Johnny-- Make sure you have a HOT iron (40 watt min. 60-80 preferred and 800+ deg.) and a wide (1/4" or 6mm min.) chisel til. Hold the Iron on the end to be soldered and touch the solder about 2mm from the tip, f it doesn't melt w/o just from touching the heated cell then the iron is to cold. I also prefer to use very thin 60/40 solder. Thinner solder melts at a lower temp. Run solder onto teh cell until you have a nice pool of molten solder the bead should begin to lay down and that s when you know it is gonna stick, also you can scrape the ends of the cells beforehand to give the solder something to "bite" into. Once you have pretinned all of the ends of the cells allow them to cool for 15-30 min.
Now place your cells into a good battery jig. hold the batterybar on the cells to be soldered and place the tip of the iron on the bar above the "bead" of solder, once enough heat has been transferred the bar will "sink" into the solder. remove the tip ad allow the soldr to cool naturally, donot blow on it.... These are the steps to properly solder your new cells.....