I've just purchased a total of 30 A123 cells direct from China. These were ordered without tabs and need to be soldered into place. I know, you are not supposed to directly solder these (or any types) of cells, but it works OK if done properly.
First attempts to solder with plain rosin core solder did not work well. The solder joints were typical of a cold solder joint.
Next, an 80 watt soldering iron was purchased, Weller model #SPG-80L. And some rosin soldering flux, Radio Shack # 64-022, and some 120 grit emery cloth, available from the hardware store. (Sand paper may not be a good idea, it can leave bits of abrasive on your work.) Last but not least, a squirt bottle of pure denatured alcohol, (Not the drugstore type, that is 20% water) some paper towels, and some rubber gloves to keep the flux off of your hands.
First, place a small amount of flux (about twice the area of the intended soldering spot) directly on the cell to be soldered, and scrub the flux and battery with the emery cloth for a second or two. No need to overdo it. When scrubbing the battery through the flux, apparently that flux keeps the battery surface clean before you hit it with the soldering iron.
Then using the Weller's 1/4 inch iron plated tip, very quickly place the iron and solder on the battery and "wet" the surface of the battery with a bit of rosin core solder. And, last, pre-tin your copper jumper strap, heat it up with the soldering iron til the solder melts. Then press the jumper strap with the hot soldering iron against the batteries pretinned area until the batteries' tinned area melts. Should not take more than two seconds or so. It helps to have an 1/8 inch diameter wood dowel or similar item to hold the strap down on the battery for a second or two after removing the soldering iron, to make certain the copper jumper strap does not lift off slightly.
And, after the soldering job is done, immediately wipe that soldering job clean with a piece of paper towel with alcohol on it, both to remove the soldering flux, and to cool down the soldering job on the battery.
Obviously, with pure alcohol, be careful of the fire hazard. And, use proper ventilation with the soldering fumes from the rosin flux.
I've tested these cells before and after soldering in this manner, and noted no difference in Milliampere Hour Rating, or voltage under load. Perhaps long term life might be affected, that remains to be seen.
My three 6S2P Dewalt DC9360 batteries are now in their third flying season, with some 300 Ampere Hours cycled through each of them. And they have the same exact performance as when new, in 2007.
OK, the pair of 6S2P A123 battery packs are now complete. As for soldering, I'd purchased some all purpose flux from this company. It worked far and above better than the Radio Shack flux. Don't know what this stuff is, so the cells were cleaned three times in pure alcohol after soldering, just in case!