N o a h ?

Building and using the kWeld spot welder

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.

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.

We con­nect the neg­a­tive wires in a sim­i­lar fash­ion.

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

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

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.

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.

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:

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.

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.

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

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 A–100 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 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.

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.

If I were to rework the case a bit, I’d prob­a­bly make it 5 mm–10 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.

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.

Parts list

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.

• Now let’s try to make a bat­tery.