So I got a Milwaukee MX Fuel battery which is a big 80ish volt battery that was bad from work. Tore it a part and found like 2 bad 21700 cells out of a total 40 cells. Putting blobs of solder on positive terminals to use them as button top cells. Am I dumb for doing this?
Also, some solder fell into some of the batteries and I am scared to mess with the ones that had this happen. Is the rim of the battery under the insulation negative? Ive had some throw sparks and scare me when messing with them so just dont wanna blow stuff up if I can help it!
No, soldering directly to batteries is not safe. There’s a reason spot-welded tabs are the standard in industry.
You’ll probably get away with it on these 21700s that have a thin raised positive contact, but statistically, you’ll probably also get away with a single round of Russian roulette.
I don’t think it’s dumb. Sure there’s a risk of things going wrong and you took a chance. Everyone harvests cells. It’s not safe either but we’re willing to take the risk. I recently did the same on some unprotected 18650s flat tops because my Nitecore Hc50 only takes button top and I don’t have any. I’ve been using the headlamp with my one and only button top protected cell so I needed a backup cells. I was scared but in the end it worked out. Did 3 cells only. Should be enough for a long time. Would do it a hundred times over before trying Russian roulette though…
But I don’t know if you should use the ones that sparked. Others with more knowledge should have tips.
Everyone is a lot of people. The problem with the internet is it’s almost impossible to know the level of somebody’s competence before providing advice. If the history of the battery is known, they have good knowledge of cells, had correct tools/equipment, my advice would be different to that I’d give to a complete novice.
Many lights accept flat tops so this seems a little unnecessary. I’ve had good luck with the Convoy “PCB Board spacer”
I don’t want to gatekeep, but at the very minimum, if you’re harvesting cells you should at least know the general construction of these. The outer “shell” is the negative electrode, so yes, you are at high risk of causing a short.
This shows me you don’t know what you’re doing or are careless, either way, this is extremely bad. Not trying to be rude, just helping you not burn your house down.
All good thoughts…
But the ones the OP got soldered are likely OK… OTH, I would do some testing for IR and capacity (while monitoring temperature) just to be (more) sure. I would be very leery about using the ones that he allowed solder to run into. He has a bunch, so why take any chances?
I have yet to tear down a battery pack without throwing sparks at least once. It seems, from my experience and testing, that very short term shorting doesn’t damage cells significantly.
Until you short one bad enough it turns into a Roman Candle that has to burn out on its own. Ask me how I know. I would guess it happened to me on my 300-400 th pack sitting at my desk. I was very lucky I didn’t burn my house down or burn myself.
What I learned-- Do the shit outside-- you can never be too careful – I have built 100s of packs etc
As far as soldering button tops, I did plenty of them. Just make sure no slag gets in the vent holes.( It’s easier to solder a small washer than make a big blob ) Now days I spot weld button tops the right way.
On the Solar Forum we have a saying, until it happens to you, you don’t understand the potential Hazzard. There, it’s in regard to big LifePo4 cells shorted across Busbars with unprotected tools.
A guy recently posted how he shorted a Torque wrench across cells. I posted a required tool list.
Torque Wrench, Sockets, ETC – Toilet Paper
I was very lucky, I was taking apart a 80v pack and some how a piece of nickel went in a vent hole. Luckily it was just the one cell. Much easier to sit on the carport and if it happens you toss it in the yard. Once was enough for me.
Keep in mind the actual Lithium battery inside the cell is a paperthin meters long rolled-up plasticy thing. It does not take much heat from soldering to mess things up.
Like many things in life safety is greatly dependent on who’s doing the work. Done properly and carefully then it’s very safe. So to say that it’s safe or not safe really doesn’t mean anything in a vacuum.
soldering to positive end is not a problem, if you do it fast, use higher wattage iron, so it takes less time, spot welding is definately safer, you canspot weld a tab and fold it on itself and weld each layer, but if welder is not available, carefully soldering is ok. negative side is a different story, too easy to melt separator inside, definitely weld the negative end, do not solder.
Nonsense, I’ve done that to more than a dozen of my 18650, 18350, and 14500 batteries without any problems, and they work like new. A drop of solder on the positive terminal, sanding or filing the dome of the solder drop to create a flat surface for better contact, and that’s it.
I have done quite a few as well… But I have to ask, How you determine that they work like new?
I have always wondered if I was somehow degrading the cells with the heat involved. But never have done extensive testing to find out whether the concern was warranted. I would think IR and capacity testing would be the minimum. Cycle life even better.
Anyway, I recognize that just because I have gotten away with it a few times, there is no potential danger involved.
How long does it take to put a drop of solder on the positive terminal of a battery? How much do you think the temperature inside the battery will have risen during that time? Do the math, and you’ll see how you laugh too.
I would still like to know how you evaluate the cells are working like new. That was my real question. I believe that if you don’t measure, you don’t know.
As long as the positive terminal needs to reach 185°C. If you take the cross-section of the material from the button to the battery, someone with the knowledge should be able to calculate the heat transfer into the battery easily.
Because I have others batteries, both with and without a button, and I notice absolutely no difference in performance using the same flashlights and batteries? I even tested batteries with a magnet on the positive terminal, compared to batteries with a flat solder droplet. I still couldn’t see a difference… Oh, yes, yes, the skeptics will say, the runtime with batteries with a solder droplet is reduced by 1 minute and 35 seconds compared to the tampered ones, a tragedy!!!
And in the long run, they die (and have to be thrown away) about 1 hour, 5 minutes and 39 seconds earlier than those that haven’t been handled after years and years of use… Another tragedy!!!
P.S: Obviously, the magnetic button solution is also perfectly valid and eliminates concerns about heat. (But it’s more uncomfortable…)
I guess most people who are screaming “NO” have never done this?
As long as I can hold the cell in my hand with no problems, the cell is far from being overheated, and soldering a blob ontop doesn’t heat up the whole cell to 400°C
The only thing you need is a powerful soldering iron and a tip with high thermal mass, so that the metal gets heated up as quick as possible and the heat doesn’t have too much time to creep into the cell.
