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Issues with Busbar Solder Joint Strength (pack too long?)
#1
Aloha,

I have just started to build my 14s172p powerwall and am at a loss with these solder joints failing to hold fuse wire to the busbars. Here's my situation.

*4 cell x 43 cell parallel groups (3' long) feel long
*Negative side utilizes 24 gauge tinned copper wire
*Positive side utilizes 2amp glass axial fuses (link)
*Busbars are 2 lengths of 8 gauge bare copper wire twisted together
*Connection to cell has been via kweld @ 10J

Is it normal for these cell groups to be so flexible? After I finish one side and flip it over, it is resting directly on the solder joints on the other side, this has caused many joints to fail. I don't see how everyone else's packs seem so durable. I cannot get these solder joints to hold properly. I have sanded the copper busbars and used quality 63/37 solder with rosin core. I melt a sufficient quantity of solder onto the fuse/busbar connection and it is weak after cooling. I am using a 40w soldering iron and let it get very hot before melting a glob onto the connection.

I would appreciate feedback on why you may think my solders are failing.

1. Is it normal for these busbar connections to be so delicate?

2. Are my packs too long?

3. How can I better solder these connections?
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#2
I am willing to bet your packs are too long. Is there a reason your trying to make them so long? The largest packs I have seen are 30p or so and they are kind of flexible. So at 172p that pack must be VERY flexible and extremely heavy. You would be better off making them smaller and using that 8awg as a common bus bar the smaller packs connect to. The 8awg wire in your specific application, the 8awg twisted will be extremely rigid parallel to the packs length, thus when the pack flexes if your fuse wires are taught they will snap. By this I mean that you cannot stretch or compress that wire, you can bend it yes at an angle lateral to its length, but it is very strong as a rope.

Another issue to consider is, battery failure is eminent although none of us want to admit it. Since this is in fact the case, repair is therefor eminent or your pack will just die and you build a new one... so what I am getting at is this, if you make smaller modular packs although it may take up slightly more room, it is much easier to take apart and repair smaller packs and easier to have a spare pack to just plug in whilst you repair a down pack. 

Not too sure about the solder, although joining a very large (twisted 8awg wire) to a small wire like the axial glass fuse can be difficult. Also consider, solder may be labeled 63/37 but its not. You can get pure flux and put a little on the bus bar and dip the end of the fuse / fuse wire in it and the solder will take to it much easier.
I hope this helps!
Griz
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#3
The fuse Wire can not be tight. It need to be floppy so to say. And yes it flexes. They Arent made to be moved around.

Image of the pack?

Solder should not get loose from the busbar. IF it does its a Cold joint.
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#4
A 40W soldering iron sounds like it is too small to heat the bussbar properly.
Can you post a pic or two?
The solder should flow not make a bead.
Try with an 80-100W iron.
Is it possible the wire has a coating on it, eg a thin lacquer or oily film, etc?
Careful with any flux, you don't want that corroding later. The rosin core flux in the solder should be plenty.
Running off solar, DIY & electronics fan :-)
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#5
(06-30-2020, 05:12 AM)Grizwald Wrote: I am willing to bet your packs are too long. Is there a reason your trying to make them so long? The largest packs I have seen are 30p or so and they are kind of flexible. So at 172p that pack must be VERY flexible and extremely heavy. You would be better off making them smaller and using that 8awg as a common bus bar the smaller packs connect to. The 8awg wire in your specific application, the 8awg twisted will be extremely rigid parallel to the packs length, thus when the pack flexes if your fuse wires are taught they will snap. By this I mean that you cannot stretch or compress that wire, you can bend it yes at an angle lateral to its length, but it is very strong as a rope.

Another issue to consider is, battery failure is eminent although none of us want to admit it. Since this is in fact the case, repair is therefor eminent or your pack will just die and you build a new one... so what I am getting at is this, if you make smaller modular packs although it may take up slightly more room, it is much easier to take apart and repair smaller packs and easier to have a spare pack to just plug in whilst you repair a down pack. 

Not too sure about the solder, although joining a very large (twisted 8awg wire) to a small wire like the axial glass fuse can be difficult. Also consider, solder may be labeled 63/37 but its not. You can get pure flux and put a little on the bus bar and dip the end of the fuse / fuse wire in it and the solder will take to it much easier.
I hope this helps!
Griz
Thank you. I decided to do this particular design because I need the packs to be 172p but also did not feel confident in my ability to fabricate a busbar other than the common type used in the 4 cell wide packs. I am in the process of reconsidering my design and may end up making smaller packs and connecting them or making the packs 8 cells wide and figuring out some sort of busbar to construct. I will keep trying to solder better but if I cannot get it to stick I may reevaluate my soldering system.
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