Will this battery case be able to sustain 3.5 amps?
This should actually be a comment, but it's too long

Sorry, but we have no idea how tight your contacts will be a) in the beginning b) after an year and numerous knocks. I guess the metal pieces itself have resistances only milliohms, but loose contact can get hot even at 3,5A. The only way to be sure is to make a test. 

Note: if your battery pack gets some strong knocks your metal stripes can get bent and the contact can become loose. In addition solder can get broken. Mechanical strength under knocks and vibration should not be based on solder.