Soldering packs
When soldering to batteries it is important to use a good paste flux as well as a hot iron.
I always tin the battery ends before soldering on the battery bars. After scuffing the battery terminals with sandpaper (or a rubberized abrasive which is what I use now), I use my finger to apply a small amount of flux to the cell, then tin it first. You then have to press down on the battery bar when soldering it on so it is contact with the cell. You really don't want any thickness of solder between the battery and the bar.