DIY spot welder: transformer, caps discharge or car battery?

[reyntjensm]

Hi,

I want to build my own 18650 pack so i was in need for a spot welder. As aliexpress is full of cheap spotwelders, i preferred to make one by myself as it would be cheap and i wouldn't have to wait for 2 months to start with my project.

After some intense research, say 3 mins on youtube i was already salvaging a transformer from an old microwave. With some 100mm2(2 turns on secondary) wire, arduino and a solid state relay i did a first test. I was not happy with the results. It was more a melting/burning connection instead of a clean spot weld. So i rewired the secondary with a pair of 16mm2(4 turns). Again i was not satisfied with the result. I need about 100-200ms for a good connection and i need to try it multiple times to get a good connection. At this point my material is already very hot and i'm worried it would heat up the cells too much.

After some more research... i found out discharging a large bank of capacitors should give better results. Any suggestions on this idea? What about shorting out a car battery? I know they sell controllers for these applications on aliexpress.

What is the definition of a good spot weld? Is there some simple math to have an idea about the amount of energy needed to do a spot weld? In my case i use 0.2mm nickle strips.

[tautech]

Suggest you study the development of Kweld:
https://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/

[Doctorandus_P]

Since you've already spent 3 minutes on this you probably already know everything and there is nothing more to add.

[reyntjensm]

Since you've already spent 3 minutes on this you probably already know everything and there is nothing more to add.

Thank you for your great feedback. I guess you are not really in too jokes. At least you got some spare time to waste

[james_s]

Capacitive discharge is what is typically used for battery tab welding, and is what I used when I built my welder. A friend of mine needed something beefier so he got one of those K-weld things and said it works well.

[reyntjensm]

Did you use some mosfets to control the discharge?

[Bud]

Unless the source battery connects via a mechanical relay or you use the welder often, i would avoid battery based welders. I have one which i use occasionally and the battery in it discharges through the mosfets when stored. I have to pull it from the shelf once in a while to keep the battery charged a bit.

[Conrad Hoffman]

We use an older thermocouple welder (that also works great for many other things, like battery tabs) and the circuit is pretty simple. A small Variac and rectifier is used to charge up a high voltage electrolytic. I think it goes up to 300-400 volts. This is discharged through the primary of a big transformer. The secondary is a turn or two of heavy copper strap, which is also brought out through the front panel for connection to whatever hand tool or fixture is to be used. There's a limit resistor between the charging circuit and the capacitor so the charging circuit doesn't get shorted out during the weld process. The switch between the capacitor and the transformer is a mercury wetted relay, but today you'd probably use something different.

[james_s]

Did you use some mosfets to control the discharge?

I used a big old stud mount SCR I found on ebay, the sort of thing that used to be used in big motor control inverters back in the 70s.

[tautech]

Did you use some mosfets to control the discharge?

I used a big old stud mount SCR I found on ebay, the sort of thing that used to be used in big motor control inverters back in the 70s.

Hardly decent control James as once it's ON it's ON until the source is depleted.
Kweld provides proper duration control.

[james_s]

Hardly decent control James as once it's ON it's ON until the source is depleted.
Kweld provides proper duration control.

It's perfect for this application because that is exactly the behavior you want for a capacitive discharge welder, which as I mentioned is the technique I used for mine. The energy of the weld is determined by the voltage the capacitor bank is charged to.

[tautech]

Hardly decent control James as once it's ON it's ON until the source is depleted.
Kweld provides proper duration control.

It's perfect for this application because that is exactly the behavior you want for a capacitive discharge welder, which as I mentioned is the technique I used for mine. The energy of the weld is determined by the voltage the capacitor bank is charged to.

Exactly but a fact easily missed by some.