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singlecells powerwall
#1
Big Grin 
Hello all,
Thought I would start a build thread.  I have been working on collecting cells for a few months now. I currently have about 700 good cells, and looking to create a 14s100p system.
I am lucky enough to have access to lots of free used battery packs from medical equipment. The great thing about them is they have only been used 2-3 years and treated quite well, the downside is that most of the batteries are designed to only power units while being transferred between mains power.


A slightly dated photo of what I have collected sorted into 200mah boxes. The first two which come from my most readily available packs only come in at 1800-2000mah. But free is free I guess. If the current ratio of battery types remain the same I should get an average of 2000mah in each pack.
The yellow bucket is full of cells under 1800mah which I dont plan to use. I now know which battery packs contain these and dont even bother pulling them apart any more. Maybe Ill use them in some smaller projects in the future.
Also collecting a few lifepo4 cells to create a 12v battery pack for camping which are those green 26650 cells.


View of my charging and testing setup. Feel free to ignore the janky wiring of the chargers. I was too excited to build it one afternoon and couldn't wait to go out and buy materials to make something better!!  

I am now starting to look into what solar/inverter/charger setup to get. I am torn between getting a cheap all in one system for an off grid setup like something from MPP Solar, or to get a hybrid inverter that could feed the grid once the battery is charged. The money you get from feeding the grid is pretty low for new installs now, so I doubt it would be worth it, I just can't deal with the thought of that wasted solar energy....
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#2
Each time i see a new post of a new build - i think to my self the community has a new baby lol Welcome to the community buddy
Korishan and BlueSwordM like this post
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#3
So things have changed around a bit over the period of collecting cells. I have moved back into a unit I own which I won't be able to install solar in. So I have decided to use some of my currently collected cells to create a 7s pack to use for camping and touring with a 24v to 12v dc-dc. I'm not sure of the capacity ill get, probably use 60 or 80 cells per pack but it's going to be enough to run everything for days on a camping trip before even adding in the recharging from the solar. Thinking ill use a mpt-7120a to charge the pack using either panels or my power supply as the input. I'll repurpose them all into a larger system once the opportunity arises.
When it comes to building the packs though, should the cells just be sorted into 50mah piles then evenly divided up or is it necessary to have some software perfectly sort them out?

I also built a super janky 4s20p lifep04 pack as I do get access to some of them from time to time. It was just a quick fun project and I didn't bother too much with checking the capacity of the cells as I didn't have the gear/time. I did run a light on it for a while and got 45ah which isn't too bad from what the packs listed as their capacity at when new. One pack charges up well before the others so it definitely could have been done better. But still, the goal was just to build and use it for fun.
jdeadman likes this post
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#4
Too bad you can't put up solar.
The more you want to maximize the usable energy, the more you have to take care to match the capacity of each pack.

eg.:
7s100p cycling between 4.2V ~ 3.0V. The pack capacities need to exactly the same, or the weakest cell will enter dangerous sub 3.0V territory (at which point the BMS should trip).
7s100p cycling between 4.2V ~ 3.5V. You can pretty much use a random selection of cells. The weakest pack might go down to 3.2V - so what.

Most powerwall people seem to use the 100mAh buckets approach.
Some use the Repackr tool ( https://secondlifestorage.com/repackr.php ) to get theoretically perfect packs.
I use random cells for my 100p packs, then afterwards swap a cell of the weakest(/strongest) few packs with a stronger(/weaker) cell.
Modular PowerShelf using 3D printed packs.  30kWh and growing.
https://secondlifestorage.com/t-AJW22-s-...PowerShelf
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