open-source-energy.org

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.

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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.

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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.

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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.

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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.

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I am revisiting this project, as a friend has requested one - in order to add solar / battery powered DC fans (i.e big PC case fans) near the vents in his attic, to help evacuate hot air out of the upper vents.

This one is also in a 50-cal ammo crate, but it only has TWO stacks of 5 PCB's (a total of nine assembled PCB's with each containing 7S 18650 Li-Ion cells (mostly LG 2200maH cells). The same website mentioned above (from 2021) - 'BatteryHookups.com' - still sells the used modem battery packs for harvesting like-new brand-name lithium cells - "don't get ripped off, get hooked up!", w0ot.

If it were full to capacity (i.e. the crate fits 3 stacks of 5 of these PCB's), it would contain 98 Li-Ion '18650' size cells. Each cell has capacity of  about 9Wh, which would be nearly 1 kilowatt-hour total (which is the number mentioned in the title of this post). However, for this latest project, the ammo crate only has TWO stacks of 5 PCB's: nine PCB's are 7S 18650 cell holders, and one PCB supports a 7S 40A BMS (batt mgmt sys) module.  The remaining space is for a solar charge controller, so that everything except the solar panels will be secured within the box; capacity is ~ 2/3 kWh.

This time instead of ordering the populated PCB assemblies from JAG35.com, I used his free PCB 'gerber file' and ordered the PCB's, cell holders, SMD fuses, and 16-pin IDC connectors - and assembled them all myself.



Also I did the same with the BMS PCB, except I just ordered a single unpopulated one from the website (instead of ordering multiple PCB's directly from a a PCB manufacturer) and the matching 'Daly' 14S 40A BMS.



Anyway, here's some pics showing my build, hopefully improving upon the experience I gained building the first one. The first BMS PCB had 3 small (20A?) BMS modules connected in parallel - but this newer one was a PCB 'gerber file' from the same designer (Jehu Garcia, JAG35.com) which accommodates one of the commonly available red 'Daly' 7S BMS modules (an easier option to source, if assembling it oneself) - see side-by-side comparison pic.

Again, I love this format (with  7S 18650 cells in spring-loaded cell holders mounted on a PCB with all wiring and connections made internally, as opposed to spot-welded bus bars), which can be scaled up indefinitely so that the capacity can be upgraded repeatedly over time. Also, this time I used blue PVC heatshrink (160mm) to hold the cells in place and insulate each PCB assy, which I think looks great!

nice