battery spot welder problem

Yeah, you’re spot on - without smoothing caps after the rectifier, the board’s getting nasty ripple, not real DC. Most of these boards need stable DC to fire properly, especially for short high-current bursts. That ripple alone can kill the weld power. Paralleling 10–20 capacitors sounds like the right move. I’ve used low ESR electrolytics for this kind of setup and it made a huge difference. Also agree on the 2S5P 18650 pack- just keep an eye on pulse loads. Looks like you’re on the right track now.

Galgso said:

I may be mistaken but I think the aluminium SMD electrolytic capacitor may be connected as shown in the schematic below:

View attachment 149593

If this is the case, the diode and capacitor will help keep the control electronics voltage the same during the weld cycles. If the voltage on the input (anode of the diode) drops, the capacitor will remain charged and will maintain the voltage that was previously there. It doesn't need to be that large, only large enough to supply whatever small amount of current is needed for the control electronics during the welding cycles. If I am correct about this, you should be able to connect another capacitor in parallel with that one to prevent the board from turning off during welding. Can you use a multimeter on continuity check mode to see if the diode and capacitor are connected as they are in the schematic I posted?

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I tried measuring between capacitor (positive side) and board input (positive). The reading read 0.7V for a brief moment, jumped to 1.8V and kept rising. So that would indicate that there is a diode between cap and input ?

I did try to put a 3000uF resistor in parallel with the existing 470uF one and can confirm that the board no longer turns off. Now i have to do a proper resistor bank, see if it helps. I tried soldering 2 0.15mm tabs together today and its no go as it is.

Ok so i finished this project, sadly as it is, it isnt usable.

- I added about 81.000uF worth of caps in parallel (about 50 caps), though it's normal caps, not super caps, so their power delivery is probably still very weak
- Transformer exit reads about 7.3V, capacitors which are connected to rectifier exit read about 9.3V, spot welding board is said to support 6-16V or 9-16V depending on which data to belive
- Board control has its own power source so even if board input sags to 0V, it will stay on
- System as it is, cannot weld 2x 0.15mm tabs together
- During the weld, killawatt reports 315W peak power draw, though cant be sure how accurate this can be due to short pulse
- If i bypass the mosfets on the board and connect rectifier negative directly to 1 of the spot welding tips and short them for a few seconds, i get a reading of over 900W. So the system is certanly able to pack a punch. That should be well over 100A considering the voltage also drops. Then ofcourse, there are losses. No part of the system heats up except the coil, so it's hard to figure out where the power is lost.

At this point, if anyone has any ideas i would be more than glad, because im stuck. I could try to increase the voltage coming from the transformer to about 14V, that would mean the rectifier exit would give me about 16V, the maximum the board allows. But it will be hard. The 6 gauge wire i used leave no space for more turns, so i would have to either use thinner wire or make a custom insulation with high temperature tape. Right now it just seems that the system isnt able to deliver that instant punch in the alloted 99ms. Sadly i cant increase that value, and its high as it is.

I tried a battery powered type one and sent it back, it worked (or not) about as well as yours. I got a supercapacitor based one instead, that uses a small AC power brick to charge the caps. That works really well, way better on a very low setting than the battery one on its highest.

Now board is shutting down again, but even at lower settings. Rectifier still reads 0.4V continuity on + and 1.8V on -

SentinelAeon said:

Now board is shutting down again, but even at lower settings. Rectifier still reads 0.4V continuity on + and 1.8V on -

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Where are you putting the probes for the 1.8v reading? The highest reading you should be able to get is two diode drops (1.4v max) with one probe on DC + and the other on DC-, less for your rectifier since it's rated for high currents and the meter test current is usually no more than a few mA. For the individual diodes the 0.4v reading or the 0.51v reading you got earlier both sound plausible for a working diode.

Also the ESR for your capacitor bank is no less important than the actual combined capacitance. A high ESR capacitor bank will still be unable to provide large pulsed currents. You can check the datasheet for your caps if you've got it.

At this point if you really want to use this board instead of a supercapacitor-based solution like rjenkinsgb suggested, I would highly recommend borrowing a car battery from someone and testing the board with it to rule out the board simply being crap.

Red on plus, black on minus gives nothing (unlimited), red in minus, black on plus gives 0.9V.
Red on any of the AC inputs and black on plus gives 0.51V, red on any of the AC inputs and black on minus gives nothing (unlimited)

SentinelAeon said:

Red on plus, black on minus gives nothing (unlimited), red in minus, black on plus gives 0.9V.
Red on any of the AC inputs and black on plus gives 0.51V, red on any of the AC inputs and black on minus gives nothing (unlimited)

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That sounds about right. I'd still check with a car battery to rule out the board being bad before you keep trying to get the rectifier and smoothing cap setup working.

That seems like the best thing to do, i will report back. Will standard car battery be able to do 0.2mm welds on this board ?

I don't know who supplied your welder, but this is what you need.



The power source ESR ought to be as low as possible to the parallel x mohm FET switches. It must be much lower than the ionization resistance of the welder arc to be reliable. The bigger the arc, the lower is the arc resistance. Faraday proved that.

Today i tested the board with a borrowed car battery. And it works - surprisingly well. I got to weld 0.2mm to sub c battery using 10ms setting (out of possible 99ms).

So it looks like i will have to abandon the transformer route sadly. I thought it was an interesting way of making a decent spot welder for under 20$.

Now i can go either super cap or car battery route, though either will probably cost me 50$ or more, which is as much as some all in one spot welder kits cost.

Engineer in me is still wondering what exactly is the reason that transformer + rectifier didnt work. Transformer should provide enough juice even with ripple.

Likely as the peak current pulses are so high, the combination of the 2 - 3V? voltage drop in the bridge rec at high current, resistive loss in the transformer and the intermittent high current availability for the partly smoothed DC.

I would like to hear your opinion on something i thought about. We talked about using 20A (30A short time) 18650 cells and we all agree that shorting a 18650 cell isn't a good idea. There is actualy a video online of someone using 18650 for battery spot welding in this way and 1 of his cells pops.

But, what if you were to use a series resistor on each cell that would limit the current from that cell to exactly 30A on short. It should be pretty safe then since cells are rated for 30A short time use, and each spot weld is <100ms ? But will such cells in parallel (maybe 3S5P) be able to supply the current fast enough for the weld ? Similar to how normal caps aren't enough, you need super caps for fast discharge ?

The last question. I have seen a lot of spot welders using lipo battery packs, supplying 100's of amps. I never saw any current limiting circuit or resistors, so when you spot weld you are basicaly shorting the battery for that 100ms. I know lipo is able to supply much higher current but why is it safer to short lipo in this manner than for instance a 20A 18650 cell ? Short is a short, isnt it ?

Cells have different maximum current ratings, depending on their internal construction. High discharge cells can be rated at massive currents - radio control flight packs are often like that.

Example:

That's 5AH, 80C rating; 1C discharge is one hour rate, 5A. 80C is 400A!

Some packs are rated 100C or more.

Some electric car batteries use high current 18650s - Tesla did until they changed to a larger cell type.

rjenkinsgb said:

Cells have different maximum current ratings, depending on their internal construction. High discharge cells can be rated at massive currents - radio control flight packs are often like that.

Example:

HCL-HP 22.2V-5000mAh-80C Flight Pack

SMC Racing HCL-HP 22.2V-5000mAh-80C Flight Pack HCL-HP 22.2V-5000mAh-80C. Our HCL-HP flight packs are made with 100% LCO cathodes for maximum cycle life and performance. You will not find a pack with better performance for the same weight. mAh +/- 5%: 5000mAh Voltage...

www.parkflyers.com

That's 5AH, 80C rating; 1C discharge is one hour rate, 5A. 80C is 400A!

Some packs are rated 100C or more.

Some electric car batteries use high current 18650s - Tesla did until they changed to a larger cell type.

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I understand that some cells are rated for much higher current than others. But do those lipo's have some kind of internal protection against short? Because even though the battery you named is capably of 400A (safely), if you short it, you will probably get even higher, unsafe current ? For instance, with my 18650 20A (30A) if you short the terminals, you get the (unsafe) 45A. I am asking because to the best of my knowledge, those spot welding board do not have any current limiting except the natural resistance of wires and traces. So its essentialy shorting the terminal of the battery.

Im in the process of getting a car battery or lipo for that spot welding board. The existing pocket spot welding will now be a backup.

So im thinking about doing some fun mods on it. It's a really nice spot welder for 20$. After improving contacts between board and battery, it does an awesome 0.12mm welds, with some patience it will even do 0.15mm. Now, it would be great if it could do 0.2mm. I have 2x 2.7V 500F supercaps. So i thought about putting them in series, do a separate charging and led voltage display for each cap. Then when the lipo and caps are at same voltage, i can safely connect them. Since the existing small lipo (4.2V) can do 0.15mm, adding the supercaps should add enough juice to do 0.2mm. And if it doesnt work, at least i remove the itch. Been thinking about this idea for a few months now. What do u think ?

SentinelAeon said:

Im in the process of getting a car battery or lipo for that spot welding board. The existing pocket spot welding will now be a backup.

So im thinking about doing some fun mods on it. It's a really nice spot welder for 20$. After improving contacts between board and battery, it does an awesome 0.12mm welds, with some patience it will even do 0.15mm. Now, it would be great if it could do 0.2mm. I have 2x 2.7V 500F supercaps. So i thought about putting them in series, do a separate charging and led voltage display for each cap. Then when the lipo and caps are at same voltage, i can safely connect them. Since the existing small lipo (4.2V) can do 0.15mm, adding the supercaps should add enough juice to do 0.2mm. And if it doesnt work, at least i remove the itch. Been thinking about this idea for a few months now. What do u think ?

Click to expand...

Depending on the ESR of the capacitors, they may or may not be able to supplement the batteries to make a noticeable difference in welding performance. Wouldn't hurt to try though. Make sure your capacitors are closely matched so that they share the voltage evenly. Supercaps usually have large tolerances and you can easily end up exceeding the voltage rating of one of the caps if you just put them in series without measuring their values or using balancing resistors.

I would measure the battery voltage and use a current-limited DC supply to charge the caps to as close to that voltage as possible. Then connect them to the battery through a low value resistor. Once the drop across the resistor is zero you can replace it with a thick wire or short across it. Screw terminals would help. In theory you may also be able to use a switch to short across the resistor and just have the battery charge the caps initally and flip the switch once they're charged but acquiring a switch with the required current rating may be difficult. Also, I may be mistaken but it may not matter that much in your case since the switch would never be used to interrupt the actual welding current in your case and you would only have to worry about the closed contact resistance.

Galgso said:

Depending on the ESR of the capacitors, they may or may not be able to supplement the batteries to make a noticeable difference in welding performance. Wouldn't hurt to try though. Make sure your capacitors are closely matched so that they share the voltage evenly. Supercaps usually have large tolerances and you can easily end up exceeding the voltage rating of one of the caps if you just put them in series without measuring their values or using balancing resistors.

I would measure the battery voltage and use a current-limited DC supply to charge the caps to as close to that voltage as possible. Then connect them to the battery through a low value resistor. Once the drop across the resistor is zero you can replace it with a thick wire or short across it. Screw terminals would help. In theory you may also be able to use a switch to short across the resistor and just have the battery charge the caps initally and flip the switch once they're charged but acquiring a switch with the required current rating may be difficult. Also, I may be mistaken but it may not matter that much in your case since the switch would never be used to interrupt the actual welding current in your case and you would only have to worry about the closed contact resistance.

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The caps are quite big, nickel strip evaporates if i short across the terminals. I thought about just using the caps for the job, but i thought, caps + lipo is better than just caps. I dont plan to keep the caps connected because im pretty sure they discharge to fast. But my idea was pretty much what you suggested. To sum it up:

Lipo is at 4.2V. I would charge each cap to 2.1V, connect them in series for 4.2V. Only after i was sure both lipo and caps were 4.2V, i would connect both in parallel. I would probably make some kind of mechanism for this, maybe bolts and screws. So after im done with the welding, i can safely remove the capacitors and prevent them from discharging the lipo.

In my idea, i would separately charge each cap, so i can avoid using any balancing resistors. I would also use 1x lcd voltmeter connected to each cap, just to make sure the lipo battery doesn't overcharge any of the caps, since caps will be in series and their resistance isn't the same. That way i can safely unplug the whole thing if i see any cap creeping over 2.5V as im doing the welding