Building an 18650 battery with individual cell fuse wires

Hi guys.

Do you guys keep you batteries in a fireproof box when left unattended? 18650 cells are safer then Lipos but the size of the batteries we build for these e-foil board have enough power to burn several houses down!

I see the most common build method is spot welding the cells. This is the easiest, and least labor intensive method but it does not allow fusing individual cells. If one cell goes bad and develops an internal short circuit the whole pack will most likely have a thermal runaway and catch fire.

Also, I see most people packing the cells together where the are touching each other. These cells handle heat quite well when kept within their amp draw limits, bit spacing the cells slightly helps the pack cool, and a cooler pack lasts longer. Also, if one cell malfunctions and overheats, spacing the cells helps reduce the thermal runaway from one cell to the next.

I am a new forum member building my first e-foil board. I have built 18650 batteries for bicycles, skateboards, and rc toys. As I started to research the voltage and amp hours needed for these e-foil boards I was shocked! I’m building a 14s14p battery with Samsung 30Q cells. It’s what the Lift board battery is.

I plan to use fuse wire on each cell, like a Tesla battery, if one cell malfunctions that fuse wire will melt and the rest of the pack will continue to function normally (just that group will have one less P).

This is just a practice group using recycled cells out of a previously soldered pack so its pretty messy.

I’m building a 14s14p battery, so this is a 1s14p. So there will be 14 of these. Finished battery I estimate will be about 1’x1’x3"


yes, cell level fusing is good idea if you have time/will. you can read more about it and get some tips here:

BUT, you do realise that you make much bigger thermal damage soldering directly on cell, than you will ever make just with heat from normal usage of cells? I am quite sure, in 14p configuration, your cells won’t go even 5 degrees up during discharge. And how much heat you transfer when soldering on negative cell end, you realise that?

if you use quality cells from trusted source, there is little to no chance that you will have any problem with cells out of the blue.

Hi, I have been building batteries for many years for our flight equipment. A note on short-circuit safety of 18650 and 21700 cells. Most brand cells have a built-in module that if the cell is overloaded, the connection breaks and then can no longer give off energy. I solder all cells with self-milled CU-E strip. It is a very special technique that does not heat the cells. yes it is important that the cells always have a distance. my packs are 1 mm. thermo measurements have shown that the packs can deliver much more power than those welded with nickel strips. in facebook you can see some pictures of the batteries. my favoriet are the samsung cells 21700 with 4 ah. I load these in my trike with 300A continuous load without problems without additional cooling. (15 S / 20 P) in my Foil I drive with the same packs only in 10 S / 10 P.

2 Likes very interesting technique! can you describe a little bit more how you solder on cells without heat transfer? As far as I know, surface has to be hot for solder to stick to it?
and can you explain those thermo measurements? what do you mean that packs can deliver more power with CU-E strip than with nickel? CU-E strip or nickel should be just way of transportation current, it should not affect power of battery pack. can you explain that in more details?

hello, I measure with high quality infrared cams. the soldering process is done with a 350 watt homemade soldering tool. This takes around 1-1.2 sec. The cells are thus pre-tinned a very small point. When soldering the cu Strip with a hole , it also takes only about 1.5 sec. Since copper conducts extremely well, the heat is immediately pulled away from the battery pole. It reaches a maximum of about 50 degrees short of the cell does not hurt. The solder used is one that has a low temp. and does not attract h2o to the flux. By copper and soldering, there are no transition resistances as with a nickel strip. the capacity is of course no longer but the entire system has a positive effect because the copper surfaces better regulate the temperature. There is no cooler cell or hotter. there is less balancing loss due to different levels of efficiency of the cells after temp. differences. e are all just my hobby but I am very very intense … the oldest battery with sony vtc 5 cells are now almost 5 years old and over 250 cycles with high load. still work fine that gives me right to do it right. I’ve soldered about 5000 cells so far. It has not been a single to change until today. For e-bikes or finished packs, I have often changed for friends cells. but there were also no name cheap cells. at sony, samsung, lg i have very great confidence in orginal ware.


The soldering iron is only on the battery for a few seconds. As soon as I remove the iron I blow on it and less then a second later I can hold my finger on the negative terminal…right on top of the solder. It is so cool I can permanently hold my finger on top of the solder joint. Have you soldered batteries before? I don’t know where this idea that the battery gets hot came from. I guess if you don’t know what you’re doing you can get the batteries hot. But yes…don’t do that.

Can you put your finger right on top of the spot welded battery immediately after spot welding it?


Thx for the tip. I will look at getting some copper strip. :call_me_hand: that makes sense! thanks for explanation, your way of connecting batteries seems really interesting and actually quite good!

@Flightjunkie I have never soldered on batteries, but there has to be some heat transfer. You started talking about space between cells for cooling and I just wanted to point out that much bigger heat is there when soldering directly on cell. I am soldering wires on nickel, always between cells or on nickel tabs. I hold cells with my fingers so I can feel that heat transfer is minimal, if any.
where did idea that battery gets got came from? well obviously, to solder something, you have to bring heat so solder will turn liquid. that liquid solder will transfer heat on battery. So yeah, there it is how battery became hot.
yes, I can put finger on nickel/cell after spot welding. have you ever spot welded?


No. I have never spot welded cells. Mainly because I don’t own a spot welder, and I have a long background of soldering stuff. So naturally I prefer to solder.

I have dismantled several batteries that were spot welded though. Looked to me like there had to be heat involved in the process. I mean, the metal end of the battery is melted! That takes way more heat then it does to melt solder when tinning the end of the battery.

The soldering of cells is a one time heat. The space between the cells is to allow cooling while being charged and discharged over and over again. The heat from use of the battery is the whole battery getting hot. The soldering to the end is a one time isolated amount of heat for a very short period of time (just like spot welding). It needs to be done with a powerful soldering iron. At least 100 watts.

The tin solder I use melts at 360°F. Sorry for the stupid statement “I don’t know where this idea that the battery gets hot came from”. What I’m trying to say is that the battery does not get hot or damaged by the heat required to tin the end when done correctly. Properly soldering to a cell is just that. You tin the end, then you tin the wire, then you melt the tin to attach wire. It takes a second, and you can touch the end of the battery immediately after soldering and the soldered terminal is not hot. My stupid statement was referring to people who have never soldered a battery before who say the battery gets hot, or damaged by the heat of soldering. Yes the solder melts at 360°F. But, very quickly. If this heat was transferred to the battery it would still be hot to the touch 1 second later. What process do you think welding uses? Spot welding is actually melting the steel metal on the end of the battery. That takes 1,370°F.

Soldering 18650 cells is done by Tesla in all of their car batteries. And the DIY power wall community solders cells when they build their large power walls. It is a tried and proven method that allows for fusing of individual cells, and does not damage the battery if done correctly.

But, it is a difficult skill to learn for someone who doesn’t have any soldering background. So, I see why people advocate spot welding. But, if one takes the time to learn to solder cells, and doesn’t mind the extra work, they can be rewarded with the added benefit of fused cells.

Just my 2c. I have been soldering 18650 cells for a long time. And I have never seen any damage from it. Google soldering 18650 cells. There is a spot welding school of thought, and a soldering school of thought. I’m not trying to start a war between the guys who solder, and the guys who spot weld. I just felt this this forum was under represented by the soldering guys.

This is a tried and proven method that works well for 1 Kw/hr batteries with the inherent risks of 196 cells connected together without any fused protection.

I will post pictures and videos as I build my batteries. This is the first battery of this size I have built. I am by no means an expert and welcome suggestions. I like what suggested about copper strip instead of nickel.

Thank you.



Tesla does ultrasonic soldering, no heat

Data sheet on 30Q for exemple , it takes 140c in a oven for 20 min
Short cut gives 130A and temp raises to more 100c

I have soldered , spot weld cells , use nese module, short cut or drop a few of them , none took fire , but he still don’t know how I will built my next battery, I don’t think fuse are really needed for 30Q at least for what I know

Thank you for the information. Can you link the source of that information / process? The solder still has to melt right? That takes heat?

I will look for it , read it on endless forum, I have this:

Here : Charged EVs | A closer look at wire bonding - Charged EVs

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The high current cell type we all use do not have short circuit protection (PTC). And cell with a continuous rating above 6 or 7 amps is unprotected.

As far as the data Samsung publishes on the battery cells showing they do short circuit testing…I don’t know what this means. Maybe I will test one of my batteries and see what happens when I short the cell. Hmmm…how to do test, should I say… safely? :rofl:

That is also the reason why replacing a faulty spot welded battery is so difficult if not impossible. Disconnecting a faulty soldered battery takes half a second. Another good point for soldering.
With YouTube videos it doesn’t take that long to learn how to make a shiny solder :face_with_raised_eyebrow:

Good point. I’ve scavenged cells from lots of spot welded packs. It’s doable. But, destroys the whole pack while trying to remove one cell.

Hey @Flightjunkie

When spot welding, current will pass through tips of electrodes, so yes it will melt, but only those two small points. welds happen because of heat obtained from resistance to electric current. there was some video on youtube where they measured temperature of spot welding on cell and compered it to soldering

You have some valid points and I have never taught about two schools. My opinion is that soldering was more popular before, when welders were big and expensive/not reliable, but nowadays, when spot welders are so cheap, reliable and easy obtainable, I just don’t see reason to solder on cells. take for example kWeld, 200 eur spot welder which can weld 0.3mm nickel and some thinner copper and I have never heard that anyone broke one!

Also, for soldering on cells, it seems that it just takes much more time than welding. You have to scrap surface of cell and nickel, pretin and solder. with welder you just have click, click and it’s done.

While I do agree that cell level fusing is great safety feature, I don’t see why would I do it on new, reliable cells from proven source. I do see you point doing it in power walls (AFAIK most power walls were built with ‘scrap’ and used batteries) - you don’t want your house to burn down because one cell went crazy. I do see point in doing it in cars - if car battery catches fire, that will bring really bad reputation to manufacturer and public media will go wild. If I will ever do it, I will do it first on nickel which will be welded to battery afterwards.

Any advantage I see in soldering on cells is that if you want to build battery or two, it will be cheaper to learn soldering than to buy spot welder only for one or two time use. Maybe you could give a chance to spot welding and see for yourself that it is really easy, much quicker than soldering and actually safer :slight_smile:

all the best and good luck with your battery! I am really curious to see it.

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For extreme performance, I built some packs with spacers and then poured in with pu - 2 k resin from the safety electronics area. The resin also means that the heat is absolutely the same for all cells. There is virtually no drift in the individual voltages. The resin is so stable that you can drive over it with a car. since no oxygen can get to the cells a fire is completely excluded. I did a lot of tests with pattern packs and tried to make a worst case. is impossible. I just want to mention that because there are different techniques to build batteries. All together, I have only soldered. my professional welding system (has cost 1500.-) for battery welding, I use only very rarely as an example to repair e-bike batteries. if I fly partly up to 2000 meters above ground, I must be sure that the battery 100% works. it is just important that everyone only the technology that he masters 100%. I know of packs eg. 14 S / 20 p burned in a mighty fire and load. the fault were bad nickel strip connections that have come to glow and have brought the pack from the outside to fail. those who built it had no knowledge of spot welding and just built it anyway. Fortunately, it was on the airfield during a test engine run on the ground. Another case was once the bms next to the battery ignited and has also ignited the battery. The bms was built very cheap with false information and lack of security interruption.


Thank you for sharing your experience and pictures.

Thank. I hope to play around with a spot welder one of these days. In the mean time… I will entertain all of you by soldering my pack together and posting the results! :rofl::call_me_hand::beers:


What up guys.

Here’s what over $2,000 worth of Samsung 30Q’s look like! :rofl::call_me_hand::beers: