Hey! It's one of the Alex guys!

Hi Bob, I used to wire up my own packs all the time. Like ken said, you don't want to hold the heat there too long. The trick is to use a soldering GUN instead of an iron. If the battery doesn't have tabs, clean the contact good and sand it to get down to really clean metal. Add a little flux, then get the gun hot and melt some solder on the tip. Touch the tip to the end of the battery just long enough for the solder to get a good grip, then remove the iron.

Do this to all of the cells on both ends. "Tin" the ends of all your wires before attaching them to the batteries - That will help keep heating time to a minimum.

When everything is tinned, place a wire to a cell, and add the gun just till they meld together.