Battery welding issues


1 mW
Apr 17, 2020
I've been playing with 18650 cells for a little bit now, recovering cells from various sources, testing them and such, and am going in the direction of starting to build some small packs. The first 7s pack I put together, I soldered (Yes, I know, not generally a good idea, but I was careful, minimized the heat time, cooled them off as fast as possible, and it was a POC - those particular cells haven't been charged since then and won't be used again - they're red wrapped cells {don't recall the mfg offhand} that are often called 'heaters' because they get hotter than most when charging). That being said, I'm looking to 'step it up' a little and do it right, so I ordered a spot welder. I ordered a Sunkko 709AD+, which is a variant of the recommended 709A in this thread (I found this forum after I ordered it, but I'd imagine all '709A' variants are 'essentially' the same at their core).

That being said, for what it cost, I'm rather disappointed. The 709 seems to be 'the' welder out there from my readings, but while I've had some success welding the nickel plated .15mm stips to cells (using 'junk' cells from my recoveries for testing), it seems pretty inconsistent, and I've had zero luck getting the .2mm thick pure nickel strips to stick. Pretty much every attempt, if you look at it hard it pops off. Sometimes it sticks more to the copper welding leads and pulls off the when you pull the welder away.... From what I've gathered, it seems like it may be at least partially related to the higher conductivity of the nickel (hence the reason I'd rather use that over nickel plated steel), causing most of the current to flow through the nickel rather than through the nickel, into the cell end, through the cell end and back through the nickel creating the welds. This theory seems backed up by the fact that I've also had some success welding some .2mm 2p pure nickel strip that has slots at each cell - when welding where there's a slot, I have gotten better welds (again, welding on junk cells to test and try to get it figured out). My concern is that without changing settings or materials, I've gotten welds that held nice, I've gotten welds that didn't hold for squat, and I've gotten welds that seemed WAY hot and like they may have blown through the battery casing a bit - those batteries went outside on a concrete patio in case they decided to 'go up'.

All of this has been with the 'pen' (I did some attempts with the fixed head, but the results weren't wildly different). Now, I've read conflicting things regarding the pen - one thing I've read says that the feeds for the pen and the fixed head are one continuous piece of braided wire (per lead) from the pen sockets to the fixed head that are soldered to the transformer in the middle, and the other thing I've read says that the wires from the transformer to the pen sockets are smaller than the wires from the transformer to the fixed head, thus introducing more drop to the pens contacts than to the fixed head. I don't know which one is accurate, but I discovered that if you take the screws and welding pin holders out of the fixed head, you can plug the pens contacts into the end of the fixed heads with no permanent modifications (you need to spread the fingers on the pens contacts a little for a snug fit, but you can still plug them into their native sockets). So I've been using it 'that way'. The problem with the fixed head is it's pretty useless unless you're only welding a single row of cells, be it Xs1p or 1pXs. Might get away with two rows, but I don't think much more than that. The leads on the pen themselves seem to be around 4AWG - while not 2/0 or 3/0, it's not quite 18AWG either. In the end, IMO, if the pen is 'useless' as some seem to feel, then in reality, the whole thing is useless.

So all that being said... As far as the nickel strips, is .2mm simply too thick, even though it claims to be able to handle it? For the machine, is there some trick to getting it to weld consistently? Or should I return it and look into another option like the JP Welder (now open sourced) that was also in the previously mentioned link (Only downside to that is needing to have at least one lead acid battery nearby)? Mains power would 'kind of' be preferred, but if there is really a better option that is driven by battery and/or large capacitors (I found one that seemed to run mainly off large caps, but can't seem to find it again), I can't say I'd be against it... 4-8 Headway LiFePo4 cells in either a 4s1p or 4s2p (or even more 'p') config could pretty easily take the place of a 12v lead acid battery.

I'm open to thoughts and suggestions... It's just rather frustrating to have so much trouble getting good welds out of a machine that seems to be fairly highly regarded, at least without spending a few K on something more commercial...
you got a picture of the handheld probe pen your using. The pictures I seen in the ads looked like 10 gauge copper clad wiring was used. When you use your handheld probe, do the cables jump all over the place everytime you do a tab weld? I notice that when I was using 8 gauge copper clad wiring.

When I first started tab welding (malectric tab welder) I was very dissapointed in the performance of the 8 gauge wiring I was using. I had to increase the power setting and the probes were getting too hot. It wasn't until I got 6 gauge pure copper welding cables for my probes that I finally got good performance. Maybe thats all you need, to make your own probe cables. 6 gauge pure copper cable will cost you less then 20 dollars. Probe tips you can make out of copper nails (also not expensive).

picture of 6 gauge pure copper probe tips. Pure copper was a major improvement for welder performance.
malectrics probes.jpg
I wouldn't exactly say the cables 'jump all over the place', but they do have a noticeable twitch to them, I'd say.

So this is the handheld 'pen' it came with:

This is the OD of one wire - granted, insulation and all.

Here's where the copper comes out of the insulation:

Based on how the insulation appears to be around .5mm thick (maybe a hair less), that places the conductor in the ballpark of the 5mm or 4AWG range.

Here's some 10ga for comparison:

So the 'pen' doesn't have 'best buy' grade speaker wire... maybe it could be better, but it could certainly be worse.... Although I suppose it's possible it's copper clad, not pure copper...
Does anyone else have any thoughts? Should I stick with this one and 'try to make it work', or should I send it back? Is the JP Welder really better (it and the 709A are the top two whose links still work in the previously mentioned thread)? I have no problem sending this back and getting the parts to build the JP welder if it really is that much better.

I did some more testing and tried out something suggesting holding the 'pen' at a 45* angle instead of straight on, and that seemed to give some good welds with a piece of the pure nickel... then tried the same thing with the same settings on the positive end of the battery... total fail. Fell right off. And actually 'dented' the cap (Probably from the heat)... The only nickel that I've really had any 'real' success with is the slotted stuff, and even that wasn't consistent. The Sunkko welders, especially the 709A, seem to be the more highly regarded welders (aside from the pulse button differences and the LCD display, I can't see the 709A and the 709AD+ being different in terms of welding capability - on the surface, the differences between the 709A and the 709AD+ seem to be the 709AD+ has a LCD display for the pulses (The 'D') and different buttons for the pulses (the '+'). Other than that, they should be the same under the hood). But I'm not seeing it...

The only thing that 'boggles my mind' regarding welders like the JP Welder is the idea that a 120v spot welder can't get the job done, but one running off a 12v car battery (Or headway LiFePo4 cells) can... But if they really can, I'm game.. What I really don't want to do, though, is the 'homemade welders' that are a car battery, a solenoid, a button and the probes (which I've seen on youtube)... THAT is a bit too redneck for me for this subject lol. That's kind of asking to burn holes in things left and right. Or something.
I had a Sunko and was very disappointed in it's performance. After looking at the JP and KWeld here on ES, I went with the KWeld and am extremely happy with it. After the first use of the KWeld, I decided that the Sunko's are junk.
BVH said:
I had a Sunko and was very disappointed in it's performance. After looking at the JP and KWeld here on ES, I went with the KWeld and am extremely happy with it. After the first use of the KWeld, I decided that the Sunko's are junk.

Thanks, that kind of input was very helpful. The Sunkko is on its way back and I'll be purchasing a kWeld.