For the most part it's usually not worth the hassle to try and make your own packs to save a couple of buck. The reason is because when soldering if you get the top/bottom of the battery too hot it melts the internal seals inside the battery, and the battery is shot after that. It will become unreliable if that seal it gone. How hot is too hot? 2-3 seconds of heat can be too much. Because of that you get a real balancing act trying to get everything hot enough to make a good solder joint. So if you are careful to not get it too hot you end up with a bad solder joint, which can cause a crash from loss of power. IMHO it's better to just buy the batteries from a reputable source and know that they are built properly. A good source of information about all things concerning RC batteries is [link=http://www.rcbatteryclinic.com]www.rcbatteryclinic.com .

Hope this helps

Ken