At our home we lose power quite frequently, no matter the season. After ending up with several hundred 18650 cells from my motorcycle build I figured this would be a great way to use them.
I figured going with 4s configuration would be the best, since as the cells lose voltage they can drop to the 12v minimum of the resources I would be running off of it. I grabbed a couple of waterproof tackle boxes, and different types of USB and 12v cigarette lighter connections.
Here is revision 1.0 and the cell layout for the second box. This was ok. But I can do better.
Here is 2.0. I changed the cell layout, and added the cig lighter plug. I charged 3 tablets, 2 cell phones, and ran 24 hour stress testing on my USB-C macbook from it and lost about .4 V on the storage.
So after discussions with a DIY board I frequented I decided that just using nickel spot welded to the cells was not enough protection. What if one of my recycled cells decides its done and fries itself into oblivion. That would be bad news. So I found a writeup about how someone used fuse wire to individually attach the cells on the + and – to a busbar and then serialize those busbars into the cell needed.
I had some Romex wire left from when I wired the attic up, so I stripped that down to bare copper and built my 18650 holders into an easy to follow shape.
I hot glued the bus bar to the cell holder, then soldered individual fuse wires to each cell
Each wire can only handle about 30 amps before burning itself off of the line, so any shorts should only remove themselves from the system.
I know a lot of people are screaming about the solder on the cells. I used a high lead content solder to reduce the amount of heating on the cells. And made sure to verify each cell before and after the soldering process.
So the small case I built has 32 Ah for a total of 472 Wh and the large case has 65 Ah for a total of 946 Wh. More than enough to keep our devices charged during any outages. I also have USB LED lights that I can run off of these boxes, and am picking up some USB fans to keep us cooler during hot summer outages.
The next project is building some DC UPSes for my wifi and modem so that if our house loses power, but the ISP still has juice the kids can still play games and watch shows until the power is back.