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.
Step 1: Material
You will need
- 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
- Your electrode cable, or make your own as mentioned before
- some lugs if you don't make your own, or these fancier connectors
- some 1 awg cable
- 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.
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.