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Wiring Plans For Modular System
Hello everyone!  

So I am in the process of planning out a modular powerwall system.  I would like to submit the following wiring plans for critique by this group.

Cost effectiveness is not a major concern for me.  I don’t mind over-sizing components to make things easier, safer, or more attractive.  I am doing this as a hobby project.

Max peak planned current draw from each cell will be 1 Amp.  Current Draw under normal circumstances will be about 250-500mA from each cell
The modules will each hold twelve 18650’s in a parallel group.  I will have a 3A fast-blow glass fuse on the positive terminal of every cell.  I plan to use stranded 10-gauge wire (insulation mostly stripped off) as the positive buss wire and negative buss wire.  These two buss wires will terminate at an XT-90 connector.  Important to note that these modules are constructed of 3D printed cell holders using leaf-spring connectors.  Fuse leads and the smaller connecting wires solder on the back of the leaf-spring connectors and to the buss wires.

My thought is that the 10-Gauge buss wires will be over-sized for the planned current draw.  They should also be large enough to allow for easy soldering vs smaller wire sizes.  The XT-90 Connectors should easily accept 10-gauge wire.   Smaller XT-60 connectors probably do not.

Here is a simplified drawing of the proposed module wiring:

The modules will be interconnected in parallel and series using 10-Gauge wire.  I think that I can just keep adding the 10-Gauge wire from each module into the bundle so that the total wire diameter keeps increasing as each additional module joins a parallel group.  The stranded 10-Gauge wires will be stripped of insulation where they run together so that they are twisted and wrapped with the other wires.  The following drawing shows an example using parallel groups of 4 modules in a series of 3.  You should be able to see how the buss wire grows larger where the current is expected to be higher.

These are greatly simplified examples of the project.  But, I believe the same model and electrical principles would apply with a larger group of parallel modules, and a larger group of series connected groups.  I just keep adding the 10-Gauge wires to the bundle with every additional parallel module.

The actual scope of this project, or physical space I have for the project, calls for an 8S configuration.  12-cell modules can be 18 to each parallel group.  So the end result would be 216P,8S (1728 cells).  I can simply add the modules 8 at a time to that maximum space capacity.

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If cost effectiveness is not a major concern, why use such small format cells ?
(08-17-2019, 09:58 PM)Sean Wrote: If cost effectiveness is not a major concern, why use such small format cells ?

Excellent point. I am harvesting the cells from laptop batteries and similar sources. So they are basically free except for my time. If I were to purchase new cells I probably would have gone with something like 26650's. Also free will be a 3/4 height network/server enclosure to enclose all of the modules. That is why I have a set space limitation.
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Some potential issues I see with your design:
* Large number of small packs probably looks messy and leads to a lot of wasted space inside the enclosure
* A lot of work/plastic/wire/parts in general vs relatively few cells.  There are good reasons why most people go with 7s60p ~ 14s200p
* Bundling up to 18x  10 gauge wires is easier said than done

10 gauge wire indeed does not fit into an xt60.  But you can make it fit by cutting a few strands at the tip.  Looks fine when filled with solder.

Modular PowerShelf using 3D printed packs.  50kWh and growing.

Thanks for the images of the XT-60 technique. I may try that instead. I have not purchased any connectors yet, so Ill try the XT-60's first and see how it goes.

I am considering your other points also. I know you are correct. I'm trying to figure out if the extra work/parts is worth it to me. I do enjoy a good project and challenge.

I will post more of the design soon. I think it will look neat and tidy if I put enough effort into the physical layout. And, even though I am talking about 144 modules max, I will just add them 8 at a time. I currently have about 500 tested good batteries. I probably have about 40 laptop packs I have not opened yet. I also get a moderate steady stream of them. I think it would realistically take me 2 years to actually fill the wall with all of the modules.

One other thing is the 3D printing time. Ill only be able to pop out a couple of modules and brackets per week.
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As you probably know, I also use 3d printed parts for my powerwall. My initial plans actually also included PCB boards with spring loaded contacts, but decided against using those for the reasons I already mentioned.
Another reason was that it would obscure the ends of the cells, making visual inspections difficult. As it turns out, visual inspection is very important when one uses very old ebike batteries that crap out way quicker than laptop or medical pack cells. I think people often keep using them until the ebike battery dies, whereas the other ones are retired much earlier.
Also, in the highly unlikely case of a cell spewing fire, I thought it'd be better if there were at least a few cm of free space for the gas to disperse and dissipate (wishful thinking?) before hitting something that could melt or even catch fire.

I'd love to see your 12-cell holder design. You're going to arrange the parallel holders like books on a bookshelf (ie the cells standing upright)? And the 8 shelves are connected in series? Are you going to 3d print some sort of slots / spacers?
One annoyance I'm facing are the cables/connectors that get a bit in the way when taking out a pack for inspection. You're going to have 8x times as many.

Some tips to speed up 3D printing time:
Use 0.5mm nozzles and 0.33mm layer height, even if you have to slow down the print to make it work. Soooo much faster than the typical 0.4mm / 0.2mm setup.
Provided you can let the printer run unattended, try to print multiple parts at once. Reduces down time when you can't set up the next print. For smaller parts, I've done up to 32 pieces at once!
Modular PowerShelf using 3D printed packs.  50kWh and growing.

Ill take some pictures of the module design soon. Right now I have a rough prototype that I have not taken the time to clean up. It is not yet something that I am proud enough to show off. PETG is stringy.

Basically the module is made up of two 6x cell sleds. They are in a triangular configuration back to back. So, the cells are not quite vertical. They are leaning back slightly. There are spacers between the sleds that facilitate buss wire tie down and provide access to solder points on the top and the bottom. Negative connections are on top with the narrow space. Positive connections are on the bottom with the wider space. This gives me more room on the bottom for the fuses.

There is a separate mounting bracket that actually screws to the wall. The module simply slides onto the bracket and locks into position. The module interconnect buss wires will actually be behind the wall (hidden but fully accessible). Those interconnect wires run through the wall and mounting bracket to the front of the module where the XT connection is made.

So basically you load up a module with cells, slide it onto an empty mounting bracket, and connect the XT connector.. If you need to remove a module you just unhook the XT connector, and remove the module from the mounting bracket. The cells also pop in and pop out of the module because they are not soldered into place.

I will pre-wire all 144 of the mounting brackets with module interconnect cables. So they will all be sitting there waiting for me to add modules as I print them and bring newly tested cells online. I will print all of the module brackets first, and just enough of the actual modules to get me started.

All of this will be mounted inside of a network/server enclosure. I am going to build "walls" on the left side, and right side, that run from front to back of the enclosure. So when you look inside of the enclosure you will see 72 mounting brackets on the left, and 72 mounting brackets on the right. The inside of the cabinet is where you load and unload modules. If you remove the side panels of the enclosure, you will see the backs of the "walls" on the left and right sides. That is where all of the module interconnect wires will be run and hidden.

Basically I am turning a network/server enclosure the size of a refrigerator into a giant battery. It is all metal with ventilation on the top, bottom, front, and back. Casters on the bottom so it can roll. It locks with keys also. I have not figured out what I want to build the walls out of yet. At first I though plywood, but I would rather find a material that does not burn as easily. Maybe something like those plastic porch/deck boards??? Not sure.
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Dont forget you'll need to manage the cell voltage every time you parallel connect it to an existing such that the voltage is exactly the same - you can't simply connect a full cell (or packs of cells) in parallel with an empty one.
Korishan likes this post
(08-18-2019, 10:29 AM)Sean Wrote: Dont forget you'll need to manage the cell voltage every time you parallel connect it to an existing such that the voltage is exactly the same - you can't simply connect a full cell (or packs of cells) in parallel with an empty one.

Yeah that would probably pop cell fuses. I would not want to do that.

Ill have this sitting next to a workbanch. I can make sure that I have charging hardware there to bring individual cells, or full modules up to charge matching the others.
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I have been working through some technical difficulties with soldering the prototype.  Basically I managed to start melting the plastic under one of the leaf springs because I got it too hot.  I need to practice a bit.  I may just twist the copper tightly in the little loop on the back of the leaf spring and then just put a tiny dab of solder.

The concept is a mount on the wall (with accessible rear side).  The cables go through the mount to the back of the wall.  The module slides onto the mount and is secured by some tabs on the mount.  Connecting the XT connector will also secure it in place.

Here is a rendering of the mount.

Here are 2 renderings of the module.  It is made up of 2 6x 18650 sleds with spacers in between them.  The spacers will hold the buss wires in place.  Negative on top and positive on the bottom:

Here are 2 renderings of the module sitting on the mount:

Printed Protype:

Leaf-Springs snap into place very securely.  You have to pry them out with a flat-head screwdriver to get them off:

Buss wire mounted:

I will post more as I get time.  I will work on this as I get time after work and stuff.
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