Diy Solenoid Battery Spot Welder, Need advice on how to limit current

Thread Starter


Joined Feb 1, 2019
just built an 18650 battery spot welder using a car battery and a 300a 12v solenoid. im triggering the solenoid with a programmable timer hooked to a footswitch.

the spot welder is functioning but once in a while it seems to be too strong and I have the timer set as low as it can go, at one tenth of a second. the car battery I'm using is rated at 540 CCA. and I do have 100 amp fuse in line with the solenoid.

I've seen online somewhere that someone recommended using a thinner gauge wire somewhere in this type of project to limit the current that is making it to the welding tabs. But I cannot find where I had seen this.

if anyone has an easy solution to limit the amount of current getting to the welding tabs/ends please let me know. if using a thinner gauge wire would work please explain


Joined Dec 29, 2008
... You might be able to use a length of graphite rod. It comes in different diameters ... 1/8" , 1/4" . .. maybe even 1/16". The difficult part will be finding a source that won't require buying a dozen or so at a single purchase. Try an industrial supply store or two.
... You can easily break them in two or smaller, as you get closer to the best amount of current. Just watch out for heat build-up, as they can get red-hot.
... I carried them as emergency 12 volt to 6 volt battery charging adapters in a 1950 pick-up with a 6 volt system. I never had to try the idea out though.


Joined Sep 17, 2013
A carbon/zinc AA/C/D cell has a carbon rod as the +ve electrode. Don't know if that would provide a suitable resistance?


Joined Jan 18, 2008
the spot welder is functioning but once in a while it seems to be too strong
What to you mean by "too strong?" Occasional burn through?

How will the resistor/resistance you plan to install know when that is about to happen? It will be in place for all welds. Will the other welds be OK?

Is the problem burn through? Let's discuss the problem, not a presumed solution that may not really address the problem.

Thread Starter


Joined Feb 1, 2019
Hey thx for all your replies...


OK well the welder is now to strong on every weld. I found that. My battery terminals I bought had a painted spot where I was making my connections before so that's why some welds were good but the welder would fire intermittently. After addressing that problems and now I have a proper connection the welds are burning straight through the nickel strip and actually starting a small fire for a sec or 2 and shoot red hot sparks....

any ideas to lower the current or would I be better off going and buying a lawn mower battery with 1/2 or less CC amps rating? I think also 0.01 seconds pulse time is too long... ill try to whip up Arduino trigger that can pulse shorter pulses...

I need a timer since I have 400+ cells to weld
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Thread Starter


Joined Feb 1, 2019
What size and length of wire are you using now?

Do you have an idea of how much current you want?
Maybe half the current . im using 8awg wire and about 16in long both pos/neg..

I was using my battery from my car rated @ 540 CC amps, so I just bought a lawn mower battery @ 220 CC amps and a motorcycle battery @ 130 CC amps.

let u know what happens shortly
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Joined Sep 24, 2015
Do you have an old heater you can scrap? Old electric dryer? The heater wire inside them can be used to limit current. Just select a long length. If there's not enough current then shorten the length. I don't think you need all those amps, but I haven't any practical experience spot welding 18650's. I'm following this thread because I will be building my own batteries; so it's good to know what you're using / how you're accomplishing the task. Fortunately I have some bad 18650's to experiment with when I do start welding them together.


Joined Sep 30, 2009
I'm following this thread because I will be building my own batteries; so it's good to know what you're using / how you're accomplishing the task. Fortunately I have some bad 18650's to experiment with when I do start welding them together.
Maybe if you would ask jpanhalt real nicely he would repost his battery welder. It if I rememeber right used a rewound microwave transformer and capacitors. By selecting the cap size it limited both the current and time for welding. Much better than using a car battery.


Joined Jan 18, 2008
Actually, I made two versions. One was a rewound MOT. I could not get the localized, fast heat I wanted. The other that I currently use was a classical capacitor discharge (CD) welder. "Tatus1969" (on another forum for which I previously gave a link) took the simple CD format and made a two-pulse format. His current version uses LiPo's as power and relatively small capacitors. The first pulse cleans; the second welds. There is no reason, other than weight, that a 12 V automotive battery won't work. My single pulse method only requires reasonably clean materials and sufficient pressure that you do not get arcing. The two-pulse method is probably better, but mine still works after >10 years experience.

Simple burn though is often the result of poor contact between the welder electrodes and the nickel strip. Arcing is the primary cause. The smoke and fire observed by the TS suggests to me some possibilities: 1) Dirt or oil; 2) Nickel strip that is plated steel; 3) Something else.

I think the TS's approach with a single pulse from an automotive battery might work, but his timing needs to be better controlled. I use relatively short 4 AWG welding cables for connection to my electrodes. The effect of inductance is clearly obvious. They jump with each weld. I don't think going to smaller and longer cables is the answer, but I haven't been asked. His problem in IMHO: You can never get control of timing with a mechanical relay that is necessary for good welds on 100's of batteries..


Joined Mar 14, 2008
im using 8awg wire and about 16in long both pos/neg
Just double the length of each wire to double the resistance.
Otherwise 11 gauge wire would have double the resistance.
Alternately 10 gauge (which may be more readily available) has a resistance about 1.6 times that 8 gauge so, if you use that, just make it about 1 1/4 longer than the 8 gauge.

Thread Starter


Joined Feb 1, 2019
Actually, I made two versions. One was a rewound MOT.
I finally got mine to work consistently. I went ahead and used the 230 CCA lawn mower battery which was just under half the amperage of my car battery. Seems to be just right for the 0.15mm nickel strip I'm using, And I have my timer controller set to 0.2sec.

I actually acquired a microwave Transformer originally but then I came across many DIY spot welders using the battery and solenoid technique. I just have so many ongoing projects and at this point I'm hard up for time trying to finish this 4kwh battery for hurricane season. I understand the safety concerns if the solenoid gets stuck closed but hopefully the 200a fuse I have in line with the solenoid will pop. It should because when using a 150a Fuse it blows when just trying to weld.

Also once in awhile it will blow a hole in the nickel strip and the reason being is I'm not making good contact between the and the spot welder the nickel strip and the battery itself? That's good to know and it makes sense because it only happens once in a great while. Also I need to get some copper nails they have a nice point on them since currently I'm just using some 12 gauge solid copper for the electrodes.

Yeah my first thought was just to increase the wire length but that gauge of silicone wire starts to get pricey and fortunately I had a Walmart gift card that I used to purchase the lawn mower battery for $24.

Again I appreciate all of your guys's input. I really need to start closing out my posts with the final outcomes of whatever projects asking questions for. I have a habit of figuring out a problem or starting another project and not going back to my Forum posts and posting the outcome.

Again thank all of you for all your help I noticed a few your names and I believe you have answered many questions/threads of mine...

Oh one other thing, normally when I would solder my 18650 battery packs I would always used the cell level fusing technique... but now that I am welding them, I can weld the fuse wire I'm to the cell but since the wire is so thin, right where the weld is is like a pinch point/stress pointand I'm afraid the wire have a tendency to break off very easily... like when you Nic a piece of copper wire and if you bend it back and forth just a couple times it'll break off right at that spot. So with that said I'm not comfortable using the fuse wire or the really thin resistor leads so I've done a couple tests using the nickel strip and I found that if I use 0.4mm wide / 0.1mm thick
Strips it seems to burn and break pretty consistently around 8amps... but it's pretty violent and shoots flaming Sparks Until It Breaks... so my question is am I okay just to use that size of strip to make up my battery pack and just use that as the fuse in case a cell goes bad it doesn't take out the entire parallel pack? I'm going to also post this as a new thread maybe people that have some experience can chime in