the same place for too long. At first, just get a spot done on each lead, just so it is secure, then let it cool.

Once it is cooled down, you can go for a second pass, this time focus on making sure that the spot where you twisted the pigtail is tinned. Keep going until you are comfortable with the amount of contact. In my case, 2 waves sufficed.

Step 5: Using It

With the electrode cables having already been fabricated, it's just a matter of assembling the parts, and learning to spot weld.

I do plan on adding a relay with a timing circuit in the future, but for now, I don't have the materials needed, so I use old school timing.... me.

Lithium cells will head up in these kids of uses, so this is meant for a few spots at a time. It really is for small jobs, with an obstructive pack. Give it a test, and allow your pack to recover between stretches of use. Otherwise, it works the same as any other spot welder. Make sure you have good contact, sand your surfaces to clean them first, etc.

Enjoy!

Step 6: Extra: Determining the Current / Power of Your Welder

If you want to know how much current is flowing, there is a test you can do. Since you know the length and specs of your lead, use it's length and resistance per foot as listed in this table: https://en.wikipedia.org/wiki/American_wire_gauge#...

Once you have your resistance for the total length between your two probes of your multimeter, you should record the voltage (as it will change fast) as you spot weld. Let me be clear, this should be on a single and same side of the "load" (the electrodes/welding).

Divide the voltage read by the total resistance of the length of wire you were using as a shunt, and that will be the number of Amperes flowing through.Your units have to be on the same scale, so convert everything to ohms and volts, or keep them in millivolts and milliohms, but be consistent.

In my case it was 176 millivolts at peak that I recorded (it probably went a bit higher though, better quality meters will refresh faster), so 0.176 volts

The resistance for 1 AWG wire is 0.1239 mOhms per foot, so 0.0001239 ohms per feet. Since I had 44.5 inches between my two probes, that is a total of 0.0004594625 ohms.

If I divide 0.176 by 0.0004594625, it gives me 383 Amps.

On a side note related to this, I also monitored the temperature of my cells and my lugs as I was testing (and quite frankly abusing) my device for the first time. The lugs are more than capable of handling the current at the duty cycle involved, and the cells were able to keep within normal operating temperatures for a significant amount of time, considering that this size pack is really meant to do just a few spots here and there.

Comments

[deluges]

Maybe a bit repetitive with the other comments but : no protection of the batteries or the human is bad and dangerous.

This is just short-circuiting the batteries and Lithium batteries expand a lot of energy that can go toward exploding, especially when being short-circuited.

I feel like a disclaimer other than "If you're not comfortable with X and Y don't do it" would be extremely useful at the beginning of this ible. People might not know just how much energy is packed in those batteries and think they are comfortable until they lose a couple fingers.

Pic related, Randall knows that a picture is worth a thousand words.

[MakinThings]

Lol, repetitive maybe, but totally worth it for the xkcd comic :p. They don't explode per say, but violently burst into flames (in the case of lico), but if you pick appropriate cells, the worst reaction from a permanent short would be swelling and venting of hot gasses (that's what those little holes on this site positive terminal are for, and one of the issues with sealed cells or battery packs like pouch cells and laptop bricks). Cell protection would have to be specifically designed; normal protection circuits would not work well here. Tracking temp and voltage are the safe thing to do in this case. A protection circuit would not make the difference between the voltage sag and dangerous discharge levels, so it would be useless as it would either prevent the device from working, or allow I to work too long while giving a horribly misplaced sense of security. Over discharging is an issue because the electrolyte becomes corrosive and eats that the cell, voltage sag does not.

[deluges]

Maybe we can control the heat by putting it in a bucket of water. Wait...

[MakinThings]

safety sand, always safety sand!

[rickharris]

I know you say this may not be such a good idea, but Lipo cells have a distinct dislike for providing huge currents. this is effectively a short circuit and my experience with RC models tells me this results in the cells bloating and quickly becoming useless if not actually igniting.

[MakinThings]

Yup, that's why I say not to use lipo cells explicitly even though they are generally considered high power ish cells. They are too severely compromised by being both high power and capacity; they are optimized for weight and cost. I have a 45 minutes video and a ible longer than this one just on selecting proper cells for high power use. I kept it separate precisely because there is so much information.

[SuperSanta]

Correct me if I'm wrong but can't this be really dangerous and bad for the cells since there is no protection circuit.

[MakinThings]

If that's what you understood from the information provided, then yes, absolutely.

[Cekpi7]

What kind of lithiums is that?
400A with only few cells is alot
Most of them can provide about 2C safely so you would need almost 200Ah at 3.7v to be safe (regular lithiums not high amperage ones)

[MakinThings]

Yup. I think I am going to update my intro a bit to reflect that; I pretty clearly say in the video that this could be dangerous if not properly done, and I do go over it in the salvaging li-ion ible, but you are right that I don't say it enough in this one. They are emoli Li-MN cells; when new they were rated for continuous use at 40A each and rated for peak (30 seconds) use at 100A. So even though they are old, I am just running them in bursts at what would have been their original rating.