I procrastinated many weeks about whether or not to solder 18650 batteries. I tried many combinations (solder, wire and flux / acid) to find the quickest method to solder these cells together. I’m pretty proud of the outcome (less than one second contact time and to tear the connection apart you need really significant forces and the battery can gives up millimeter or two when applying a huge pull)
A quality 80w soldering iron
Lötwasser (soldering acid)
Stranded copper wire, I took mine apart from an audio cable
The actual cells
What I couldn’t demonstrate in the video is how I applied pressure to the joint with a wooden block to really maximize the surface area of the contact. Please enjoy the short video!
This is a demonstration on how flexible this kind of installation is. Leaded solder can also withstand the huge forces on a crossbow’s bow’s tip… Torsional forces are probably something solder can’t withstand but I don’t see how the contacts would come loose from the 18650’s.
And yes I’m aware of the fact spot welding is a bit safer but I find it interesting to really understand the real differences between spot welding and soldering if done right. Thanks for the inputs guys!
The damage that can be done to the electrolyte by the heat, even for a couple seconds of contact, can mess with the properties of the cell. It can change the internal resistance of each cell, leading to (best case scenario) a pack that drifts more than usual. Additionally, it is risky in terms of electrical connection and may be a critical point of failure. If the contact is compromised, may result in arcing, then heat, and possibly fire. I would never attempt to build a pack without a spot welder.
@PatRocks Is the small difference in internal resistance how relevant as some people have even successfully built well-functioning packs with different cells from different manufacturers. Correct me if I’m wrong - parallel packs (1s6p) difference in cell resistance won’t matter so much as the parallel cells are always forming one pack with precisely the same voltage.
And my intention is to never assemble a BMS. I’ll always balance charge the pack and monitor it externally and I’ll set the low voltage cutoffs etc to the VESC and in addition I’ll use low voltage alarm buzzers.
@FinnishSnowmobiler I’m on your team! Most of my packs have felt the heat of a soldering iron. Skinny strips of nickel just ain’t enough!
I look at my A123 26650, and look at the .20 nickel, and I just can’t bear to do do it. 3 packs built with solder, no problemo. I really think soldering is the correct way to go with low p count packs built from high power cells.