i just use good ol kester “44” 63/37.
done in about 2 seconds.
and thats with a battery iron.
But the temperature is much higher, which is why soldering is not used or recommended for Li batteries.
Kester 44 Lead Solder Wire - +682 F Melting Point - Sn/Pb Compound - 37 % Lead
Rose’s metal consists of 50% bismuth, 25–28% lead and 22–25% tin. Its melting point is between 94 °C (201 °F) and 98 °C (208 °F).
yes its a somewhat safer way of doing it.
but if you know how to solder its pointless to order expensive exotic stuff to do this.
i get it done fast enough that the cell end is barely warm afterward.
trick is a good hot iron,and sanding the target area a bit.
if you do it right it will take solder right now.
if you are not sure just practice on junkfire cells.
run them down in something just in case you short it with a solder blob or hold it long enough to melt the seal.
i tried to do that on a scrap cell and i had to keep the iron on it over a minute to see the seal start to melt and extrude.
That’s good to know, thanks for doing the test.
Copper wires and conductors should work with your method, but Nickel-plated strips might be difficult to solder due to the need for an active flux and higher tip temperatures.
i worked on a project where some phd’s claimed the wire was defective and not solderable—it was just nickel-plated and they didn’t know the tricks of the trade…
Sorry jmoots, didn't noticed your question before.
I bought the stuff from eBay seller zhuykoff a few months ago. It came in a sealed bag, in such pip form. Weighed 'em recently, they vary between a bit above 200mg and a tad less than 250mg. Maybe ≈120 pieces per ounce. :-)
Paid $10 for a 75g bag, though I received 85g. ;-)
Cheers my fellows :-)
Very good method Barkuti,
Link those small solder balls when available.
For solder blob I use large flat horse shoe tip.
Then I just sand battery surface, flux it, and when large horse shoe tip is loaded with solder i just make fast 1- 2 second down-up move.
luminarium iaculator, this is my current sale link of choice: http://www.ebay.com/itm/Roses-Rose-alloy-metal-Lead-Bismuth-Tin-low-temperature-melting-90-gramms/132292315369
There are small formulation differences, or so it seems. Melting temperature shouldn't vary much, though.
Cool stuff, I like it. :-)
Cheers 
one place i can see this stuff being useful is making a thermal fuse in a pack.
solder the interconnects in spring tension.if pack overheats alloy melts and opens circuit.
like a grasshopper fuse.
this alloy is often used in sprinkler heads.it melts and activates the valve.
What about for making the individual cell fuses—that is what i was considering to use it, but was concerned about loose hot solder dripping about in the event that one cell went thermal. Maybe a small blob to hold the cell fuse wire would not be a significant amount to worry…?
If you assemble a battery pack, you can hang a small weight from the interconnecting wires. This results in an open circuit condition if the batteries reach 93 - 96°. Excellent from a safety standpoint. ;-)
snakebite, :THUMBS-UP: .
Cheers ^:)
Barkutti, thanks for your video and explanations!
I see myself in the possibility to have to solder the top of some unprotected 18650 batteries, with not extras, apart from the solder.
I have flux (JBC), I have solder wire (Sn60, 0,8mm), but no pre-made blobs!
Is it “safe” or appropriate to just make some blobs with the wire I have, and them proceed as you did (add the flux, put the blobs on the top, and let it “melt”)?
I’ve never put solder on the cells, so I don’t know if “normal” components will do the work correctly…I didn’t want to spend money in the blobs or in other “tops” for it…
Thanks in advance!!
One rather unorthodox method I use is to melt a hanging blob of solder on the tip of the iron, put organic flux on the surface I’m going to hit with it, then hit said surface with the molten solder. It’ll sizzle away and hopefully quickly transfer enough heat to cook the surface and get the solder to stick.
The conventional method of using the iron to heat the surface and melt the solder draws away that much more heat from the tip, potentially taking longer, or even cooling down everything to the point where you’ll end up with a cold joint or one that just doesn’t stick.
Heating up the surface takes X amount of heat. Melting room-temp solder to its liquid phase (and latent heat of fusion) takes Y amount of heat. You have to hope that your iron has enough temperature and thermal mass to at least exceed X+Y by a good margin.
Take Y out of the equation, and you need only X amount of heat!
MascaratumB, solder in blob form is not a requirement, it is just the way the solder I use comes as: Rose's metal / Rose metal (Bismuth, Lead, Tin alloy) 75 g. LOW MELTING SOLDER @ zhuykoff
You'll need appropiate flux for steel (aggressive, acidic), but you can also use some fine grain ”wet” sandpaper to slightly sand down the surface to be soldered over.
With regards to using standard Sn60Pb40/Sn63Pb37 solders, the problem here is you'll need to raise the cell surface temperature nearly 100°C higher, which is a lot. What this means is if you don't have a powerful iron and a fat tip to transfer the heat fast better forget it. You can try, but if in at most ≈5s your solder hasn't properly pooled yet holding onto the steel surface, I'd say better give up. Alternatively, buy some Rose's metal, makes your life muuuch easier and safer. 
Cheers :-)
Thanks for the answer Lightbringer! My iron is 30W, and so far ir only had 1 or 2 problems with some solder. I hope I can do a similar process like you, or like Barkutti!
I’ll try make a clean job not to damage the cells! 
Gaaaah! Get a hotter one. It’s a one-time investment, so make it good. I got an AEIOU variable-temp iron, and crank it up to maximum when doing that kind of thing.
Hit it fast, hit it hard, get in and get out. You’ll never have to linger for 20min cooking everything else in creation just to make one little solder-blob.
I had a 30W iron long ago, tried to solder with Sn63Pb37 onto Ni-MH AA cells unsuccessfully: cold joints and damaged cells.
Also, while Lightbringer's X + Y tip is fair please note that, in this case, heating up the cell takes away most of the heat and, in any case, you should at least pre-moisten the iron's tip in some amount of solder in order to maximize contact surface and thus heat transfer.
Cheers :-)
The big hanging solder-blob is moisture enough.
Thanks for the answers, to both 
I guess I will try first in some old batteries (from a powerbank) and see how the solder iron behaves! I didn’t want to buy another one, budget is becoming short for some things on the hobby
I’ll experiment! Thanks again
Well, just tried it and it was not the most perfect surgery, but so far the cell is working
Further work will be needed, namely to combine the flux and the soldering time over the battery! I’ll learn!
Again, thanks to the answers!
Huh, it looks like in a local shop it costs about $7 for 100g pack.