I saw a youtube video where the guy heated both cells with a double sided barrel iron and slid them together when he pulled the iron away. This method along with low temp solder would be perfect for my application. I just need two cells in series.
Maybe avoid any chances of overheating the cells/batteries? 
The heat a battery can sink is quite substantial, and proportional to its mass. It is easy to inflict permanent damage to a cell with standard solder stuff.
Added security? With 183+°C solder melting point your battery will for sure cook itself in case of a short (not all packs have BMSes, and even less with temperature monitoring).
Packs with vertical cells connected with Rose's Metal would be very safe, as the tabs underneath would readily detach at 98+°C in case of overheat emergency.
0K, not gonna insist any longer. If any of you can't see the point, I may be able to say it louder, yet I doubt it'll sound any clearer. :!!!:
Cheers fellows ^:)
I sometimes make battery packs. But not much experience.
Barkuti,
The Rose metal appears to come in little pellets. I’ve never used it. Would you just solder a pellet to the battery and the connecting tab then join them or is there some technique I’m not aware of to use these?
The primary thing I like about the lower melting point is less heat applied to the batteries? Wouldn’t it take less heat to the batteries to use this solder?
But then the question arises….How well does rose metal adhere compared to high heat solder?
Sounds like something I would like to try.
Thanks for the thread.
The important thing is to have a powerful solder station which applies a lot of heat rapidly in the cap or bottom, so the rest of the battery stays cool
Both ends of the battery are a few millimeters away from the rolled Lithium foil
Zebretta, since it comes in pellets, in the first place I'd wet the iron's tip with a pellet or two, in order to maximize contact surface. I also do it when wire soldering anyway. Then, I'd grab a tab/pack of wires, place enough pellets over it, then heat it from above in order to properly wet the surface. After this step, turn the “tab” upside down, reapply solder over the tip if needed and, after pre-heating the tab from above (with the underneath solder layer hot and fluid), place it all over the battery terminal. Should grip on it immediately, so hold it on with needle nose pliers or a screwdriver while you move the iron away, and let it cool down.
Please notice that flux may be required for this to work, so go make yourself a small bottle of it with some resin and ethyl/isopropyl alcohol. Go thick (10%+) with it. And thoroughly clean the cells' terminals.
Cheers ^:)
With regards to the anode, that is true. But I'm not so sure if we speak of the cathode… 
Cheers ^:)
Ok, I’m not into making battery packs, but I have a faint idea of where heat goes.
The underside of batteries is made of iron/steel or something close and is not in direct contact with the core of the battery (or it would short, right? Have a look at HKJ’s dissections of li-ion batteries).
With a good hot solder iron, it takes 3 seconds to add a (normal type rosin core solder) solder blob on the battery (top or bottom), then let it cool down while an opposite blob on the lip is made, and then again 3 seconds to attach the lip to the battery, fusing the solder from both sides.
With the distance from the heat source, the spread out of the heat to a larger volume, and the relatively bad heat conductance of iron, there is no way that the core of the battery where the chemistry is taking place is getting anywhere near 100 degrees C (which is the temperature that HKJ stops his li-ion tests).
I have no back-up measurements for this but I’m confident that I’m correct and would have no hesitation to do it this way if the need arises.
Anyone more knowledgable than me who disagrees?
Well there is definitely a limit! I lost an eneloop aaa to the heat of a quick mot spot welder blast as well as a Samsung 30q from iron soldering a tab. If it’s done right and quick as you are saying then I’m sure it’s ok, but I was surprised at how little I had to “screw up” before the battery took of in thermal run away. This battery was not hot in my hand at first but after a few seconds it started getting warm and then too hot to hold. It didn’t vent or anything but I concidered it trash.
When soldering to a battery does the charge state matter? The 30q I lost was freshly charged
After it cooled down it was somewhere between 3.8-3.9v. Don’t remember exactly but it had lowered as a result of the incident.
Thanks for the story 
, hmm, then that contradicts my gut feeling quite a bit. I will be careful whenever I do my first battery pack!
I would be interested to hear if anyone else has had similar experiences?
I solder blobbed 6 x 18650”s & a couple of 26650”s a couple of weeks ago as I had measured a 0.5A drop in current on a 2 x 26650 in series light on turbo using a magnet as a cell connector.
It was the first time I have done it as I have always been worried about solder blobbing cells (hence the use of magnets).
I measured the voltage of the cells before & after the solder blobbing, due to my concerns, & measured no difference.
I also measured capacity after which was also fine.
I used 60/40 2% rosin core solder which with the addition of a little extra flux from a flux pen flowed nicely forming a nice “button”.
I’ve just finished a little more reading on the subject. I think I may now have the courage to give it another try. I now how a high power soldering station and quality solder. Both of which have helped me greatly!
I was one time told that any solder will do the job. While this is true if you just need to solder a few wires or connectors here and there, if you are using solder daily good solder saves a lot of frustration!
I was also once told that any wattage iron will do the job if it’s done right. Again, it will do the job. I was also told that too much watts will ruin your work if working on sensitive parts. This is true ONLY if using an iron WITHOUT temperature control. With temperature controll you can solder smd parts with 500watts. In fact the more watts you have the lower the needed temp and the faster your piece heats up. All this measures up to both faster and safer soldering. There are some that will argue this but after my research and experience I can boldly say, they are wrong! :disguised_face:
I think this is my first rant on the forum:) just to say I’m going to give soldering batteries another try:)
I can read dozens of succesful anode blob solderings nearly everywhere. Not so much for the cathode, it doesn't seems to sport the same interest.
Now think about this: if batteries were immune to the heat stress of a soldering iron, why in hell would I need a chinese Roman on a spot welder to build battery packs? This is the usual method, and I'm pretty sure there are reasons to back it up.
It is advised nearly everywhere that heat stresses/kills batteries. I already killed a Ni-MH AA 3-pack in the past. It is required to heat up to fused solder temperatures the surfaces to join. This doubtlessly sets a lot of thermal inertia in the cell/battery, at least for common Sn63Pb37 & Sn60Pb40 alloys for electronics. And you need a powerful iron and actual soldering skills and experience, don't even try without it.
Maybe we could seduce HKJ to make some sort of test in this regard. Take a bunch of cells to test (various sizes and chemistries: AAA/10440, AA/14500, 18650 & 26650; Ni-MH, li-ion and LiFePO4, at the very least), record a few “before” discharge curves at low/high amp draw, solder tab/wires on the cell bunch and let 'em rest for a few hours, and finally proceed to record an “after” set of discharge curves. We'd have objective material for evaluation.
Cheers ^:)
I used a calibrated 60watt digital temp control soldering station set at 400c iirc & a big 3.2mm 45” degree tip.
This allowed me to work very quickly with each blob only taking a second or 2 so minimal heat applied to the anode.
Interesting about anode vs cathode. It was the cathode that set that 30q off? I will say the solder wasn’t melting well so I’m betting I had the iron on there a total of 6-7sec?
bella-headlight correct procedure in using both big tip and temp, as a cell sinks heat fast thence a lot of thermal inertia is required to stand a chance of raising the battery terminals to solder fused temp quickly, minimizing overall thermal transfer (if the heat source is removed immediately, of course).
LightRider, I said what I said (anode vs cathode) because I observed a hollow space beneath the anode terminals of LiitoKala 26650-50A/B cells. This for sure cripples heat transfer, a desirable effect here because we do not want the body removing the heat from the flat top fast if at all. :!!!:
Cheers fellows ^:)
Solder wire low melting point 124C 1m/1mm Bi55.5 Pb44.5 alloy good desoldering @valentinas33
5/9 parts bismuth, 4/9 lead, 255°F fusion temperature.
Unknown alloy properties, of course. 
If we were to add a 20% of readily available Sn99.3Cu0.7 to this, we'd end up with 41.6͡7Bi (5/12), 33.3͡3Pb, 24.825Sn and a 0.175Cu trace. Close to Rose's Metal I'd say.
Cheers ^:)
FWIW I’ve also lost a few eneloops when making replacement packs for cordless phones. They go into a runaway type situation and one shot a burst of flame out. Thankfully I’d thrown it into the yard.
Happens fast doesn’t it? 
I would never use low temp solder for anything that even remotely heats up.
Just another unnecessary point of failure, and I doubt it is as good quality as the 3% silver solder I use.
With a good soldering iron and decent soldering skills, the 200C+ melting point of regular solder won’t damage anything.
When soldering cables to batteries you just need to preapply solder to both the contact and the cable, then heat the cable and fuse it to the contact.
A few seconds of touching is not enough to significantly heat up the battery, just the solder on the contact.
Same for any other sensitive electronic part.