Building and using the kWeld spot welder

Con­fi­dence
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Domain expe­ri­ence
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Audi­ence
Bat­tery junkies
Sum­mary
I strug­gle to put together a spot welder kit.

I bought the kWeld kit and the com­ple­men­tary kCap mod­ule to spot weld con­nec­tions to cells. Both of these are boards with com­po­nents pre-sol­dered, so they require a bit of assem­bly to be func­tional.

Preparation and other components

In addi­tion to the kWeld and kCap boards, we’ll need a power sup­ply and I’m really only com­fort­able oper­at­ing this thing with a case. (I’m clumsy enough that just hold­ing the elec­trodes is intim­i­dat­ing.)

You could buy a kSup­ply mod­ule; it’s designed to be attached to a server PSU, which you’d still have to buy sep­a­rately (or have on hand, as more seri­ous tin­ker­ers seem to). The kCap doc­u­men­ta­tion lists a few other options, includ­ing this MEAN WELL power sup­ply, which is about half the price of the kSup­ply mod­ule by itself (but charges the capac­i­tors more slowly). MEAN WELL sup­plies have worked for me in the past, so this feels like a good start­ing point.

Simul­ta­ne­ously, I found a very suit­able case design pub­lished by a fel­low named Oscar Olan­der. I had been think­ing about mak­ing my own case any­way as I didn’t really like the design of the first-party kWeld or kCap cases (effec­tively still sep­a­rate units, and no room for the power sup­ply). This enclo­sure is even designed for the MEAN WELL unit.

I rec­om­mend print­ing the case before work­ing on any­thing else as it is very use­ful to see how the parts will ulti­mately fit together. I printed it in PETG and it took about 7 hours.

Putting it together

Main module

We sim­ply fol­low the assem­bly instruc­tions from the kWeld man­ual. We don’t need to attach the stand­offs for the main board.

The unassembled main kWeld module, the daughterboard for the display, and associated wires shown on a heat-resistant mat.

The unassem­bled main kWeld mod­ule and its daugh­ter­board. This is how the kit arrives.

The display board attached to the main kWeld module with a standoff and screws.

The dis­play board attached to the main kWeld mod­ule.

The two busbars attached to the main kWeld module with six large screws each.

The bus­bars prop­erly aligned and attached to the main kWeld mod­ule. We’ll attach the neg­a­tive elec­trode on the right side and the cir­cuit will con­tinue back to the power sup­ply out the left side.

Rather than the unter­mi­nated wire from the kWeld kit, we use the wire with a ring ter­mi­nal that came with the kCap. Also note the odd angle; this allows it to fit in the enclo­sure.

The positive electrode and power supply wires connected to the main kWeld module. A 300 amp fuse is between them.

The power sup­ply con­nects to the pos­i­tive side of the main kWeld mod­ule through a 300 A fuse, included. We also bolt on the pos­i­tive elec­trode just past the fuse.

We con­nect the neg­a­tive wires in a sim­i­lar fash­ion.

The negative electrode and power supply wires connected to the main kWeld module.

The wires for the foot switch need to pass through the case before we con­nect them to the ter­mi­nal block.

A top-down view of the stouter side of the 3D printed enclosure with the wire from the foot switch pushed through one of the holes.

You can take the elec­trodes off their cable assem­bly to pass them through the side of the case. The com­pleted stand­alone kWeld mod­ule:

On the left, the kWeld module, and on the right, part of the enclosure with the electrodes passed through it.

The side of the enclo­sure only stands upright because the elec­trode wires are slightly com­pressed in the holes. It’s actu­ally still pretty wob­bly at this point. The foot switch con­nects to the kWeld mod­ule per the instruc­tions.

kCap

Fol­low the kCap man­ual to con­nect it to the kWeld. The out­put side of the kCap also needs wires at a weird angle to fit prop­erly. Again, we don’t attach the stand­offs.

The kWeld and kCap modules on their side, the kWeld facing forward and the kCap facing backward, connected with a pair of wires.

The kWeld and kCap mod­ules will need to be on their side from now on. I don’t think there’s any other way to ori­ent them while keep­ing the wires short enough.

I tested the fit of the two boards in the case. It’s really tight. I thought I had done some­thing very wrong until I real­ized I had to take the knob off the encoder. Then it went in with only slightly more effort than seemed appro­pri­ate.

A side view of the case with the kWeld and kCap boards roughly positioned.

The larger side of the enclo­sure with both boards slot­ted in. Notice how close the encoder is to the edge. With a bit of wig­gling, the dis­play does sit flush against the case.

A top-down view of the case with the kWeld and kCap boards roughly positioned.

It’s hard to align the boards and the wires that con­nect the kCap to the kWeld. I tried to cap­ture the ori­en­ta­tion of the wires, but they’re still dif­fi­cult to see.

Enclosure

I put the heat set inserts into the case. Two of them go on the left side to hold the top of the case together:

A top-down view of the empty case with heat set inserts at the top.

The other four are for “acces­sories” (for now, only an elec­trode holder). I inserted these from the inside of the right side. Unfor­tu­nately, when I did that, it melted the plas­tic and obscured the hole from the out­side, so I had to do some addi­tional cleanup.

Also, in ret­ro­spect, I prob­a­bly should have done this step before I put a bunch of wires through the case. I had to hold the case awk­wardly while I pushed the inserts in with the sol­der­ing iron.

A top-down view of the other side of the case with the heat set inserts for the accessories.

The exterior of the case, showing the remnants of the plastic over each hole where I placed a heat set insert.

I used some long M4 screws to push the melted plas­tic back out from the inside, but it left quite a mess.

A top-down view of the exterior of the smaller side of the enclosure with the electrode holder in place.

A cheap debur­ring tool (or prob­a­bly even a dull kitchen knife) cleans it up nicely. I test fit the elec­trode holder here.

Case fan

The fan wires are unter­mi­nated, so I had to crimp them into Molex KK 254 ter­mi­nals and insert them into a com­pat­i­ble hous­ing. These are really small crimp ter­mi­nals; I ended up buy­ing (and would rec­om­mend) an Engi­neer PA-20 crimper for this step.

A slim case fan with the positive, negative, and tachometer wires in their housing.

The fan ready to go into the enclo­sure.

M5×10 mm screws, which are stan­dard for case fans, work just fine for our appli­ca­tion. The screw will bite into the fan frame.

The case fan attached to the side of the enclosure.

Power supply

Next, I pre­pared the wires for the power sup­ply assem­bly by cut­ting them to size and crimp­ing them to the appro­pri­ate ter­mi­nals.

The power inlet has quick dis­con­nect ter­mi­nals, which I hadn’t used before. I ini­tially bought these (appar­ently nice) TE Ultra-Fast ter­mi­nals, but I didn’t real­ize the bar­rel had a flat side. That made it incom­pat­i­ble with my generic crimper, and I wasn’t in the mood to shell out hun­dreds of dol­lars for their mag­i­cal pro­pri­etary crimper and die. I replaced the ter­mi­nals with a dif­fer­ent brand, but I still had a hard time align­ing them; they also seemed to require a ton of force to close all the way.

Nev­er­the­less, the crimps turned out solid. How do I know? Well, I cut one open to see, but I also real­ized I could actu­ally test the resis­tance of the wire because I had bought a microohm­me­ter from Aliex­press to check the inter­nal resis­tance of my bat­tery packs. Cal­i­brated to Amer­i­can stan­dards, these suck­ers will run you some­where around $5,000. But the Chi­nese spe­cial? That’s right, I paid $23.13. Granted, the $5,000 microohm­me­ters let you test with some­thing like 10 A100 A and mine pulls 50 mA. Any­way, the wires mea­sured cor­rectly for their lengths through the ter­mi­nals, and I was happy (even though my hands hurt).

The power wires for the PSU and kCap laid out on a mat.

From left to right: fuse to switch, switch to PSU, PSU to neu­tral, PSU to ground, PSU DC out­put to kCap and return. I used 14 AWG for all of these wires because it’s what I had on hand, but even for the kCap side 16 AWG would prob­a­bly be fine since the dis­tance is so short.

The power inlet with the live, neutral, and ground wires attached through the fuse and switch.

The power inlet wired up. The switch is DPST; we’re only using one pair.

The power inlet connected to the PSU and fit into the enclosure.

The spade ter­mi­nals I bought were on the longer side, so I had to bend them a lit­tle to fit them into the enclo­sure next to the PSU.

The kCap needs at least 8.1 V, so we use the adjust­ment pot on the PSU to select the proper out­put volt­age before we con­nect it. The kCap doc­u­men­ta­tion rec­om­mends 8.2 V for this PSU, so that’s what I set it to.

My multimeter connected to the DC terminals of the PSU. The power inlet switch is off.

The mul­ti­me­ter care­fully con­nected to the DC ter­mi­nals of the PSU, leav­ing enough room to adjust the pot with a screw­dri­ver.

The multimeter reading the proper voltage after adjusting the pot.

The PSU online and adjusted to the desired volt­age.

The PSU and kCap connected.

Once the volt­age was set, I dis­con­nected the mul­ti­me­ter and con­nected the kCap inputs to the PSU.

Fitment

Finally, I put every­thing into the case together. Get­ting the right side of the case to fit onto the left side might have been the most dif­fi­cult part of the entire project. In the test fit, you can see how the 6 AWG wire pushes the kCap away from its desired align­ment. I ended up squish­ing the wire a bit to get the case to close; I’ll have to recheck every­thing after a few uses to make sure the screw for the kCap input isn’t dam­ag­ing the insu­la­tion.

Speak­ing of that screw, I ini­tially wired the input side of the kCap on the bot­tom of the board, just like the out­puts. But there’s no space to run wires between the kCap and the PSU: the board is flush with the case on 3 sides and sit­ting on the PSU itself on the other side. So I ended up hav­ing to pull every­thing back out of the case to swap those con­nec­tions to the top of the board instead.

The wires were also annoy­ing to wran­gle, and I ended up using some strate­gic zip ties to keep them together as appro­pri­ate. I pushed most of the AC wiring around the hole for the foot switch stor­age bucket.

A top-down view of the kWeld module, kCap, and PSU inside the left side of the case.

The mod­ules and PSU inside the left side of the case. In this photo, the kCap input wires are still con­nected on the wrong side, and you can see it will be impos­si­ble to close the right side of the case over them.

If I were to rework the case a bit, I’d prob­a­bly make it 5 mm10 mm longer to increase the dis­tance between the kWeld and kCap boards slightly. I’d also put some wire guides on the inside to help keep the DC and AC wires apart and every­thing away from the capac­i­tors and the fan.

The complete kWeld assembly with power to the display. The display reads CAL.

The moment of truth. It boots!

Fol­low­ing the cal­i­bra­tion guide in the oper­a­tion man­ual, I got about 1200 A from the kCap and about 2.8 mΩ for the inter­nal resis­tance of the whole cir­cuit, which is to spec. Now we’ll see if I need to buy another kCap to put in par­al­lel or if my appetite will be fully sated by 0.2 mm-thick nickel strips.

The kWeld after calibation. The display reads E= 10.0J, indicating it is ready to weld.

The unit ready for its first weld.

Parts list

PartModelQuantityPriceLink
Wire~1 m
Size (Ga)
14 AWG
Insu­la­tion color
Black
Insu­la­tion mate­r­ial
PVC
Wire mate­r­ial
Cop­per
Wire type
Stranded
Wire~1 m
Size (Ga)
14 AWG
Insu­la­tion color
White
Insu­la­tion mate­r­ial
PVC
Wire mate­r­ial
Cop­per
Wire type
Stranded
Wire~1 m
Size (Ga)
14 AWG
Insu­la­tion color
Green
Insu­la­tion mate­r­ial
PVC
Wire mate­r­ial
Cop­per
Wire type
Stranded
Wire~1 m
Size (Ga)
14 AWG
Insu­la­tion color
Red
Insu­la­tion mate­r­ial
PVC
Wire mate­r­ial
Cop­per
Wire type
Stranded
Cable tie~5
Power supply assembly
Power sup­plyMEAN WELL RSP-150-7.51
  • USD 38.60
DigiKey
InletAdam-Tech IEC-GS-1-1001
  • USD 5.31
DigiKey
Fuse1
  • USD 1.09
DigiKey
Dimen­sions (⌀ × L)
5 mm × 20 mm
Rated for cur­rent
10 A
Rated for volt­age
250 V
Insu­lated quick dis­con­nect ter­mi­nal5
  • USD 0.24
ea.Ama­zon
Size (Ga)
14 AWG
Tab dimen­sions (W × T)
4.8 mm × 0.8 mm
Tool
Insu­lated ter­mi­nal crimper
kWeld assembly
kWeld kit1
  • USD 249.00
Grid Rewired
kCap kit1
  • USD 197.00
Grid Rewired
Screw2
  • USD 0.28
ea.Bolt Depot
Dimen­sions (⌀ × L)
M6 × 16 mm
Screw head style
Socket head cap
Screw type
Machine
Insu­lated ring ter­mi­nal2
  • USD 0.69
ea.DigiKey
Size (Ga)
14 AWG
Stud size
M6
Tool
Insu­lated ter­mi­nal crimper
Washer2
  • USD 0.06
ea.Bolt Depot
Dimen­sions (⌀ × H)
12 mm × 1.6 mm
Stud size
M6
Hex nut2
  • USD 0.11
ea.Bolt Depot
Stud size
M6
Case fan assembly
FanDelta AFB0812VHB-F001
  • USD 13.83
DigiKey
Dimen­sions (L × W × D)
80 mm × 80 mm × 15 mm
Rated for cur­rent
500 mA
Pin 1
Ground
Pin 2
12 V
Pin 3
Tachome­ter
Screw4
  • USD 0.18
ea.Bolt Depot
Dimen­sions (⌀ × L)
M5 × 10 mm
Screw head style
Coun­ter­sunk
Screw type
Machine
Molex KK 254 crimp ter­mi­nal3
  • USD 0.06
ea.DigiKey
Tool
Unin­su­lated ter­mi­nal crimper
Engi­neer PA-20
  • USD 31.95
Ama­zon
Molex KK 254 crimp hous­ing1
  • USD 0.14
DigiKey
Con­nec­tions
3
Enclosure
3D model1
  • ~USD 10.00
Thin­gi­verse
Heat set insert6
  • USD 0.19
ea.Ama­zon
Dimen­sions (⌀ × L)
M4 × 6.3 mm
Tool
Sol­der­ing iron
Screw6
  • USD 0.11
ea.Bolt Depot
Dimen­sions (⌀ × L)
M4 × 12 mm
Screw head style
Coun­ter­sunk
Screw type
Machine
Screw2
  • USD 0.18
ea.Bolt Depot
Dimen­sions (⌀ × L)
M4 × 25 mm
Screw head style
Coun­ter­sunk
Screw type
Machine
Down­load the parts list in TOML for­mat.

Conclusion

Hap­pily, this turned out to be more like putting LEGO bricks together than a com­plex elec­tron­ics project. Fol­low the instruc­tions and you’ll be fine.