High-power battery welding device - Page 1
Forum section: electronics / projects

author: daniel89pl
author_rank: Contributor
timestamp: September 25, 2025, 09:57:45 am
content: Hello,I'm currently working on a device for connecting copper plates to cells. It's a prototype. I want to achieve a short-circuit current of 6000A on two power boards, with precise measurement on the shunt. If anyone would like more details, please feel free to discuss.Best regards,Daniel.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: September 25, 2025, 12:43:45 pm
content: Dzien dobry Daniel89pl, It sounds to me like you are seeking a 'spot welder', but if your 3rd from the top photo is of the batteries, then what is the threaded extension on the bottom of each cell

author: daniel89pl
author_rank: Contributor
timestamp: September 25, 2025, 01:17:10 pm
content: Maybe I'm phrasing my first sentence incorrectly. I'm doing a DIY project and this welder is my own design.The supercapacitors I want to use, shown in photo 3, are each 3000F and 2.7V. I want to connect them in a package of 4 in series and 2 in parallel. If anyone is interested in the electronic diagram and program, I can show it.The first two photos show a 1000A shunt at 75mV and a differential circuit to convert the voltage drop to a 0-3.3V range for measurement on the microprocessor's ADC.The 4th photo shows the control board not yet fully soldered, and the last one shows one of the two power boards.

author: David Hess
author_rank: Super Contributor
timestamp: September 25, 2025, 04:52:45 pm
content: I have used capacitive discharge spot welders in the past, but was thinking about doing something similar to your project on a smaller scale for spot welding metal strips to battery ends. I was going to use 18650 cells as a power source, but with full linear control of the current waveform which requires careful attention to forward bias safe operating area.At these current levels, it is important to minimize loop area and the layout becomes more like copper plumbing than printed circuit layout. Careful attention is required to balance currents between parallel power devices, and if linear current limiting is implemented, then devices must be either matched or controlled independently.I seem to remember that advanced spot welders use a double pulse with a short higher voltage pulse used to blow away oxidation followed by the high current welding pulse.Quote from: BILLPOD on September 25, 2025, 12:43:45 pmThose are supercapacitors. There is also a threaded terminal on the other end.

author: daniel89pl
author_rank: Contributor
timestamp: September 26, 2025, 06:36:21 am
content: Regarding parallel connection, you're right:1) First, I need to measure the resistance on one power board with the power supply switched on.2) Then, measure the same on the other.3) Compare them to see if they're the same. A slight deviation will cause an uneven current distribution, according to Kirchhoff's law.4) The contact pressures are also very important in such an installation, as is the quality of these connections, i.e., their internal resistance.5) Only with equal load on both boards can I be sure that the current is distributed evenly.6) There's also another issue: there may be a slight deviation on the MOSFETs, but I've already compensated for this with the number of MOSFETs rated for a given current.Regarding the control, you're absolutely right:1) I have a control program for a pre-pulse for oxide removal and a main pulse for grouting.2) There's another control option I've implemented, which is Joule power control. This involves measuring the specific current and controlling it to deliver the appropriate power regardless of the voltage drop across the supercapacitors. According to Joule's law, Q = I²RtI already have a device ready for welding nickel sheets, but the project I'm most excited about now is to test the feasibility of welding a very difficult material, namely copper. I want to use tungsten electrodes and precisely measure the current applied to the weld. This is a challenge for me.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 26, 2025, 10:45:42 pm
content: Might it make more sense to store energy at a higher voltage, then use a H bridge and transformer to step it down?

author: daniel89pl
author_rank: Contributor
timestamp: September 27, 2025, 03:39:11 pm
content: This isn't the path I'm following. It doesn't make sense to me. Too much power loss at every stage. A special HF transformer is needed. There are too many delays and distortions, and it's difficult to control the pulse shape. It's less safe, has a higher risk of failure, and requires more expensive components. It complicates control and repeatability. In short, it's not my cup of tea.

author: voltsandjolts
author_rank: Unknown
timestamp: September 27, 2025, 04:02:48 pm
content: Some circuits and pictures for reference maybe...https://www.eevblog.com/forum/manufacture/hughes-hcd125-capacitor-discharge-welder/https://www.eevblog.com/forum/buysellwanted/fs-uk-miyachi-ub25-dc-resistance-welder/UB25 output side is something like shown in picture here:

author: daniel89pl
author_rank: Contributor
timestamp: September 27, 2025, 07:09:25 pm
content: The device above is very cool. I like the ramp setup. Can I ask what exactly this ramp does for welding?and is it controlled by PWM or analog signal?

author: voltsandjolts
author_rank: Unknown
timestamp: September 28, 2025, 07:10:56 am
content: I think it's cool too. The copper busbars are awesome.There are a lot of things that can affect weld quality; materials to be welded, material coatings, weld tip material, tip pressure, tip diameter, current, tip voltage, weld power...the list goes on. Anyway, there are quite some settings on the UB25, one is the ramp up/down rate. Some materials can be liable to sputtering when full weld current is applied (sparks fly), ramping up the weld current in a controlled manner can reduce sputtering.The array of output fets are controlled by an analog signal, which is the output of a pid control loop. The feedback to the control loop is pre-selected as weld current, tip voltage or weld power (IV). The waveform created by the user on the front panel is output by a dac to the pid control loop, which then controls the selected parameter accordingly.

author: daniel89pl
author_rank: Contributor
timestamp: September 28, 2025, 07:51:53 am
content: I understand this means a PID controller after the current measurement. My initial idea was to create two control options: 1) time control, an initial pulse based on a percentage of the main pulse. However, I was inspired by this ramp, and I'm thinking of adding a PWM-controlled, 20%-averaged current at full power, with a five-step increase and a five-step decrease. I'll see if this improves electrode wear and weld quality. It would be nice to have feedback in the loop to see how the current is increasing—I just don't know if it makes sense. It's better to only enable this option at the beginning of the current measurement to see what the actual current will be at any given moment. 2) The second option is to apply the appropriate power in Joules in a closed-loop current measurement on the controller, ensuring consistent results regardless of the push quality. I admit, I hadn't considered such a ramp, as it might help. But I've read up on it, and the material cooling process is indeed better with a ramp, and it doesn't cause shock, stress, or any potential spalling. Well, this needs to be tested in practice because I don't like theory and books, but practice.

author: voltsandjolts
author_rank: Unknown
timestamp: September 28, 2025, 08:12:37 am
content: PWM with hundreds of amps or more sounds like alotof trouble.I would try on/off control first and see if that meets your needs. Dual pulse certainly works well in many setups.If you want a slower cooldown phase, you could do triple pulse, with a short pulse after the main weld pulse - haven't heard of this being done before, just my thoughts.

author: daniel89pl
author_rank: Contributor
timestamp: September 28, 2025, 08:30:51 am
content: You're right, PWM isn't the best idea for this power. But I can achieve cooling and preheating as you described, i.e., with an initial pulse, the actual pulse, and the final pulse. This should simulate a ramp. I can use a current loop to set the time according to the formula J=I2*R*t. Based on the current, I can select the time for the given Joules required for a given connection.

author: voltsandjolts
author_rank: Unknown
timestamp: September 28, 2025, 08:58:11 am
content: Suggestion:Sometimes with hand-held weld tips you burn through the nickel tabs or whatever your welding - this mostly happens on the last weld of a thus far perfect battery pack. Since you have a current measurement you can monitor for erratic current and abort the weld to reduce the damage.

author: daniel89pl
author_rank: Contributor
timestamp: September 28, 2025, 09:42:20 am
content: Is the initial impulse controlled from 0 to 50 percent of the actual impulse completely sufficient? Is the final impulse the same (0-50 percent) ?

author: voltsandjolts
author_rank: Unknown
timestamp: September 28, 2025, 11:23:11 am
content: For spot welding nickel tabs on 18650 cells, I would typically use dual pulse with the first about half the energy as the second, say 20J and then 40J. But on the welders I've used you can set the pulse energy however you like for both pulses, e.g. 100J followed by 20J.

author: daniel89pl
author_rank: Contributor
timestamp: September 28, 2025, 02:43:23 pm
content: For Cooper 0.1mm how many Joules ?

author: voltsandjolts
author_rank: Unknown
timestamp: September 28, 2025, 03:18:40 pm
content: I've never spot welded with copper strip but it's electrical conductivity is around 4x that of nickel, and so is it's thermal conductivity (sucking heat away). So going to need at least 4x weld energy, maybe 8x, IDK.Are you sure you need copper rather than nickel strips?Double up on the nickel strip?

author: daniel89pl
author_rank: Contributor
timestamp: September 28, 2025, 07:31:59 pm
content: I want to try welding a 0.1mm copper plate with current on the supercapacitors' landing pad. I'll see if it works. (Maybe 6k Amper is ok?)

author: voltsandjolts
author_rank: Unknown
timestamp: September 28, 2025, 07:37:28 pm
content: I use 400A to 800A for nickel strip, on the UB25.

author: daniel89pl
author_rank: Contributor
timestamp: September 28, 2025, 07:52:40 pm
content: I used 2000A once, and it barely caught a 0.2A nickel plate. 0.3A was a problem. Hence my question: how much current is needed in a fraction of a second to weld copper like that on tungsten electrodes? At 500A, nothing welded, at most nickel-plated steel. Maybe I was doing something wrong or the ramp wasn't there?

author: voltsandjolts
author_rank: Unknown
timestamp: September 29, 2025, 08:06:53 am
content: Quote from: daniel89pl on September 28, 2025, 07:52:40 pmI guess you mean 0.2mm and 0.3mm nickel strip.2000A sounds high.What is the pulse duration?Are you sure you're measuring current correctly?What is the probe tip diameter?

author: daniel89pl
author_rank: Contributor
timestamp: September 29, 2025, 08:36:24 am
content: Look hereShunt 500A 60mV

author: voltsandjolts
author_rank: Unknown
timestamp: September 29, 2025, 09:10:05 am
content: OK, that 850A should be ample for welding nickel tabs but the pulse width is short at around 1ms. I tend to use 6 to 10ms.

author: daniel89pl
author_rank: Contributor
timestamp: September 30, 2025, 07:08:18 am
content: Controlling the time intervals between these 3 pulses should also be adjustable, I understand.Is this okej ?How to configure this in the control (interface suggestion):• Prepulse: % (e.g., 0–100% of the main time)• Pause after prepulse: 1–100 ms• Main pulse: 1–100 ms (base parameter)• Pause after main pulse: 1–100 ms• End pulse: % (e.g., 0–50% of the main time)

author: voltsandjolts
author_rank: Unknown
timestamp: September 30, 2025, 05:56:25 pm
content: Seem reasonable. For comparison, here is an excerpt from the Miyachi UB25 manual;