When soldering battery bars, first of all, place a small amount of foam around the outside of the cells that are being soldered, and clamp it in a vice. Then take your battery bar, and using at least a 75 watt solderer and some relatively hard soldier (e.g. silver solder), tin the bar.
Then tin your cell, but make sure you only keep the solderer on the cells for less than 5 seconds, otherwise you could damage the cells. then press the battery bar onto the cells, and apply the solderer and a little solder to each end of the bar. Hold the battery bar still with a pair of pliers. This should result in a solidstrong connection with little resistance.
It works well, so give it a go!!