Hey guys, I recently tried soldering my 18650s together, it worked for a while but now the solder is snapping off. So I was wondering if anyone would be willing to let me use/rent their spot welder. I’d even buy a used one if the price is right. Hell if I could send my batteries out to someone who would weld them I’d do it.
Sounds like you have cold solder joints. A good solder joint on a battery will bend or damage the battery significantly if you try to remove it - or the solder it’s self will break, leaving half still on the battery.
I did a lot of testing getting solder to stick to 18650 cells using dead laptop cells.
I found that a 80-100w soldering iron with rosin/flux core solder worked best. The little 30-40w soldering irons lose too much heat and don’t produce strong solder joints. You have to heat it long enough for the solder to “flow” across the batteries surface, I find about 10 seconds with a 100w gun?
I pre-tin the cells, then come back and solder the wire to it.
It’s better to solder to spot welded tabs if you have access to a spot welder. You still need to be quick when soldering to tabs since they will transmit plenty of heat into the cell if given the chance. Ideally you would solder fuse wire to a parallel buss bar connection just in case a cell decides to go thermal later in life. Using cell level fusing also allows the parallel packs to flex without breaking connections, I assume it helps with vibratory stress also.
On the subject of broken connections and vibration. It is a good idea to stabilize all wired connections with a natural cure silicone to keep the soldered connection from repeatedly bending or vibrating excessively. This stuff can be hard to find, if you need something found locally you can use Versachem Mega Grey. It is important that you do not use regular silicon since it can cause corrosion from the curing process.
To add to this, some of my cell fuses broke due to excessive vibration and rubbing against the deck. +1 on the recommendation to secure any fusing with a shock absorbent material like silicone. I also padded my pack with some quarter inch soft craft foam as well and it’s been holding up great!
I like to use 2 layers of fishpaper with adhesive backing. One under the bussbar and one on top. I fold about 1/8th of an inch over the edge of the cells to prevent the fuse wire from shorting out. The whole package is then shrink wrapped and siliconed in place to prevent shifting.
It is important that some air gap is left around the fuse wire
I’m buying a spot welder this morning. I jumped on here quick to see if there was any recommendations between the 709 in the 788. I don’t know if you got your issue resolved but I’ll have it in a couple days and I can show you how to solder so it sticks. I’m in East Bethel Mn.