For the past year or so, I have been collecting laptop batteries and processing and sorting the 18650 cells inside. My laptop is getting old now, with a 2dn gen i7, it eats power, so I needed something to charge it on the go, though carrying this battery around is definitely not ideal. Now that I've made it, I also use it a ton to power my soldering iron, a hakko T12 clone kit from Aliexpress. I rarely use the computer power supply on my workbench anymore, and just use this 4S 10P battery.
Selecting the Cells
All the cells I used in this battery have been tested in my 76 cell charging and testing station. This was the first pack I made, so I used red Sanyo cells in the 1900-2000mah range to save the better cells for other projects coming up - I'm thinking an e-bike and small powerwall or portable power station. This pack is 4S 10P, 40 cells in total.
Making and Adding Bus Bars
The bus bars for this pack are made from 4 pieces of 20AWG wire from old Christmas lights, twisted together with a clamp and a cordless drill. I made three rectangles to connect the cells in series, and two straight bus bars for the positive and negative connections.
Tinning the Cells
After putting all the batteries into 4 4x5 cell holders, 2 on top and 2 on bottom, I used a flux pen to add flux to all the cells. Soldering to 18650 cells is perfectly fine, as long is it is done quickly. Don't hold the soldering iron on the cells for more than 2 or 3 seconds. I use a 60W Nexxtech soldering iron. It takes almost 10 minutes to heat up, but it works great. Just add a small dot of solder on both end of each cell.
Fusing the Cells
I used 2A Glass Axial fuses on all the positive ends of the cells to connect to the bus bars. Given that these are not amazing cells, 2A each might be pushing them, but 1A fuses would be enough. I need this battery to be able to provide over 200W continuous, so using 1A fuses would not have been suitable. For the positive ends of the cells, I used a fuse to connect them to the bus bars, and on the negative end, I used resistor legs.
Connecting it all together and adding balancing wires
This pack can output a maximum of 20A continuous, so an XT60 can handle the current easily. Positive to positive and negative to negative connected with 16AWG wire and some 3mm heat shrink, which according to this chart, can handle 20amps, and with only about 1% voltage drop, which is perfectly acceptable. I did not have a 5 pin JST connector on hand for the balance connector, so I used a regular female pin header cut to 5 pins. It has the same pitch, so it is perfectly compatible, but it can be plugged in backwards, which can be dangerous - direct short circuit. I used 24AWG stranded wire for the balance cables and 1.5mm black and red heat shrink to label the positive and negative ends.
Cleaning it up and making it look good
I hot glued all the power and balance cables to the battery, leaving them as long as possible, but still securing every one. 2 pieces of plywood were cut slightly bigger than battery to protect the connections. Pieces of 5mm MDF were used as standoffs between the battery and the plywood so that there is no direct pressure on the connections to the fuses and bus bars. I sealed all the edges of the plywood (or chip board) with duct tape so that the edges do not fray or break in transport. I added my logo to the top of the plywood by printing it out mirrored on a sheet of sticky labels with the labels peeled off. The printer must be an inkjet printer, a laser printer will not work for this, as the ink (not toner) will not be absorbed into the label sticker backing, and will come off easily when pressed against the plywood. The writing did not come out as I had hoped, but that is due to the inconsistencies in the chip board. I finally sealed the ink in with a quick coat of clear acrylic spray paint.