18650 Packs - Why Not Low Temperature Solder?

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Punx0r

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Some basic info on low-melting point bismuth-based alloys: http://en.wikipedia.org/wiki/Wood's_metal

A quick google search reveals there are many industrial low-temperature solders available. The lowest I saw claimed to be liquid at 7°C!

I suspect mechanical strength may be lower than the standard lead-tin solders we're used to, but this could be overcome by simply designing the pack so the electrical connections are not expected to be structural.

What do we reckon an 18650 cell can withstand for, say, 5 seconds without suffering electrolyte decomposition? 100*C? 70*C?
 
Punx0r said:
Some basic info on low-melting point bismuth-based alloys: http://en.wikipedia.org/wiki/Wood's_metal

A quick google search reveals there are many industrial low-temperature solders available. The lowest I saw claimed to be liquid at 7°C!

I suspect mechanical strength may be lower than the standard lead-tin solders we're used to, but this could be overcome by simply designing the pack so the electrical connections are not expected to be structural.

What do we reckon an 18650 cell can withstand for, say, 5 seconds without suffering electrolyte decomposition? 100*C? 70*C?
Click to expand...


The amazing thing, is clean surfaces clamped tightly conduct better than most not-silver solders.
 
Its not just the solder, but the cell's conductive end-caps that need to heat to insure a bond. The various experienced I consulted before deciding to solder my battery pack, all said high heat for the shortest time possible. I set my digital solder station to 450, the highest setting (typical Sn60Pb40 electrical solder has a 376 Degree F melting point). Still, I needed to hold the wand a full 10 seconds on the cathode side to see the solder bond to that surface. All wires jostled loose were because of an inadequate surface bond. I'll not know whether or how much damage to the electrolyte happened until I approach end-of-life for the cells, either premature or as expected ~2000 cycles. As of now, however, the cells continue to perform well, so inferring no damage done. FYI, I'm in the process of making my own spot welder from an automotive 12V 2-farad capacitor, so as to avoid having to solder cells again. Mostly because of the nuisance, rather than fear of cell damage, but that too. I think with the right spot welding setup I can spin out a 12S string of 26650 cells in 20 minutes and with no muss, no fuss. But yea, for those making their own packs, having the right equipment and approach is essential.
 
If you needed 10seconds, your solderiron had too low mass to rapidly transfer the required heat. Also need to have decent power for soldering these cells, at least with "standard" solder.
I did 180cells with a 100w fat tip iron, was only a few seconds of high temp exposure. But in the end, the solder method is not optimal.
There is no denying the heat will affect the cells. But how much is a question, on which I havent found reliable information.

I can still se a benefit in being able to solder the cells with no damage over clamping the cells together in some cases.
Even if the resistance is slightly higher.

http://en.wikipedia.org/wiki/Solder I had no clue there were so many different solders out there. :)
 
r3volved said:
If your solder liquified at 7.C then wouldn't you have a bunch of loose cells and a puddle of metal on a hot day? Or a ride down the street?
Click to expand...


You would get it on a cold day even.

He is just suggesting some blend of a lower temp solder may be better suited towards cell connections. I agree.

My preference is not to solder in battery pack construction whenever possible though.
 
I have done alot with heat as I have an infrared/hot air work station I use for computer repair. In my opinion I really don't think soldering will damage a cell. I have opened a 18650 cell before and the paper that is used for the ion filter or whatever looks to be quite able to handle heat. I say this because it looks to be similar to high temp flexible paper like stuff I have seen that I can heat up to 180-210c. Also heat will transfer fastest thru the sides of the can on the negative side and the top positive side is isolated basically from the can as I has a small copper plated circuit board that attaches to it then into the can to connect to the lithium paper deal so it should remain much cooler inside the can. I assume the filter is the number one threat for a cell to start on fire, if it is made strong enough to last many years without corroding or... then IMO it will handle heat above 150c.

Here is some stuff I picked up to use as a thermal paste for my cpu, it melts at 160c. Was going to use my infrared station to heat the cpu/board enough to melt it creating seal that goes into the tiny "craters" inside the cpu die/copper heatsink contact but never got around to it.

IMG_0724.jpg


http://en.wikipedia.org/wiki/Indium
 
JB Weld works good for me. Just got more at walmart for $4.67.
Great to repair a pack, where the new cells don't have tabs.
I would not build an entire pack. Too much work! Sure with practice anything gets easy, but it is like rebuilding an engine for a car. VERY FEW can learn how to do it right.
Low temp solder is an interesting topic! But JB doesn't melt 'til about 400F, works as a thermal fuse.
I've got 2 cells with JB in my scooter. works great! I put together a runt pack for experiments. :twisted:
 
Matt Gruber said:
JB Weld works good for me. Just got more at walmart for $4.67.
Great to repair a pack, where the new cells don't have tabs.
I would not build an entire pack. Too much work! Sure with practice anything gets easy, but it is like rebuilding an engine for a car. VERY FEW can learn how to do it right.
Low temp solder is an interesting topic! But JB doesn't melt 'til about 400F, works as a thermal fuse.
I've got 2 cells with JB in my scooter. works great! I put together a runt pack for experiments. :twisted:
Click to expand...
Never heard of this before. I spent some time looking into conductive epoxy and experimenting, without success. Are you saying that JB Weld is conductive, so can be used instead of solder or spot welding? If so, just how conductive is it?
 
Matt Gruber said:
JB Weld works good for me. Just got more at walmart for $4.67.
Great to repair a pack, where the new cells don't have tabs.
I would not build an entire pack. Too much work! Sure with practice anything gets easy, but it is like rebuilding an engine for a car. VERY FEW can learn how to do it right.
Low temp solder is an interesting topic! But JB doesn't melt 'til about 400F, works as a thermal fuse.
I've got 2 cells with JB in my scooter. works great! I put together a runt pack for experiments. :twisted:
Click to expand...
I've heard the JB weld thing several times...
If a cell were to die, how easy/impossible would it be to remove from a JBwelded pack design? Are you JBwelding right to the cell or cell tab?
 
arkmundi said:
Never heard of this before. I spent some time looking into conductive epoxy and experimenting, without success. Are you saying that JB Weld is conductive, so can be used instead of solder or spot welding? If so, just how conductive is it?
Click to expand...
Search for Matts tread about JB, it is not conductive, he found a way to hold together contacting surfaces with JB.
 
parabellum said:
Search for Matts tread about JB, it is not conductive, he found a way to hold together contacting surfaces with JB.
Click to expand...
OK, did that and believe this ...
Low-power no-solder idea...
Looks like the idea is to use a smattering of JB Weld and then clamp down so the conductive surfaces meet, then allow to cure and whambamthankumam, a no solder, no weld approach! Hmmm, may try that.
 
liveforphysics said:
He is just suggesting some blend of a lower temp solder may be better suited towards cell connections. I agree.
Click to expand...

Spot on.

liveforphysics said:
The amazing thing, is clean surfaces clamped tightly conduct better than most not-silver solders.
Click to expand...

Interesting. Is contact pressure highly significant? There's a big different between a bolt connection and a spring/magnet/epoxy fixing.

I suspect the conclusion to this thread will be: Stick with welding or a well-engineered mechanical joint - low temp. solder is a sticking plaster for an inappropriate method.
 
Punx0r said:
I suspect the conclusion to this thread will be: Stick with welding or a well-engineered mechanical joint - low temp. solder is a sticking plaster for an inappropriate method.
Click to expand...
I'd agree with that assessment. The real test is on the road. I find that with the significant road vibration and jostling of the pack, even my high-temp solder joints have been popping loose. Nothing to force me to walk my bike home, yet. Hence my initiation of a home-brew 2-farad capacitor welding doohickey, which if it works, I'll replace all the splices in my pack with.
 
r3volved said:
Matt Gruber said:
JB Weld works good for me. Just got more at walmart for $4.67.
Great to repair a pack, where the new cells don't have tabs.
I would not build an entire pack. Too much work! Sure with practice anything gets easy, but it is like rebuilding an engine for a car. VERY FEW can learn how to do it right.
Low temp solder is an interesting topic! But JB doesn't melt 'til about 400F, works as a thermal fuse.
I've got 2 cells with JB in my scooter. works great! I put together a runt pack for experiments. :twisted:
Click to expand...
I've heard the JB weld thing several times...
If a cell were to die, how easy/impossible would it be to remove from a JBwelded pack design? Are you JBwelding right to the cell or cell tab?
Click to expand...
JB direct to the cell. If it has a tab, that is wonderful, since only the tab needs to get melting hot with solder. So i haven't given up soldering, just soldering direct to the cell. it only takes 3 or 4 seconds to solder, even direct, with my 110w iron. MUST cool as soon as solder sets. wet rag or sponge sizzles as it cools. That sizzle warns you the electrolyte may be sizzling too!
How easy to remove? on ONE cell, not too bad, i use a file, grinder, razor works too. I don't recommend building a pack from scratch with jb, but a repair YES. I buy tool packs, too lazy to build from scratch.
JB is a good insulator, so the repair is less likely to ever short.
 
It causes contamination of the cell that will cause premature failure. "Premature" could be anytime between tomorrow and many years hence...
 
In my early e-bike builds, I used pure indium to solder leads onto my controllers (after mechanically crimping and attaching spade connectors first). Indium is electrically and thermally superior to normal solder, and it flows at 300F. It's so soft that wire strain does not seem to be much of an issue. It works with normal flux.

I can't say that it made any real difference versus using 60/40 solder or even using crimps alone, but it did seal and permanently secure my connections.
 
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