Battery welding - Page 1
Forum section: electronics / general

author: throbscottle
author_rank: Contributor
timestamp: November 23, 2024, 01:56:00 am
content: I've just got one of those little rechargeable battery tab welders (this one:https://www.temu.com/uk/battery-spot-welder--adjustable-5000mah-spot-welder-automatic-and-manual---welder-spot-welder-for-18650-battery-build-battery-packs-repair-ebike-pack-2m-nickle-sheet-g-603402822065701.html). to re-build some batteries. Haven't received the new cells yet.This is my first foray into battery welding - I have no experience with it at all.So I did some tinkering around with the nickel plated steel strip that came with the welder. I certainly looks to be doing the business, but I can pull all the welds apart with varying degrees of effort, so there's something amiss there.So is this something in the nature of plated steel strip being like this, or is it just that the welder isn't grunty enough?

author: David Hess
author_rank: Super Contributor
timestamp: November 23, 2024, 03:21:00 am
content: The welds should rip the base material before coming apart.

author: throbscottle
author_rank: Contributor
timestamp: November 23, 2024, 04:28:33 pm
content: Yes, I've tried to pull manufactured ones apart - they do exactly that.After my initial question I tried it with some nickel strip pulled from the dead battery pack - barely touched it! Is this due to lower resistance?

author: voltsandjolts
author_rank: Unknown
timestamp: November 23, 2024, 05:14:43 pm
content: One of my battery welders is theFNIRSI SWM-10, which is similar to what you have. Both are 'cheap and cheerful' units with limitations but can be useful for small packs. The main limitation with the FNIRSI is the thickness of the tab material, which as you say changes the resistance. I use it with the thinner materials, 0.1 or 0.15mm thickness and this pushes it to the limit really. But it works for making low-ish current battery packs.If you make a tab weld, then pull the tab off of the cell, there should be pinholes in the tab material where it tore away. That tells you the weld was sufficiently strong. If it tears off and the tab material is intact, you need more weld power or thinner tab. Technique also plays a role, don't press down too hard on the weld tips, just enough to maintain good contact.

author: voltsandjolts
author_rank: Unknown
timestamp: November 23, 2024, 05:24:16 pm
content: If you want some introductory reading on the subject of resistance welding (..its a whole science really) try the attached document, which is an extract from a Miyachi welder manual.

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 23, 2024, 06:17:34 pm
content: I bought something similar (https://nl.aliexpress.com/item/1005006727554963.html) and are not very happy with it. Sure it makes welds, but as others wrote it is only ok ish for lower current packs. I have used it to make a new battery pack for my Husqvarna chainsaw and had to double the strips (0.1mm) to reduce the resistance and then welding becomes a bit rubbish with these cheap automatic things.I also burned through a couple of batteries on setting 4. On 5 is just blows holes in the 0.1mm strip and the battery. On 3 the weld is ok, but it is tricky with the automatic thing. I would rather have one where I trigger the welding myself.Sure one can save a lot of money on such a battery pack. A new one costs about 250 euro and getting new batteries is only about 75 euros, but it was a hell of a job to get the BMS properly removed and hooked up again and fit it all in the case. Had to use my Dremel to grind away some of the plastic to make room for the new battery holder.I bought 5x4 battery brackets (https://nl.aliexpress.com/item/1005005428528216.html) to make it easy for myself and make sure the batteries would have enough ventilation.But it worked, and failed after a couple of uses. Had to re weld a strip of the BMS that had come loose. Knock on wood, but it is still working since.

author: throbscottle
author_rank: Contributor
timestamp: November 23, 2024, 07:04:04 pm
content: Wow - better already! Even got the nickel to stick! My first pull-apart hole has appeared! (in the steel one) Thanks!For reference, the supplied steel strip and old nickel strip are both 0.15mm.Onwards and upwards...OAN - Years ago, I started experimenting with building a capacitive (but, high voltage cap + transformer) welder. Made zero progress with it. Interesting though@pcprogrammer - I read someone else could only use 0.1mm or thinner strips (the general experience seems to be that these welders are too weak), but bulked them up with a bead of solder for extra thickness.I'm puzzled though - if your welder is capable of blowing holes in the strip and the battery, how is it not able to produce adequate welds using thicker material? Surely you have a relatively good one!

author: voltsandjolts
author_rank: Unknown
timestamp: November 23, 2024, 07:54:53 pm
content: Quote from: throbscottle on November 23, 2024, 07:04:04 pmFor a while I had a Hughes HCD125 which works as you describe, cap discharge into transformer, high current on secondary winding output. The voltage on the caps determined the output energy, 125J at the most but other models had 300 or 1000J outputs. The transformer was a beauty. See pictures here:https://www.eevblog.com/forum/projects/homemade-spot-welder-keeps-blowing-mosfets/msg3580031/#msg3580031They are also branded as MicroJoin or Palomar HCD125. If you see one pop up on ebay, buy it. Very well made, and very repairable too, which many more modern tools are not.

author: wilfred
author_rank: Super Contributor
timestamp: November 23, 2024, 08:56:47 pm
content: If you're finding yourself stuck at the thinner strips being all your welder can do then it would be better to use full nickel strips. Depending on the current requirements of course. I bought some on AliX and paid the required premium for it. Whilst that isn't a guarantee mine did pass the deep scratch and salt water test without rusting.But you can be sure the cheaper strips are only nickel plated.

author: throbscottle
author_rank: Contributor
timestamp: November 23, 2024, 11:25:28 pm
content: Interesting thread. What does the HCD125 use as it's firing device? SCR or MOSFET? (or spark gap - eek!) I'm feeling inspired to tinker again...I will get some nickel strips now I have a better idea of what I'm doing

author: pcprogrammer
author_rank: Super Contributor
timestamp: November 24, 2024, 07:25:47 am
content: Quote from: throbscottle on November 23, 2024, 07:04:04 pmI don't have thicker material at hand, so can't test it. It does seem to be strong enough, but with only the 5 steps in adjustment it is harder to make a good weld. I also found it to be unpredictable with the automatic trigger, leading to crap welds too.It will be an experience thing, I guess.Have to read the document voltsandjolts posted to gain some more knowledge about the process.Using solder to improve things does not seem to be a good idea due to the heat sensitivity of Liion batteries. That is the whole point of using the spot welder, to not overheat the battery terminals.

author: voltsandjolts
author_rank: Unknown
timestamp: November 24, 2024, 10:29:56 am
content: Quote from: throbscottle on November 23, 2024, 11:25:28 pmIt uses an SCR, with a bypass mosfet that's used momentarily to reduce SCR current to zero, and hence switch it off under mcu control.

author: throbscottle
author_rank: Contributor
timestamp: November 24, 2024, 01:55:26 pm
content: Ah, cunning! Thank you for that - I didn't think it would be possible to control the on time with a SCR, but I see now it is! Learn something every day...I successfully welded a bit of tab strip to a broken off bit of craft knife blade last night - by successfully I mean I couldn't pull it apart. So I'm getting better at it, though I realise the steel helps.What do people do when replacing funny shaped strips? None of the pre-cut ones I've looked at really match most of what's in the packs I'm rebuilding. So do you fold/cut straight strip, join pieces together, just try to re-use the originals, something else???Also, a lot of the pre-cut shapes have a slot (as do the existing tabs in my batteries). Why is this? Do you weld with an electrode on either side of the slot (for more resistance), or both on the same side?The more I learn, the more I have to ask

author: David Hess
author_rank: Super Contributor
timestamp: November 24, 2024, 07:39:18 pm
content: Quote from: voltsandjolts on November 24, 2024, 10:29:56 amWith power MOSFETs as cheap as they are now, why not use the power MOSFET in place of the SCR rather than to just commutate it?

author: voltsandjolts
author_rank: Unknown
timestamp: November 25, 2024, 08:30:02 am
content: Quote from: David Hess on November 24, 2024, 07:39:18 pmThat schema is from the HCD125 which was likely designed in the 1980's or early 90's. The SCR was a good choice then, and the 25+yo units I used worked perfectly. For anyone self-building such a welder today, I still think SCR is an option worth considering. Show me a robust fet and I'll show you a more robust scr that's cheaper!

author: jpanhalt
author_rank: Super Contributor
timestamp: November 25, 2024, 08:50:20 am
content: Quote from: David Hess on November 24, 2024, 07:39:18 pmThere's a lengthy thread here with a successful CD welder using mosfets. As I recall, it does a short pulse to clean followed by a longer burst to weld.https://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/msg1232857/#msg1232857

author: David Hess
author_rank: Super Contributor
timestamp: November 25, 2024, 08:31:06 pm
content: Quote from: voltsandjolts on November 25, 2024, 08:30:02 amI had this problem looking for high power bipolar transistors versus MOSFETs. The MOSFETs are available in larger packages and have a greater economy of scale. So for instance the highest current SCRs without going to pucks are TO-247, 80 amps and about $5. But Infineon power MOSFETs in the same package and about the same price are 300+ amps, and if you do not trust them, IXYS has 500+ amp ones in the TO-264 package for about $15.Even higher current surface mount MOSFETs are less expensive, but I prefer something I can more easily solder.Modern designs all seem to use several smaller surface mount MOSFETs, but I am dubious about good current sharing with a layout for more than 2 or 4 parts, and they do seem failure prone.

author: jpanhalt
author_rank: Super Contributor
timestamp: November 25, 2024, 09:21:25 pm
content: For this application, isn't the pulse rating of the SCR applicable? That's in the 1000's of A's and is what I used. I went the high capacitance low voltage (<25V) route. It works fine, and I usually use about 15V.The additional winding to the commercial toroid is for my logic circuit.

author: David Hess
author_rank: Super Contributor
timestamp: November 26, 2024, 04:18:51 am
content: Quote from: jpanhalt on November 25, 2024, 09:21:25 pmThe pulse rating of those MOSFETs is 1000+ amps and actually limited by the thermal capacity so should be better than the SCRs in the same package.I have used capacitive discharge welders before but have always wondered if they would benefit from some pulse shaping. The next best thing might be to either use a half bridge buck output to regulate the high current, or use big power MOSFETs in linear mode to regulate the current.Of course capacitive discharge welders can work well so it is probably not worth the extra effort and complexity.

author: voltsandjolts
author_rank: Unknown
timestamp: November 26, 2024, 06:20:05 am
content: Quote from: David Hess on November 26, 2024, 04:18:51 amCap discharge welders either dump capacitor energy to an output transformer, or directly into the weld head. The former may have rudimentary pulse shaping by selecting transformer taps (see Hughes HCD schema above), while the latter method can offer more control.Quote from: David Hess on November 26, 2024, 04:18:51 amIt is worth the effort, and commercial welders have been using linear mode mosfet control for a couple of decades. A low voltage capacitor bank feeds dozens of paralleled mosfets used as a resistive control element. They have programmable pulse shapes for either weld current, weld head voltage or weld power under PID control.