the way i do it is
scrach up the battery ends with a key or something to make sure the solder sticks. then put some solder onto it.
then you put some solder on the battery bar
solder one sidedown while the other side is tilted. then push on the tilted side right away
i know what your talking about but i dont think anyone has had trouble with this. i usually look for a 60w soldering iron.
when i fisrt started i had a 30
once the solder is in a big chunk i cant get it to melt
i hope this helps