i have always used a 60w weller soldering station (currently known as WTCPT) the tip is magnetic to control temp so you buy a 6 7 or 800F temp tip. i use an modified deans battery jig and deans solder.

i am using a 800F 3/16" tip but i think used to have a 1/4" one for batteries.
the solder ball on the cell gets flattened quickly when i heat the bar on top, however the solder just doesn't want to flow out from the bottom of the bar.
i cleaned the surface but did not scuff it. i never used to use flux just what was in the solder (deans)

i will try scuffing and flux. maybe the temp is high enough but the solder won't flow without help.

thanks again for the replies.