guys i know how to assemble cells but i want to know if there is a secret to making them better, like what kind of tip should i get? and i already have a 60wt soldering iron
A big chisel tip seems to work the best. The prep is the key to getting a good solder joint. Use some sandpaper or a Scotchbrite pad to lightly scuff both ends of the cells, and clean them with motor spray or alcohol (to remove and grease/oil). The hotter the iron, the better. I pre-tin each end of every cell, let the cells cool, then use a battery jig to hold the battery bar in place. You don't want to heat the cell with the iron any more than necessary. Pre-tinning the cells helps here. If you do it right, it should only take 1-2 seconds to get a good solder connection.
I just use the 60/40 rosin core solder from Radio Shack. Also, keep the tip clean, and lightly wipe the tip on a wet sponge to keep it that way. The tip cleaner that Radio Shack sells seems to make the tip last longer if you clean the tip with it, then immediately unplug the iron.