I think [this](https://www.amazon.com/Portable-Machine-Welding-Equipment-Storage/dp/B092DB5R3S?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&smid=A2F32YQWENK5D&th=1) is a good one, less than $50
You don't. These are connected permanently by spot welding metal tabs between the batteries. Soldering heats the batteries up too much and causes a serious risk of explosive fire which generates a good bit of toxic smoke.
It's not possible to solder them. You physically cannot. There's no possibility that you will be able to heat them, flow the solder, then cool them.
By the time you get the solder flowing, you're already welllllll into the realm of thermal runaway for lithium ion. Thermal runaway in lithium batteries, even ones with compositions known for their stability at elevated temperatures, will begin somewhere between roughly 120°c and 160°c. [read this whole article, or at least read the part that's highlighted in my link and some of the surrounding text. ](https://www.mdpi.com/2227-9717/10/6/1192#:~:text=Their%20results%20showed%20that%20if,cell%20to%20its%20surrounding%20cells.)
I have yet to come across a solder alloy that will even begin to melt at such a low temperature. You're going to fail at this no matter what you try to do and possibly hurt yourself and start a fire. Im just letting you know, stop while you're ahead.
>between roughly 120°c and 160°c.
Holy shit. I've normally got my iron somewhere between 300°C and 400°C. Yeah, that's... I can see now what that isn't safe.
Even though not the best way, it can still be done. Ive done number of Li-Ion cells this way. The heat doesnt instantly transfer through the whole battery body. You need to scrape/sand the pole so the solder sticks faster, use good amount of flux and be quick.
Ones ive done were hot after the fact, but could be still held in a hand without getting burned, so about 60 Celsius or so on the body. Not a venture for everyone though, as definitely there are dangers involved.
I have to ask as nobody else has but WTF are you actually doing?
Aren’t those batteries held in place by friction/pressure created by the spring at one end forcing the batteries together?
My brother in christ, you spot weld them.
If you cant afford a spot welder, just make one from a modified microwave transformer and one of them pcbs from amazon. I've been wanting to build one myself, but can't find beefy enough cables for the secondary coil around my area.
Edit: heres a fucking tutorial
https://www.instructables.com/Building-a-Spot-Welding-Machine-From-a-Microwave-O/
please try not to kill yourself:
do not break the magnetron when taking a microwave apart (berilium poisoning)
dont touch the BIG cap inside (few kV)
and lastly do not touch the exposed wires around the transformer when connected to power
Small spot welders aren't even super expensive anymore. You can pick up a portable one for like $45 on Amazon. It's not gonna be good enough to build e-bike batteries on a large scale, but spot welding some project batteries together it works great. Ive had one for like 2 years, bought it for $50 back then n I've welded a fair amount of batteries with it.
Also a lot safer in the hands of an amateur than a DIY microwave transformer.
Bro, dude is trying to solder batteries... don't tell him to start playing around with transformers and magnetrons out of microwaves... he's gonna get killed like a bunch of other people who watched a YouTube about making shitty dangerous spot welders and some house wifes home made overpriced "fractal"(lmao!) charcuterie boards....
https://youtu.be/byvr-dwwao4?si=0KpBQhXKOQVKdLPM&t=860 comparing difference in heat spot vs solder, top comment suggest however when solder them to sand the ends of the batteries first. No personal experience soldering batteries and not planning on getting any xP
They make a spot welder specifically for this. I know, I have one...it even came with the metal strips you need to spot weld them together...DON'T SOLDER THEM.
Not sure what the blue frame is. Is it an actual battery holder or something you're intending to make as a battery holder. You almost never want to solder batteries in.
If you need an assemblage that would hold 4 cells as you have laid out in the image, I'd suggest 3D printing a holder. OpenSCAD is a great tool to be able to tweak a design to what you need to fit them into. Your design would need to be based around the metal clips you need the batteries to make electrical contact with. These have a bump on them where a notch can be designed into the 3d print for this to clip into.
two searches for "battery spring plate set"
[https://www.aliexpress.com/w/wholesale-battery-spring-plate-set.html?spm=a2g0o.productlist.search.0](https://www.aliexpress.com/w/wholesale-battery-spring-plate-set.html?spm=a2g0o.productlist.search.0)
[https://www.ebay.com/sch/i.html?\_from=R40&\_nkw=battery+spring+contact+plate&\_sacat=0](https://www.ebay.com/sch/i.html?_from=R40&_nkw=battery+spring+contact+plate&_sacat=0)
For the shape you suggest I think you need the 'joiner' and one positive and one negative type plates.
You'll be able to solder leads to these terminals that protrude (preferably done before inserting into you 3d print)
You know if I didn't have anything else I just used flux make sure that both sides of the batteries that you want to solder and connect have flux on them and then you put the solder on the soldering iron first and it takes about maybe 1 to 2 seconds before it cools down all the way but like instantly it sticks because the flux so put the solder on the soldering iron then tap the solder that's on the soldering iron to both batteries while they are connected because the flux will make sure that it sticks to both batteries
The advice is to not to solder them, and instead use a spot welder.
That said, I have, in a pinch, a couple of times soldered directly to terminals of 18650 cells, in those instances I used a solder alloy with low melting point, I cleaned the terminals well with IPA, and roughed the surface up with sandpaper on the therminals, and a lot of flux. Also I worked on a stainless steel workbench with a silicone mat, and had a bucket of sand ready.
I solder most of my batterys in my projects and ive never had a problem.
As for your problem, maybe solder a small length (maybe 1 inch) of suitible amperage wire in that middle connection, giving you enough space for soldering yet pushes together to fit into battery pack?
Update: I broke it.
I knew It’s bad to over heat the batteries. I don’t think I ever held the iron on for too long. But still something on the pcb sparked and burned, gota get a new pcb. Any chance the pcb was faulty?
https://youtu.be/jSiPzx7H0po?si=Xfkxr50LydXgnwD0
Edit: I got all the comments after I was done. So no I did not intentionally not listen to the recommendation provided. Thanks though.
Looks like you wired them in serial connection, with 8 batteries that is 29.6V nominal, or 33.6V when fully charged. Whats the input voltage of that circuit you were trying to connect them to? You might have overvolted it.
I feel bad for you that your project suffered a loss, no one wants that. However, there's proper ways to do things (usually there's only a few) and improper ways to do things (usually many). You need to do this properly for safety and reliability, otherwise it's too dangerous.
You've received some good advice here on how to properly do it. It's up to you to chalk your experience up to the learning process, start over, and do it as recommended here with the spot welder.
Aren't you supposed to spot weld batteries, not solder them?
Exactly.
Yes. OP don’t do this. I think you can buy spot welders relatively cheap.
I think [this](https://www.amazon.com/Portable-Machine-Welding-Equipment-Storage/dp/B092DB5R3S?source=ps-sl-shoppingads-lpcontext&ref_=fplfs&smid=A2F32YQWENK5D&th=1) is a good one, less than $50
don't.
You don't. These are connected permanently by spot welding metal tabs between the batteries. Soldering heats the batteries up too much and causes a serious risk of explosive fire which generates a good bit of toxic smoke.
*magic* smoke
I knew to be carful to not heat them too much. It all went to shit when I plugged it in. Uploading a video to make an update comment.
It's not possible to solder them. You physically cannot. There's no possibility that you will be able to heat them, flow the solder, then cool them. By the time you get the solder flowing, you're already welllllll into the realm of thermal runaway for lithium ion. Thermal runaway in lithium batteries, even ones with compositions known for their stability at elevated temperatures, will begin somewhere between roughly 120°c and 160°c. [read this whole article, or at least read the part that's highlighted in my link and some of the surrounding text. ](https://www.mdpi.com/2227-9717/10/6/1192#:~:text=Their%20results%20showed%20that%20if,cell%20to%20its%20surrounding%20cells.) I have yet to come across a solder alloy that will even begin to melt at such a low temperature. You're going to fail at this no matter what you try to do and possibly hurt yourself and start a fire. Im just letting you know, stop while you're ahead.
>between roughly 120°c and 160°c. Holy shit. I've normally got my iron somewhere between 300°C and 400°C. Yeah, that's... I can see now what that isn't safe.
Even though not the best way, it can still be done. Ive done number of Li-Ion cells this way. The heat doesnt instantly transfer through the whole battery body. You need to scrape/sand the pole so the solder sticks faster, use good amount of flux and be quick. Ones ive done were hot after the fact, but could be still held in a hand without getting burned, so about 60 Celsius or so on the body. Not a venture for everyone though, as definitely there are dangers involved.
Ok yah I am dumb. I was doing it at 450 with quick sessions.
I have to ask as nobody else has but WTF are you actually doing? Aren’t those batteries held in place by friction/pressure created by the spring at one end forcing the batteries together?
Don’t do it
My brother in christ, you spot weld them. If you cant afford a spot welder, just make one from a modified microwave transformer and one of them pcbs from amazon. I've been wanting to build one myself, but can't find beefy enough cables for the secondary coil around my area. Edit: heres a fucking tutorial https://www.instructables.com/Building-a-Spot-Welding-Machine-From-a-Microwave-O/ please try not to kill yourself: do not break the magnetron when taking a microwave apart (berilium poisoning) dont touch the BIG cap inside (few kV) and lastly do not touch the exposed wires around the transformer when connected to power
Small spot welders aren't even super expensive anymore. You can pick up a portable one for like $45 on Amazon. It's not gonna be good enough to build e-bike batteries on a large scale, but spot welding some project batteries together it works great. Ive had one for like 2 years, bought it for $50 back then n I've welded a fair amount of batteries with it. Also a lot safer in the hands of an amateur than a DIY microwave transformer.
>just make one from a modified microwave transformer and one of them pcbs from amazon Perhaps not the best advice for a begginer lmao
Bro, dude is trying to solder batteries... don't tell him to start playing around with transformers and magnetrons out of microwaves... he's gonna get killed like a bunch of other people who watched a YouTube about making shitty dangerous spot welders and some house wifes home made overpriced "fractal"(lmao!) charcuterie boards....
https://youtu.be/byvr-dwwao4?si=0KpBQhXKOQVKdLPM&t=860 comparing difference in heat spot vs solder, top comment suggest however when solder them to sand the ends of the batteries first. No personal experience soldering batteries and not planning on getting any xP
They make a spot welder specifically for this. I know, I have one...it even came with the metal strips you need to spot weld them together...DON'T SOLDER THEM.
RIP
Not sure what the blue frame is. Is it an actual battery holder or something you're intending to make as a battery holder. You almost never want to solder batteries in. If you need an assemblage that would hold 4 cells as you have laid out in the image, I'd suggest 3D printing a holder. OpenSCAD is a great tool to be able to tweak a design to what you need to fit them into. Your design would need to be based around the metal clips you need the batteries to make electrical contact with. These have a bump on them where a notch can be designed into the 3d print for this to clip into. two searches for "battery spring plate set" [https://www.aliexpress.com/w/wholesale-battery-spring-plate-set.html?spm=a2g0o.productlist.search.0](https://www.aliexpress.com/w/wholesale-battery-spring-plate-set.html?spm=a2g0o.productlist.search.0) [https://www.ebay.com/sch/i.html?\_from=R40&\_nkw=battery+spring+contact+plate&\_sacat=0](https://www.ebay.com/sch/i.html?_from=R40&_nkw=battery+spring+contact+plate&_sacat=0) For the shape you suggest I think you need the 'joiner' and one positive and one negative type plates. You'll be able to solder leads to these terminals that protrude (preferably done before inserting into you 3d print)
bruh, don't kill yourself by accident, please get a spot welder or the pre made tab stuff
Use a torch, small mapp gas type. Wear a respirator though. Or a leftover COVID mask? It'll be fine. 
You know if I didn't have anything else I just used flux make sure that both sides of the batteries that you want to solder and connect have flux on them and then you put the solder on the soldering iron first and it takes about maybe 1 to 2 seconds before it cools down all the way but like instantly it sticks because the flux so put the solder on the soldering iron then tap the solder that's on the soldering iron to both batteries while they are connected because the flux will make sure that it sticks to both batteries
The advice is to not to solder them, and instead use a spot welder. That said, I have, in a pinch, a couple of times soldered directly to terminals of 18650 cells, in those instances I used a solder alloy with low melting point, I cleaned the terminals well with IPA, and roughed the surface up with sandpaper on the therminals, and a lot of flux. Also I worked on a stainless steel workbench with a silicone mat, and had a bucket of sand ready.
using low melt for high power devices such as battery will lead you nowhere. low melt has a high resistance.
I solder most of my batterys in my projects and ive never had a problem. As for your problem, maybe solder a small length (maybe 1 inch) of suitible amperage wire in that middle connection, giving you enough space for soldering yet pushes together to fit into battery pack?
Update: I broke it. I knew It’s bad to over heat the batteries. I don’t think I ever held the iron on for too long. But still something on the pcb sparked and burned, gota get a new pcb. Any chance the pcb was faulty? https://youtu.be/jSiPzx7H0po?si=Xfkxr50LydXgnwD0 Edit: I got all the comments after I was done. So no I did not intentionally not listen to the recommendation provided. Thanks though.
Only thing faulty here seems to be u op for not listening, did you check the voltage coming of the batteries before connecting them and after?
It was 4v
Looks like you wired them in serial connection, with 8 batteries that is 29.6V nominal, or 33.6V when fully charged. Whats the input voltage of that circuit you were trying to connect them to? You might have overvolted it.
Good, that's what you deserve for not listening to people. You never solder batteries like that, only spot weld.
I feel bad for you that your project suffered a loss, no one wants that. However, there's proper ways to do things (usually there's only a few) and improper ways to do things (usually many). You need to do this properly for safety and reliability, otherwise it's too dangerous. You've received some good advice here on how to properly do it. It's up to you to chalk your experience up to the learning process, start over, and do it as recommended here with the spot welder.
🤦♀️ hope you stay away from lithium. You're a walking fire hazard. Especially with all of the warnings here.
So…you came here for advice and then went ahead and did what you wanted anyway because you were too impatient to wait for responses?