hi
for electrical connections use rosin core solder and for mechanical non electrical connections the best is silver solder. this takes a special flux. heat the joint and not the solder, let the joint melt the solder so it flows smoothly over joint.
on battery pack again rosin solder and I cut thin brass strips and solder to batteries to construct a pack. the best method here is to apply solder <tin> the top and bottom of battery and do the same to brass strips <a small spot of solder on each end> then the can attach strip to batteries quickly avoiding over heating batteries.
good luck
joe