Ok,
Yea, get a smaller Iron. You may consider making a PCboard, just put holes in it below the battery terminal and fill with solder, acts as a mount too, see here.
Ok so the “Battery PAK” is the issue, are you saying you put 3 battery’s together?
Let’s see that art work. I would bet you exceeded the max on the PMIC if so.
Did you top or bottom balance the batteries. , Do they power the whole system?
HTH
GL 
PJ 