Spot welding fuses to individual 18650 cells?

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garolittle   100 W

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Spot welding fuses to individual 18650 cells?

Post by garolittle » Mar 31 2018 2:35pm

I have been asking several sources for their opinions on this question. For an electric go kart project, my plan is to use the following:
  • Brand new Samsung INR18650-25R cells (20Amps maximum continuous discharge)
    Battery packs: Using the Samsung cells, I plan to have two separate 20S5P packs connected in parallel
    Motor: 72V Motenergy Brushless Motor (ME0201014201). Continuous current - approximately 50 - 60 amps at 72V
    Charge Controller: Sabvoton (svmc72150) DC Current Limit: 100A
The main objective is speed so driving range is secondary. With that in mind, would you recommend that I make the parallel and series connections using a spot welder and the .15mm x .70mm pure nickel strips or should I individually fuse the cells? To avoid excessive heat, I have been practicing using a spot welder to connect thin fuse wire (which blows at about 5 amps) to the cells instead of soldering. I would think that spot welding the fuses improves safety (since thermal runaway would be less likely) but I also want to avoid adding too much resistance into the circuit. Any opinions or suggestions would be greatly appreciated.

Gary
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Re: Spot welding fuses to individual 18650 cells?

Post by spinningmagnets » Mar 31 2018 3:19pm

If it is going to be parked and stored indoors, I would add fuses to each cell. It doesn't protect against all possible incidents, but...it mitigates the possibility of the entire pack catching fire due to one cell developing an internal short. If that happens, the one bad cell will still boil over, but...at least the rest of the pack has a chance to avoid that. It's not about saving the rest of the cells in the pack, it's about saving the garage and house.

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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Mar 31 2018 4:10pm

Thanks for the response. I may use the method demonstrated here:

https://youtu.be/H_b3OJuQpEE

Do you foresee any problems regarding too much resistance within the circuit if I use individual fuses on the battery pack with the 72v motor/charge controller? Thanks again for all of the information.
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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Apr 02 2018 11:04am

One other question regarding the type of individual fuse to use: Given the motor and charge controller specifications listed above, I thought the following fuse wire would work well:

36 SWG Tinned Copper Wire 500g FUSE WIRE 5 AMP 0.20MM

Any thoughts or opinions? Thanks in advance for any ideas.

Gary
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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » May 04 2018 10:18am

So I just had an idea that I wanted to share with the forum. Rather than having to 2 separate 36V battery packs connected in series (one on each side of the go cart seat), why not have 3 separate 24V battery packs as depicted in the drawing. Please forgive my drawing abilities. The 24V battery packs could be connected in series which would allow me to achieve the necessary 72V. The risk of thermal runaway over at a large single or double battery pack is reduced simply because there would multiple smaller packs. Also, this provides a “modular approach” which would allow me to use the individual 24V battery packs for other purposes when not attached to the go kart. My plan would be to connect each of the 24 V battery packs using 4-AWG since I could pull a maximum of 300 amps (50-60 Amps Constant Current). I could still spot weld the individual cell fuses as discussed but I wanted to see what the forum experts thought about this battery configuration. Thanks again for all the great ideas and support.
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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » May 04 2018 11:01am

I forgot to mention that each 24 V battery module would have a 4S20P configuration. Thanks for any suggestions or ideas anyone may have.

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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » May 04 2018 12:09pm

I apologize for the incorrect battery configuration. It would actually be 7S20P.
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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Jun 28 2018 9:54am

I am intrigued by the idea of individual fuses so I am going to run some experiments. Here is a small prototype I built last night. I am going to spot weld the individual fuses the this weekend. I used a Dremel to cut a small groove in the copper bus bar so that the individual fuse wires would fit snugly inside the bar. I then used a 100 W solder gun to coat the wire with solder. It’s a small test but it should be interesting.
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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Jun 30 2018 3:50pm

I am really starting to like the idea of spot welding fuses to individual cells. I’m still experimenting with soldering the fuse wire to the copper bus bar but it just seems like it will be the best solution for my application. I have not yet determined the optimal size of the fuse wire in terms of amps but I will do so in the near future.
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Re: Spot welding fuses to individual 18650 cells?

Post by spinningmagnets » Jun 30 2018 5:06pm

Have you considered the option of running two 36V packs on series to get 72V? A lot fewer headaches, and I think safer...

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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Jun 30 2018 8:34pm

Yes Sir. I am still considering that idea. I like using the “modular” approach since it would allow me to use the batteries for other applications (ie camping). I even gave consideration to the idea of connecting three 24v modules in series to get the 72v needed. Would that be advisable?
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Re: Spot welding fuses to individual 18650 cells?

Post by methods » Jul 03 2018 1:16pm

garolittle wrote:
Jun 30 2018 3:50pm
I am really starting to like the idea of spot welding fuses to individual cells. I’m still experimenting with soldering the fuse wire to the copper bus bar but it just seems like it will be the best solution for my application. .....
Rad... :P
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(in case the pics get lost)

This is a clear win


-methods
Increasing battery voltage and controller current limit will result in a non linear experience

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Re: Spot welding fuses to individual 18650 cells?

Post by Addy » Jul 03 2018 3:19pm

I also really like the idea of spot welding fusible wire to cells. I bought some 30 AWG tinned copper wire for this purpose and I've used it to build 2 small battery packs so far.

I'm finding it easier to spot weld these wires to recycled cells. You don't need to grind away the old spot welds, you can just weld the new wire in an empty space. I also like that you can easily see whether the weld is good or not. Here are some close up views:
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Also, here's the wire table with fusing currents which I got from wikipedia. This is what I used to pick which wire gauge I wanted to use.
wire_table.png
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While the 30 AWG wire is easy for me to work with, I would be cautious with larger wires. I attempted to use tinned 24 AWG copper wire for the series connections on a low current pack. First I practiced with some garbage cells and I was able to successfully weld the 24 AWG wire to the tops and bottoms of these cells. After that I tried to do it on the battery pack I was building. I completed a couple welds with no issues, but on the third weld I must not have seated the welder tip properly, as it made a gnarly pop when I made the weld, which ended up making a tiny hole in the cell can. Electrolyte was leaking out of the cell so it had to be replaced. Thankfully the damaged cell didn't heat up or have any reaction from that.

I think this kind of problem is less likely with advanced spot welders. My spot welder is homemade from a microwave transformer and just applies a single pulse without knowing if there's good contact or not.

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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Jul 04 2018 3:20pm

Great discussion. :) Thanks for sharing your experience. I found that cutting a thin groove into the copper bus bar makes soldering the fuse wire much easier.

I am using 28 awg copper tinned wire which seems to work well. I am also using the Sunkko 788 Spot welder but I may upgrade to a better one soon. https://www.amazon.com/dp/B01MF8LKYK/re ... pBb8F2FD3J
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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Jul 04 2018 9:12pm

I have found it easier to use painters tape to tape down the ends of the bus bars and ends of each fuse wire. I then hold the solder gun (100 watt Weller) on the bus bar for 5 seconds before applying the solder on top of the wire so it will melt down into the groove. So far it seems to work well.
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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Jul 05 2018 8:46pm

So the spot welding of the individual fuses went well last night. There were a few instances of sparks that prompted me to clean off the electrodes of the welder but otherwise everything went well. I added a BMS to complete the experiment and overall I am pleased with the results. Here is my 4S5P 12v battery complete with copper bus bars and spot welded fuses. :)
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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Jul 06 2018 9:06am

By the way, I used a roll of copper tubing from Lowe’s and a hammer to create my homemade busbars. Much cheaper and easy to get the diameter needed (about 16-18 mm) for this application. Not sure if it will work well, but I literally used crazy glue to attach the copper bus bars to the plastic cell holders. This allowed me to solder everything to the bus bar first with zero heat applied to the individual cells. I then spot welded the fuses and added the BMS.
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Re: Spot welding fuses to individual 18650 cells?

Post by fechter » Jul 06 2018 1:08pm

It would be good to load test the pack and see how warm the fuse wires actually get. You could also measure the voltage drop across them to determine how much is lost in the wires.
"One test is worth a thousand opinions"

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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Jul 06 2018 6:18pm

Thanks. I will do it. I am new to this field so I will have to do some research on how to load test but I will certainly post the results.
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Re: Spot welding fuses to individual 18650 cells?

Post by Willow » Jul 06 2018 6:53pm

In the search for low resistance, safe battery construction techniques for high power output, this seems to be neither low resistance or robust enough for e-bike/go-kart application. Heat will be an issue because of the resistance of the fuse wire... and I wouldn't trust the wires to maintain contact with the cells. Spot welding may make the wire brittle enough to simply snap the wire off - remember there is going to be significant heating and cooling through these connections, which equates to the wires expanding and contracting accordingly, adding to the mechanical stress of the connections.

If you have a spot welder, IMO you would be much better off either using nickel strip or copper to spot weld directly to the cells, and save yourself some serious headaches down the road.

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Re: Spot welding fuses to individual 18650 cells?

Post by Addy » Jul 06 2018 8:01pm

Willow wrote:
Jul 06 2018 6:53pm
In the search for low resistance, safe battery construction techniques for high power output, this seems to be neither low resistance or robust enough for e-bike/go-kart application. Heat will be an issue because of the resistance of the fuse wire... and I wouldn't trust the wires to maintain contact with the cells. Spot welding may make the wire brittle enough to simply snap the wire off - remember there is going to be significant heating and cooling through these connections, which equates to the wires expanding and contracting accordingly, adding to the mechanical stress of the connections.
Do you have experience with fusible wire? Where are these claims coming from?

Of course you need to select the correct wire size for your application, so that the wire is not heating significantly during operation.

You can see from the pictures I shared how good the contact is between the wire and the cell. These connections are not brittle - I've practiced with junk cells and I've tried to rip the wires off the cells after welding them. The wire is what breaks, not the connection to the cell.

For the 30 AWG wire I'm using, a 2cm piece would have a resistance of 6.77mOhm. I would assume I have a 2cm piece connected to the top and bottom of each cell, so that's 13.5mOhm. This is low enough for the packs I'm using it on. For higher current per cell, I would choose a larger wire gauge.

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Re: Spot welding fuses to individual 18650 cells?

Post by garolittle » Jul 06 2018 8:08pm

Willow wrote:
Jul 06 2018 6:53pm

If you have a spot welder, IMO you would be much better off either using nickel strip or copper to spot weld directly to the cells, and save yourself some serious headaches down the road.
Thanks for the good information. I have used spot welding many times and I enjoy the ease with which you can make the series and parallel connections
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The final battery pack (72v) will need to put out about 45-55 amps continuous and I had read that nickel strips may have too high of resistance. However, your post makes sense since all that current running through fairly thin fuse wires makes heat an obvious issue. I just wish there was an easy way to “”nickel plate” copper strips (about 7-8 mm wide). That would be the best of all worlds.... ease of spot welding and low resistance.
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Re: Spot welding fuses to individual 18650 cells?

Post by spinningmagnets » Jul 06 2018 9:13pm

"DIY nickel-plating onto copper cell connections"
viewtopic.php?t=85310

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Re: Spot welding fuses to individual 18650 cells?

Post by Willow » Jul 06 2018 10:28pm

Addy wrote:
Jul 06 2018 8:01pm

The wire is what breaks, not the connection to the cell.
Is exactly what I was getting to - the wire may fracture at that point where it has been spot welded and joins the cell. There is a good chance that the wire is hardened by the intense heat (localized) from the weld, and as such presents a point of mechanical weakness. Vibration, heating and cooling over numerous cycles will certainly stress the wire, maybe to the point where it fails.

To me, those close up photos of the wire that has been spotwelded (and vaporized at points) is reason enough to not go down this road.

A properly welded (good current sharing like the pics that has just been posted) nickel spotwelded pack will easily deliver 50Amps safely.
At least double that for Copper.... which can be welded if you have the right setup (high current pulse for a very short time).

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Re: Spot welding fuses to individual 18650 cells?

Post by Addy » Jul 06 2018 10:55pm

Willow wrote: the wire may fracture at that point where it has been spot welded and joins the cell. There is a good chance that the wire is hardened by the intense heat (localized) from the weld, and as such presents a point of mechanical weakness. Vibration, heating and cooling over numerous cycles will certainly stress the wire, maybe to the point where it fails.
I recommend you try spot welding fusible wire, at least to get a feel for what happens to the wire after welding. The wire breaks when pulled because it gets drawn too thin, not because it was brittle.

Willow wrote: To me, those close up photos of the wire that has been spotwelded (and vaporized at points) is reason enough to not go down this road.
The welds don't have to look like that with a good welder that's configured properly.

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