For building batteries the bigger the tip the better. I swear by Weller irons, made in the good ol USA and WAY better then any hako I've used. With my weller I only have to touch the iron for a second and it's hot. With the hako in my experience it put too much heat in the cell trying to solder properly.

Do you have a local Fry's Electronics? They carry Weller soldering stations and all the tips.