You need to move to at least a 40 Weller or Hakker fixed iron. I keep one of these to just do batteries with. Big chisel tip. $20 - $30. If you get too hot of an iron the solder evaporates and you end up trying to tranfer heat with a dry tip.
On that note I use a Hakker Station 936 for my regular duty stuff.
Scuff the ends of your cell. Positive and negative are different materials and are different thicknesses.
Tin the ends lightly with no clean 60/40 or no clean silver solder 5%.
Take your bar and scuff the mounting surfaces. Apply a drop of flux to both solder tinned areas and set your bar on with pinted pliers.
WET the tip of your iron to transfer maximum heat and apply the heat to the negative end first and the flux will fuse the bar as it boils out. This is a very fast process. The reson you do negative first is because it is the lowest point between two cells side by side and the thicker material will be better if you have to reheat the battery bar.
WET the tip again and do your positive side.
When you get good the whole thing will take you less than 20 secs with an applied heat time of less than 5 secs.
Hope this helps.