I pre-tin the back of the battery bar that will be in contact with the cell and then apply flux to the cell positive or negative of the cell. I then drop the bar onto the cell and heat the bar from the top so it re-flows the solder underneath and solders to the cell surface. I also 'inject' some solder at 45 degrees whilst i am doing this.
I big hot iron is the trick. Cells HATE being soldered like this and i have to hit my head against the wall each time i do it because we WELD millions per year at work and i have written technical manuals saying NEVER SOLDER TO CELLS. The problem is that the heat does three nasty things:
(1) when soldering the negative, it can melt the seperator material between the anode and cathode
(2) when soldering the positive, it can effect the properties of the rubber slug that forms part of the vent mechanism which can lead to the cell not venting at the correct pressure
(3) also on the positive, it can melt the insulation ring that sits around the top cover and keeps the positive button insulated from the negative can.
but if you want to win races you have to entertain the devil... (or something like that)....
Seriously, a copper, braid welded joint between two cells would be just as good as some fancy gold plated battery bar.. but none of us have £10,000 cap discharge, water cooled cell welders in our homes so we are stuck with soldering....
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