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Modular 12V Battery System
#1
Greetings!

Over the last few years, I've tested a few different battery setups.  It mainly has been using deep cycle SLA batteries with 12v inverters to power various camping trips and tailgates.  But after awhile I got tired of the weight and low weight/performance ratio of this type of setup and had been looking for alternatives.  After researching for the last year or so into DIY battery technology and designs, I think I've come up with something that should be the ultimate battery and power setup for my situation and needs.  It's also given me a great outlet to nerd out on various things to build and play with!

My Requirements:
  • Modular batteries that can be expanded upon depending on expected power needs
  • Portable and easy to deploy/pack up (but weight not really a concern)
  • Sufficient power to run a high demand (focusing more on duration and not power output) tailgate or camping trip
  • Serviceable and relatively easy to modify
What I've come up with is a series of 4S20P 26650 LiFePo4 Cell battery modules that will be housed in a portable case with an accompanying case for power distribution.

The 4S20P "Modules" Design






The 20P Individual Packs


These 4S20P "packs" will be capable of powering things on their own, with each one roughly 35Ah at 14.4v.  But, I plan to also group them in parallel into a larger setup shown below.  I have purchased enough supplies for 6 of these "Modules" to start with, which should all fit in the large housing.  All total, the battery system will be roughly 3kw in this first iteration.

They will be housed in Milwaukee Packout containers:


With the batteries in the larger, lower unit, and the distribution in the smaller, upper unit.  They will be connected through large gauge wires and Anderson Powerpole connections.  I'm still working on the power rail design for the battery container though.

Next post I'll detail all the battery cell testing and reconditioning I've done so far and am currently in the process of completing, but would love to start getting feedback on the community's thoughts about my plans!
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#2
It looks like a great idea.
See if you can keep the waterproofing and external connectors for stacking/usage at the same time.
External charge/load/capacity/voltage of the pack would be nice as well. Given that the used packs will be discharged at the same rate the indicators will only be needed at the main pack
Will you be able to charge 2-3 packs at the same time or individually only ?
I would recommand BMS that uses same leads for load and charge. it will simplify the build.
When designing it, think about standardized cable used to connect multiple modules, therefor you will need 2 set of connectors on each add-on modules.

My 2 cents.
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#3
(09-26-2018, 01:33 PM)marcin Wrote: It looks like a great idea.
See if you can keep the waterproofing and external connectors for stacking/usage at the same time.
External charge/load/capacity/voltage of the pack would be nice as well. Given that the used packs will be discharged at the same rate the indicators will only be needed at the main pack
Will you be able to charge 2-3 packs at the same time or individually only ?
I would recommand BMS that uses same leads for load and charge. it will simplify the build.
When designing it, think about standardized cable used to connect multiple modules, therefor you will need 2 set of connectors on each add-on modules.

My 2 cents.

Thanks for the feedback marcin, lots of good points.

The standardized cables will def. be incorporated and that's what the Anderson Powerpole connectors were planned to do. I know they're not waterproof, but this setup will not be ran in the rain, so I think the limited weather protection they provide will be sufficient. But there will be multiple ports in order to potentially daisy-chain more packs in the future.
The packs will most likely need to be removed and charged individually due to their size. I have built a 24v 500w charging setup to be able to charge at up to 20A, but with all 6 modules installed, that would still only be about .04A per cell, or roughly .02C. I think it would be much more time efficient to charge them at 20A per module.
Finally, I did look into a BMS that will charge and load on the same port, but due to the high amperage rating, it seemed that all of them use different ports for each function. It shouldn't be a big deal to use different ports on the modules, and it should help prevent some improper hookups if I use different connectors for different purposes.
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#4
After doing research, I knew LiFePo4 would be the better setup for me since I plan on utilizing a lot of 12v equipment.  So for this build I purchased 490 Fullriver 26650 LiFePo4 cells from eBay.  Before buying, I found the other threads detailing the capacity issues others have had with these cells, like THIS one by thumper338, and THIS ONE.  But I determined that the price savings compared to other options out there and the fact that I would be using a large number of these cells was a worthwhile trade off with performance.  I also knew that I would have to really test each and every cell in order to build my packs properly.

So while I'm still designing the final details of the modules and pack design, I did get started on cataloging and testing the cells about 2 months ago.  I built a testing board using 8 capacity meters and 8 charging spots.  It runs off an ATX power supply and breakout board.  It worked great and has allowed me to test almost 500 cells in about a month.  Here are some images:




And the underside:




I tracked the cells at initial test, first charge, two week rest voltage, recharge voltage, and finally capacity.  Here's what the spreadsheet looked like:

I can post a link to the GDoc if anyone wants to view it or use it.

In the end, the cells were around my expectations of half capacity.  They averaged around 1700mAh with some as high as 3400mAh and some as low as 850mAh.  I was pleasantly surprised to only find 1 bad cell that wouldn't hold a charge out the almost 500 tested.  I had ordered 490 knowing I would find some bad ones and still needed to have 480 cells total.  This meant I could ignore the lowest 9 tested and this meant all cells used will be at least 1100mAh.  And I do think that once the cells have been cycled a few times, their capacity should improve marginally, maybe 5-10%.

Next I'll show why I'm ditching the cardboard and dressing up the cells for the pack builds!
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#5
This is pretty much exactly what I'm doing for my first "big" battery pack. Although mine is slightly different being 18650s (4S80P) still with a 12V inverter but housed in Pelican case which will hopefully have not only power but various audio/video connections and contain a laptop so that my company can make use of them in the field as portable workstations for when power isn't available. The main problem I've been running into is that I had intended the entire case to act like a laptop battery where I could plug up a regular AC line to the case to not only charge the batteries but also power the outlets built into the side of the case. But should a power outage occur while plugged into the mains, the batteries would automatically kick in and the inverter would then take over to power those same outlets. There's a few ways to do it I think and I've found a few charging circuits on a small scale but I'm not sure what's the most practical or what's best for larger currents like I'm hoping to run.

Anywho, I fully plan on reading your thread so that I can steal any good ideas you might come up with. 

Oh one thing, the Dewalt Tough System with the detachable dolly (that's the key because there are some units with a built in dolly that aren't nearly as functional) I think may be something you want to look at. The advantage is that you can stack all the cases on top of each other and lock em like the Milwaukees BUT with the dolly I mentioned, you can actually put space between each stackable case so that you can open any one of the cases without having to unstack the entire tower. 

Should I run out of room in my pelican case, I'll prob upgrade to the Dewalts. Worth a look maybe.
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#6
(09-28-2018, 11:03 AM)OhmGrown Wrote: This is pretty much exactly what I'm doing for my first "big" battery pack. Although mine is slightly different being 18650s (4S80P) still with a 12V inverter but housed in Pelican case which will hopefully have not only power but various audio/video connections and contain a laptop so that my company can make use of them in the field as portable workstations for when power isn't available. The main problem I've been running into is that I had intended the entire case to act like a laptop battery where I could plug up a regular AC line to the case to not only charge the batteries but also power the outlets built into the side of the case. But should a power outage occur while plugged into the mains, the batteries would automatically kick in and the inverter would then take over to power those same outlets. There's a few ways to do it I think and I've found a few charging circuits on a small scale but I'm not sure what's the most practical or what's best for larger currents like I'm hoping to run.

Anywho, I fully plan on reading your thread so that I can steal any good ideas you might come up with. 

Oh one thing, the Dewalt Tough System with the detachable dolly (that's the key because there are some units with a built in dolly that aren't nearly as functional) I think may be something you want to look at. The advantage is that you can stack all the cases on top of each other and lock em like the Milwaukees BUT with the dolly I mentioned, you can actually put space between each stackable case so that you can open any one of the cases without having to unstack the entire tower. 

Should I run out of room in my pelican case, I'll prob upgrade to the Dewalts. Worth a look maybe.

That sounds like a great all-in-one solution that has a lot of parallels with this project, I'll be looking forward to seeing it come to life!

And great tip on the DeWalt system.  I did look at their case solution, but liked the Milwaukee packs a little better.  Plus, the battery case will be the base and hardly be opened on a regular basis.  The top case will house the inverter, solar charge controller, and distribution components and will get opened much more frequently.

While testing all these cells, I began playing with the configuration of the 20P packs.  I ordered 26650 plastic holders in a 2x5 configuration and will combine them into a 2x10 configuration.

The problem was that the cells in their original cardboard sleeve didn't fit into the holders.  The top could be snugged into place, but because the cardboard ended at the side of the cell, it would push against the lip of the holder and wouldn't let the cell fully seat.  So i soon realized I was going to need to rewrap all 490 cells in order to keep to the original pack design  Confused...  But bright side was that it would allow me to make this build look really professional and that made my inner engineer happy, haha.  So I progressed:

After cataloging all the cells, I created labels for each cell with it's cell testing number, factory serial number, and capacity.  These were designed on small residential return address labels, which turned out to be a perfect size.  Here's a test cell I did.




And here's it fitting nicely in the cell holders:



I ordered almost 25m of 26650 heatshrink off eBay and cut it up into 73mm sleeves.  490 times.  But it actually didn't take that long after I used a jig setup with a paper cutter.

Next I designed and 3D printed a holder for 4 cells at once.  It holds the cells up a little and will allow me to consistantly place the wrap at the correct location on the cell while a heatgun shrinks the wrap to the cell.  I numbered the posts, because once I unsleeved the cardboard I wouldn't be able to tell the cells apart until I replaced a label on the new shrink:



Now I'm just going through the LONG process of rewrapping all these cells.  Here's some progress shots:




I'm only about 100 cells down, and have struggled to find some more time to continue with GT Football season starting up, but hopefully will find some free time here soon to power through the rest!
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#7
Quick update!

I'm progressing through the cell wraps and have hit the 2/3rds mark with over 320 cells rewrapped!  Here's the progress from a few days ago when I'd hit the half-way point


As I've been rewrapping these cells, I keep thinking about the bus bars for each pack and a way to make them truly modular.  Instead of soldering on the brass rods across the packs after they're tied together for the series connections, I would like to have bus bars that can be tied together no matter what the arrangement.  This way if down the road I want to move to a 7S for 24v packs, it would be a lot easier to disassemble and then reassemble these "packs" into the new configuration.

So in that mind, here's a new bus-bar design I'm thinking of


It's made of 8 AWG solid copper wire with bare copper butt connectors at the intervals, like these


These would be soldered onto the wires, then crimped to create a flat spot where I could drill and tap a small M3 hole.  Then the connecting wires would have simple ring terminals on the ends that could be fastened to the bars with brass M3 screws.

Here's what the new batteries would look like


I know this avenue would be a bigger time investment into building these packs, but I think it would pay off down the road and stay true to the "spirit" of this modular build.

Feel free to give me some feedback or thoughts on this new approach!
Korishan likes this post
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#8
Nice. For the butt connectors, you would want to flatten then solder. Otherwise the solder could crack and not make good contact Wink
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#9
That really looks great - wish i had cad skills to flesh ideas out first ... well done
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#10
(10-23-2018, 10:33 PM)Korishan Wrote: Nice.  For the butt connectors, you would want to flatten then solder. Otherwise the solder could crack and not make good contact  Wink
Thanks! And great tip for the connectors.  I'm working on a jig to give a consistent flattening and profile for the connectors that should help too.

(10-23-2018, 10:51 PM)hbpowerwall Wrote: That really looks great - wish i had cad skills to flesh ideas out first ... well done
Thanks hbpowerwall!  I use SketchUp which is a great tool for hashing out ideas and is pretty user-friendly to pick up!

I appreciate the heavy-hitters of this forum weighing in on my build and the great tips you guys give.

Now for the new update:

After a month of finding an hour or two here... an hour or two there... I'm finished!  All 490 cells rewrapped and ready to start building into the packs!


I knew it would take extra time, but I def. did not think it would take roughly 24ish hours total to rewrap all these cells.  I'm glad I did it, but very glad it's complete.

Next up: This weekend I hope to test out building the bus bars using the new design.  The 8AWG butt connectors came in and I found that they have an opening of 5mm, which is much bigger than the 3.26mm diameter of 8AWG wire.  So in order to get more capacity built into these bus bars, even if just for a safety factor, I've been thinking I need to do a stranded bus bar instead of solid.  With a 5mm opening I can get either a 3-strand 12AWG braid, or a 7-strand 14 AWG braid:


I'm going to try the 7-strand first.  With 14AWG I estimate the capacity of each bus bar should be around 105A (15A per each strand).  That should be perfect for the 100A BMS board I'm planning on installing on these modules.  Again, I don't think I'll ever run these packs at that level individually, but it'll be nice to have that capacity and headroom should the need ever arise.  Here's what the new packs would look like:


One issue with these bigger capacity though, is that it will make the bus bars much bigger than expected and now wont fit the overall case and front plate size I previously designed:


So that's the next design challenge to hurdle...

While I know this is probably overkill and over-engineered for the use I will be subjecting these cells to, and is causing a lot of added time to this build, I think it's better to build in as much quality as possible upfront to save some future hassles.  We'll see if I still feel the same when it's all-said-and-done though haha  Tongue
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