[Here's a re-post of a 2021 topic @ my workbench, so as to have the content linkable for non-members]
May 13th, 2021, 03:19 PM
I took a break from 'tesla coil stuff' to finish a stale project from last autumn - a backup lithium-ion  battery bank, housed in a military surplus 50-cal Ammo Crate (steel, waterproof, fireproof). Here, I want to describe the build thoroughly enough that one might get a good idea of how to reproduce it (and to help cement my own understanding of the hardware as I summarize everything).

For this project, I chose a hardware setup featuring modular/stackable PCB's which contain the batteries and battery management system (BMS) chips - 7 cells in series (7s), for nominal "24 Volt" battery bank.

To house it all, I opted for the 'Ammo Can' setup, with 70 lithium ion cells (10x 7S pcb's). Generally, you can use these same PCB's to make a "DIY PowerWall" as big or as small as you want, if you keep stacking 7s Modules with enough BMS's mixed in. Wiring is pretty clean, mostly just ribbon cables and brass standoffs serving as buss bars. Cool stuff!

I'll be referencing some open-source PCB designs, which are NOT my own original PCB designs; the inspiration and hardware for this project all came from jag35.com (jehu garcia on YouTube), see included links. I appreciate that he sells the kits, yet also allows open-source access to his designs for those interested. I have attached his gerber files (free from his website) for the PCB's (battery holders and BMS, with parts lists in links below).

Here's link to page where I purchased mine from, which contains further links to parts etc (I'll put in a screenshot pic in case link goes bad).
https://jag35.com/products/18650-battery-module-diy-pcb-kit <---- these are the ones I purchased, good description
https://jag35.com/products/24v-ammo-can-battery-kit <--- I didn't use this exact kit, but it has a good parts list

Here's the walkthrough vid I roughly followed:

https://www.youtube.com/watch?v=bAB5A_ZR5M0

*The original plan was for this 24v (nominal) battery bank to be part of a 'solar generator', connected with 24V A/C Inverter & two 100W flexible solar panels, hanging inside a window inside my 2nd-floor apartment in Austin, Texas. The whole 'solar panels in window' part didn't work out, but the battery works great! With the AC inverter, I have working power outlets for outdoor use or in case of emergency.

May 14th, 2021, 09:31 AM
First step was to source the lithium ion 18650 cells. You can buy them new for a couple bucks per cell, or you can harvest/recycle barely used Samsung/LG/Sony cells from old Modem battery packs, for about 50 cents each as of this writing (although breaking them apart can be time consuming).

So in my case, the modem battery packs were dismantled and the top/bottom terminals of the loose Li-Ion cells were all filed smooth. The 'best practice' would have been to measure the capacity of each cell with a discharge test, but I just sorted the cells by make/model and checked all the voltages. The goal is that each 7S PCB contains 7 closely matched cells (brand, capacity, voltage).

I had enough populated PCB's and li-ion cells for a total of 70 li-ion cells in my 24v/7s Ammo Crate build. Once installed, I fully charged all the 7s PCB's to 29.4 volts.

Pics show the batteries I sourced from modem battery packs, installed into PCB battery holders.

September 12th, 2021, 03:42 AM
Next, I assembled the PCB's into 3 stacks of 5, to fit in the ammo crate. Two of the stacks are all battery-filled PCB's, and the third stack has the BMS in the middle of some blank/unpopulated PCB's (to give structural stability to the hex standoffs).

XT90 pigtails were soldered to the appropriate PCB's of each stack, to interconnect the 3 stacks.

To prepare the ammo crate, I coated it with gray primer/paint, and marked where the holes would need to be drilled by placing my PCB stacks in the box and determining 'empirically'. These holes are for M4 screws. Then I added bigger holes in the top lid and front panel, for XT90 solar connections and an LCD meter & binding posts.

September 12th, 2021, 05:02 AM
Finally, to wire it all up. Each of the PCB's is connected with 16-pin IDC Cable/Connectors, which I made using the parts shown in attached pic.

I also wanted to add a BatteryGo BG-8S module to monitor the voltage of the cells and to balance the cells as needed. This unit has an 8-pin output, but by referring to the BG-8S pinout, I was able to connect it to 16-pin IDC cable by clipping off every other conductor, and crimping on JST Connector pins to the remaining conductors. Then I inserted these into two JST 4-pin plugs, which fit perfectly into the BG-8S.

Now, the BG-8S is able to extend to the outside of the ammo crate, for monitoring the cells without opening the case.

September 12th, 2021, 05:26 AM
Here's some pics of the final assembly, and connected with the 24V AC Inverter. The last pic shows how I had originally hoped to use it, with flexible solar panels hanging in window.

However, unlike solar-powered calculators from school days, I learned that REAL solar panels need full direct sunlight to put out any amps. The couple hours daily of partial sunlight I get proved insufficient for me to draw any current, so unfortunately they could not charge the battery at all when hanging in the window. But when tested outdoors in full sunlight, it worked great.

It works great outside as mentioned - but don't take my word for it! Here's some "live action" shots, since I've tested a variety of solar panels & solar charge controllers with it over the last couple years.