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I agree with flatulent it is possible to solder batteries, just remember to use a soldering iron with enough wattage, you do not have time to heat it for a minute or so. You need to be quick and use a lot of heat just as flatulent said. BUT remember that batteries can explode so if you would like to be on the safe side and be able to see Santa use eye protection. I do not know if it is neceserry, I have glasses so I trust they will stop the most if the batteries explode.
As said before it is necessary to tin the wire and battery before soldering.
To tin the battery, file end of the battery and spread a little flux before tinning with a high wattage soldering gun (my is 75Watt. The flux helps to do a quick tinning.
The effects of extreme overheating have been described above as an explosion. There is a lesser damage from overheting a smaller amount. This is disconnecting the inside connection to the external contacts. It may be less risky to use a battery holder if this is possible.
I have replaced the insides of two HT battery packs using the soldering method described above and they have lasted for several years by now.
I know it's late but : I had the same problem when I tried to solder a flat piece of meta on the cell , but when i used a wire instead of the that piece it just soldered fine , so just solder wires on the cells instead; and i used a 30W soldering iron
i think the problem is that the tap will take the heat from the solder joint so fast that it cant be that good.