Also use a low-wattage soldering iron. By such means, you are less likely to ruin the components that get re-used, and less likely to compromise the brand new cells,

Click to expand...

em er ... maybe I always recommend a mid wattage iron, the idea goes:

With a low wattage iron on larger parts (ie battery cases) the iron can not keep up with the heat sinking ability of the object and so if you want solder to flow you have to heat the whole thing up until the object is saturated, electronics (especially ones that are chemicals in sealed metal cases) usually do not like this

with a mid powered iron you dont run into this problem, you can heat just a spot to high enough temperature for soldering and move on, reducing soaking