A few things I make sure to do when soldering a battery pack, is:

First - use a Soldering GUN, not an iron.

A gun gets a lot hotter, so you don't need to keep it in contact with the battery for as long.

Second - Lightly sand the area of the battery where you are going to solder, add a dab of Flux, melt some solder on the gun tip, and touch it to the battery. THis will adhere some solder to the battery with minimal "Time under heat". Next, "Tin" the end of the wire.

Now you have solder on the battery and the wire. Put the two together, and add the gun just long enough to melt the two solders (On the battery and on the wire) And they will merge into one lump. The battery never gets a chance to get over heated.

And RichD, yes, they are rechargeable