DIY soldered li-ion pack

The goal of this project is to make a battery for charging my drone LiPo packs. It should be as cheap as possible and preferably lightweight.

I decided to go with used 18650 li-ion cells, because they are really cheap and have good energy density. I started by buying 50 used Samsung INR18650-29E cells for about 35 USD. The seller tested the capacity and internal resistance of each one, which saved me a lot of work.

My battery charger has 200W of power and maximum current of 10A. This means that to get the maximum power, you need to supply at least 20V. For this reason I decided to build a 6s battery. 50 cells would allow me to build 6s8p, but I chose 6s7p to have some cells left as a backup. The resulting capacity of the battery is around 26Ah or 420Wh.

I don't have a spot welder to join the cells together, so I decided to solder them. The key to safe battery soldering is good surface preparation and large heat transfer. To prepare the battery surface, I am using a flux specifically made for nickel. High heat transfer requires high contact area between the heating element and the li-ion cell. For this reason I am using a 5mm bevel tip (C245-C5), high power soldering iron and temperature of 400°C. With this setup I can solder the cell in under 2 seconds, which means that the cell doesn't get too hot.

I made every bus bar out of a single bent copper wire. This allows me to solder each side of cell only once, reducing the risks and work. I came up with this pattern: I 3D printed a jig to help me bend these wires:

I placed the cells in 3D printed grid to hold them together. Soldering them with my method was quite easy. The starting and ending bus bar are made from 4 strands of this wire to handle the high current better:

I decided to not use a BMS and just add a balance lead to reduce the cost even further. The main lead uses XT60 and 14AWG silicon wires:

In the end I printed a case to protect the battery and secure the leads with zip-ties:

In the end the battery weighs around 2.5kg, costs around 40 USD and I can charge 12 of my 6s LiPo packs with it.