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Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Hi all,
So recently I built a battery tab spot welder to weld together a large battery pack (NiMh 'D' cells) for another project. I've been trying to get a decent weld all day, but haven't managed it yet so I'm hoping I can pick someone's brain for tips on what I might be doing wrong - I thought maybe someone here might have built one themselves, or knows a bit more about welding than I do.
First, a bit about the welder I've built - it's basically a large IGBT driven by a PIC microcontroller, and a bank of 4x 7.5Ah 12V SLA batteries in parallel to provide welding current. I have two brass electrodes on 10mm^2 pieces of cable, the power-electronic side is quite robust and I have no concerns there.
The pulse timing is 50mS ON (to clean/soften the material without welding it), followed by 350mS OFF, followed by an adjustable pulse width (up to 512mS on a pot) to perform the actual weld. This is the results of my researches into similar welders including commercially available ones. This part is all working fine, and I'm currently operating at maximum pulse length of 512mS.
Now onto the problem - I've spent a good few hours today experimenting with different energy storage methods, originally I had a bank of 3x 97,000uF capacitors charged to between 12 and 25VDC but all I got was either nothing at all (when applying lots of force to the electrodes) or a shower of sparks and vaporised strip/terminals (when applying less force, and creating an imperfect contact). I quickly abandoned capacitors at that stage, and started using SLA batteries instead - with the current being limited by the ESR of the battery. This seemed a lot more promising, but despite lots of current (glowing contact area and warm cables all round after only a couple of pulses) I'm just not getting any weld at all! I haven't tried to measure the pulse current yet, but it seems unlikely to me that I still don't have enough current but I might be wrong. I've put a video at the end of this post showing my latest attempt - showing the heating of the strip with moderate force applied to the electrodes.
Theories are: - Thermal mass of electrodes is soaking away the heat from the weld area - Tip shape is wrong - Pulse is too long/current too low (but energy ok) so heat is soaking away before it can create a hot enough localised area - ..?
Setup showing the 4x 7.5Ah batts in parallel. Cables are 10mm^2 (or equivalent) all round
Battery voltage during attempted weld, fully charged at start (~13V) dropping to 7.5V under load
Latest attempt (calling it a night now!) with above setup and maximum pulse width:
Registered Member #2989
Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94
Battery can't supply current? I have some 7Ah lead acid batteries and the initial current may be limited to 2Amp that's what it says on the side of them. And as you see a dip in voltage that must be the case. Maybe you could take a resistance measurement before hand to see how well the electrodes are doing. Maybe clean the metal before welding? Seem to remember seeing battery tabs with 4 small spot welds holding them on. spring loaded electrodes then slowly increasing the power, I think this is going to take a fare amount of goes to get right. You may also be able to solder to them using arax flux solder, you just need to clean it well after.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
... either nothing at all (when applying lots of force to the electrodes) or a shower of sparks and vaporized strip/terminals ...
I think this may be a good thing. it means "control-ability" is what you need next, your system has whats needed to work, and your revised system just seems inadequate i believe, for the reasons you correctly state.
- Pulse is too long/current too low (but energy ok) so heat is soaking away before it can create a hot enough localised area
Yes i think so.
Perhaps the electrodes should produce mechanical force enough for good heat, and another pin or rod the mechanical fusing force? So three rods total. Maybe like a rod of tungsten carbide pressing down. You can round off tungsten carbide with a diamond dremel wheel. Ive done it.
Dont kill yourself, i presume here on 4HV we all know how dangerous batteries can be.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
Brass is not conductive enough. Use copper and up the diameter and then sharpen to about 1/8" dia for both electrodes. Careful as the copper will allow about 5 times more current to flow than your present arrangement
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
The current is too low. I calculate about 600 A to begin with given the paralleled SLAs have about 20 mohms internal resistance each, giving 10 mohms between them when paralleled, and as the voltage drops by about half, there must be another 10 mohms in the contact. If the contact area is too big 600 A sounds like too little current. Also if the pulse time is too long, the nickel will expand as it heats and break any bond formed.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Thanks all.
I've taken everyone's tips into account (even my own!) and changed a few things:
- All 10mm wiring replaced with 35mm^2 - Ditched the brass electrodes and made some new ones out of copper bus bar, sharpened to a decent point - Used my van battery instead of the 4 small SLAs.
The result - I'm getting welds now, although I still think there's room for improvement. Looking for second opinions too on the result - third pic below. The rise time of the heat is a lot more noticeable - the material is glowing red just on the pre-weld pulse alone (50mS) and can form an actual weld in just 50mS (not supposed to happen with the pre-weld pulse). Interestingly, the electrode pressure seems to matter a lot as well - too much pressure and I don't get a weld at all.
The current is too low. I calculate about 600 A to begin with given the paralleled SLAs have about 20 mohms internal resistance each, giving 10 mohms between them when paralleled, and as the voltage drops by about half, there must be another 10 mohms in the contact. If the contact area is too big 600 A sounds like too little current. Also if the pulse time is too long, the nickel will expand as it heats and break any bond formed.
This is interesting, and I think your estimate was a lot more accurate than mine. I'd originally aimed for 1000A, and I'd calculated from the ESR of my batteries and the various wiring resistances that I should get about 900A. That was based on about 7.5mR of wiring, an IGBT drop of about 0.7V in saturation. I think I was over optimistic on... everything? and your estimate seems a lot more realistic.
I've also demonstrated the effect of a too-long pulse time, 50mS was enough to form a weak weld but with 300mS I didn't get a weld at all and it just fell apart. 150mS seems like the sweet spot with the current setup, but I still don't think the current is high enough (the heat is 'leaking' away and you can see evidence of this on the nickel strip).
The setup now. I had to put a chunk of wood under the IGBT to space it away from the box containing the microcontroller - the intense magnetic field of the pulse was resetting the microcontroller! This is definitely not built to any kind of EMC standards.
New electrodes
First weld - still think the pulse needs to be higher current and shorter.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
Conundrum wrote ...
Useful tip. Have you tried using pyrolytic graphite or a mu-metal casing (stolen from old audio amplifier transformer) as a magnetic shield?
I haven't tried any kind of shielding at the moment, but pyrolytic graphite sounds like interesting stuff.
I'm probably going to revise the layout to keep the magnetic field away from the micro, that plus some kind of fault tolerance around the software. Because the micro is controlling the IGBT, and it can lock up as I've already demonstrated, I need to add some kind of hardware 'catch' to switch off the gate if the micro has stopped. I'm thinking either some kind of hardware monostable with a set time slightly longer than the micro (although then it seems a shame to have the micro at all!) or another solution is to have the micro outputting a square wave (generated in software) on one of the pins that I filter to 2.5VDC then have a window comparator to always look for this 2.5V and switch off the IGBT if it isn't there (if the software locks up, the voltage will go either to 5V or 0V when the square wave stops). That was a big dump of my current thinking! The PICs do seem to be susceptible to locking up though, it's happened to me before but rarely seems to happen with other microcontrollers.
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Battery tab spot welder advice
