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Modular 12V Battery System
#11
BACK FROM THE DEAD!

I know it's been awhile from my last update, but the project never died, just fell on the back-burner.  After completing the cataloging of the cells, I had other projects take priority, and next thing I knew, it was 4-5 months later!  But a couple of months ago, I picked the project back up and have almost brought it to completion!  FAIR WARNING, these will be longer posts due to the much overdue update, haha.

First update, the bus bars:

As mentioned in a previous post, I decided to move forward with the new modular-style bus bars and think they turned out to be a success.  I started by taking 14AWG solid, bare copper wire, and cutting them into 90" segments x7.  This was calculated to allow all 6 bus bars per battery pack to be cut from one segment.  All 7 wires were clamped at one end, and then chucked into a hand drill at the other:


This could then be twisted into a tight bundle, which also helped straighten out the wires into a cable:


All total I used around 315 ft. of 14AWG to create the 90" cables:


Next, these were cut down into (x96) 4.5" segments:


Now came the somewhat tedious task of creating the connection points that will allow assembly/disassembly.  I started with about x150 8AWG bare copper butt terminals and used a hydraulic terminal crimper with the dies reversed to create a flat spot:
 

I then created a jig that would help with the cross-drilling on the drill press.  First creating a center-point using a punch, then using a 1/8" drill bit and some cutting oil:
 

And finally, to finish creating all the parts, I took the cross-drilled connectors and tapped each one with a #8-32 tap and some tapping fluid:
 

It def. took a sizeable time investment creating these parts, but this design has already proved to be valuable for it's modularity and adaptability.

Next up, putting all these pieces together!
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#12
Now that I had all the parts, it was time to solder everything together into the bus bars.  I thought I would be able to soldering everything together in one go, but quickly found that everything was too loose and flopped around.  So I decided to solder the bus bars in halves, and would then bring them together.  I bought a new high-power 100w soldering iron to help heat up all this copper properly, and use a second-hand soldering tool to keep everything aligned.  Then put on some music and soldered all 96 halves:


To bring the two halves together, I realized I was going to need a better jig to make sure they stayed straight.  So this is what I came up with using some scraps of wood, a piece of mountable t-track, and some knob hardware.  The pine blocks have a dado cut into the backside that rides on the t-track and has 2 alligator clips set into drilled holes.  The whole jig is clamped down to the work table to prevent tipping.


The sliding action of the jig allowed me to clamp in a half, and then slide them together with a connection in-between, before locking them down to solder.  Due to gravity, I ended up having to solder a half from the top, and then flip the bus bar to solder the other half, but the jig worked great and the bus bars all came out really well!
 
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#13
I think you need more solder between the wire and the lug. You may not realize it, but constant changes in current flow will actually loosen connections, even bolted connections. With little solder, it could actually cause the solder to weaken and break causing a loose connection between the wire and the lug.

If you can, I would recommend crimping them before adding more solder, tho. This creates a tight hold on the wire, and reduces the amount of solder required to finish the connection.
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#14
Korishan - Thanks for the input.  I agree that it would have been better to crimp and then solder, but I tried a test and there wasn't enough of a socket on the connectors to fit the cable.  When I pressed on the ends, it would flare the twist and push the cable out of the socket.  So I went with flux to properly flow & fill the joint and made sure that there was ample solder in each joint.  It does look like some are a little light in the pictures, but I ensured a proper joint on each soldered connection.  I will keep an eye on the potential for loose connections if anything develops.

With the bus bars completed, it was time to assemble the 24 individual 20P packs.  I prepped each of the 10-battery holders by drilling two holes, centered on the packs to accept the 55mm socket head bolts that hold the opposing holders together.  Once again using the drilling jig on the drill press.
 

Then I needed to assemble the packs according to the catalog created by rePacKr.  I laid out all 490 cells and then picked the cells into the 20p packs.  Previously taking the time to label the cells made this go pretty quickly.  As I assembled the packs, I took the opportunity to sand the contacts of the batteries with 1000 grit sandpaper.  This helped down the road when soldering to the cells and giving the solder something to grab.  Here's the difference between a sanded and un-sanded cell


Now that all the prep work had been completed, I was left with (x24) 20P packs, all with an equal capacity of around 35Ah!


To aid in the final assembly, I took the opportunity to tin every cell before the bus bars got in the way.  This would help keep the contact time of the soldering iron to a minimum during final assembly.  I fluxed each contact and then briefly touched it with the 100w iron.


In order to attach the bus bars to the packs, I realized I had forgotten to drill the holes needed.  So I had to put the whole packs on the drill press to drill the small holes for the cable ties to secure the copper rails.  The ties pass from one side to the other and loop around the rails on either side.


Finally, with all the prep work on the packs finished, next we're on to final assembly!
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#15
Now came the time to complete the assembly on each of the 20p packs.  This entailed connecting all 20 cells together, and A LOT of soldering for me.

I started by building a jig to hold the packs in place for soldering.  Since the bus-bars protrude from the center of the packs, they tended to flip-flop and I needed them stable.  So i took more scraps of wood and glued two guide rails, with ledges for the edge of the pack to rest on.  This left the center open for the bottom rail to float in, while I worked on the other side.  It also was a way for me to clamp everything down to the table for a stable work surface.


To connect the cells to the bus bars, I'm using the resistor leg method for fuse wire.  It was economical, and seems to be a perfectly acceptable method, even if it doesn't offer the surest of fail-safe methods.  I think it's enough protection for me and these LiFePo4s.  Plus it worked out that the pre-cut legs were the perfect length needed to span cells.
I would prepare a batch of resistors by pulling them off a roll, and bending a small offset on the end


Then work down one side of the pack with a quick touch of the iron to attach the resistors


Next, cutting off the extra at the base of the resistor body


Then I flipped the pack around and applied a small dab of flux to the end of the wire before pushing it over the rail and soldering to the opposite cell


Then I flip the entire pack over and repeat the process with the remaining leg of the resistor on the opposite contact of the cells


To solder the fuse wire to the rails, I found a different iron tip helped achieve good flow of the solder and a solid connection to the rail, so I saved all the bus bar connections till the end.


I did a pick test of every solder connection point and then a electrical continuity test of every cell to ensure there was a solid mechanical AND electrical connection to the copper bus bar.

After all that soldering I had (x24) 20p packs ready to be assembled into their final 12V Battery forms!  (And a ton of left-over 1/4-watt resistor bodies, haha) 
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#16
With all the 35Ah parallel packs assembled, it was time to utilize the modular nature of this design and build up some 4S 12V batteries!

To physically attach the batteries together, I had originally designed some 3D printed end caps that would bolt together with connector plates.


But after holding the packs themselves, I came to realize that this would be overkill on the design and manufacturing required, while only providing a marginal improvement.  So I went with the simpler option which has turned out great.  I recently replaced the screen in a monitor and had to purchased a roll of the very thin, but very sticky double sided tape used to adhere the LCD panels to the bezels.  I used this tape on the frames of the packs as an initial bond.  This was surprisingly strong and could even hold the packs attached when picked up by the top.


But to really keep everything together I purchased some extra long stainless steel zip-ties to hold everything together.  They came with this great tool to really tighten the bands and make the packs feel very secured together


Now that I had them together, I had the length needed for the series connectors.  I cut up 56 sections of high-strand, silicon 12AWG power wire and soldered brass ring terminals to each end.


I then added these cables to the 3 connection points of each pack with a brass washer and a brass #8 x 1/4" screw.  I finally had the 4S20P batteries I had designed in my head and on paper over a year ago!
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