Well from what i have read it's a no-no. But I have soldered a small blob and made battery packs with copper strips solder to the battery tops and bottoms with a soldering iron. I usually solder the battery then very quickly put a damp rag on the joint or blob of the battery. Trying to keep as much heat as possible out of the battery. You will need to clean the area where your going to solder to. You can use sand paper and slightly rough up the area. I use a 150 watt soldering iron for the job. You may get by with a 30 watt. I have never tried with anything but the 150watt. I always figured the less time I am on the piece the less heat will be absorbed into the entire piece. Clean the area, apply a little spot of flux on the area, then add the appropriate sized solder blob to the already hot iron, touch the solder iron blob to the area where you applied the flux and it should melt on to the area very quickly. Then wrap the rag around the area very quickly to avoid over heating. The damp rag should help to cool the area (battery) down before anything bad happens. If it doesn't seem to stick within a few seconds, then you may need a more powerful soldering iron. I do not think anyone is really going to tell you this is safe because it is not. And if you try this you should already know how to solder to some extent. Try at your own risk. But it can be done.
. The main thing that you are at risk of, is battery explosion from the heat. Thats why I try to keep the soldering time only to maybe 3 sec max and cool it down fast. If it doesn't take in less than three seconds then stop and cool the battery down and try again once cool. You need to practice. Don't solder on any battery until after you have practiced on something else. And remember this is dangerous. Be careful.
