I used to solder up all of my battery packs. I had learned to solder by building my own Heath Kit TV set and later my own 8 channel single stick Heath Kit R/C radio. About 8 years after I put together my Heath Kit R/C radio I crashed a plane due to total loss of signal and found a cold solder joint at a battery solder joint (I believe I had replaced one of the four batteries in the pack). I could have killed someone with that plane, including myself. I was very good at soldering, but sometimes I did not solder for a few years. That was my problem. After that crash, back in 1981, I have never soldered a battery pack again. Now I always purchase complete battery packs and I believe I am much happier for that. I also sleep better.