One false move, one battery misaligned, one spot-weld gone wrong and you will be sitting in front of a literal bomb. So, please don’t try this at home. Practice for years with two 18650. Only then can you move on to three 18650s. No joke.
Here is a fun slo-mo video of an 18650 exploding. (Again, there are many ways they can explode, this is just one.)
Mechanical engineer Adam Bender has put together a detailed guide on how to create a lithium-ion battery pack using a series of 18650 cells and some clever engineering.
“I’ll walk through step by step how I build a 48 cell lithium battery pack out of 18650 cells. I’ll talk about the mechanical structure, how I welded the cells together, setting up the battery management system, and the push button on-off switch.”
His build is pretty straightforward and begins by arranging the Samsung 30Q 18650 cells into a series of plastic cell holders and coupling them together to form both parallel and serial groups. The top and bottom of the cell holders also act as a handy guide for cutting nickel-based busbars for the electrical connections. These, in turn, are then cut to size, and spot-welded (over soldering) to minimize heat distribution, which can lessen a lithium-ion cells life.
With the welding complete (on both top and bottom busbars), copper output wires are then soldered to the busbars at each end of the battery build, which is done before being spot-welded to the battery. To protect the battery pack, Bender incorporated a BMS (Battery Management System) to protect the battery when charging/discharging, as lithium-ion batteries tend to get cranky when drained below 3V or charged over 4.2V.
At this point, the battery block is zip-tied together at specific points to keep the wiring neat and to provide some mechanical support. The battery pack is then encased in heat shrink plastic for a final layer of protection, and to guard against foreign objects that could cause a short. Those interested in replicating the lithium-ion battery, Bender provides a detailed walkthrough of his project in a two-part series that can be found at the link provided at the top.