When it comes to soldering, I try not to. I'd suggest going solderless. You can get solderless battery packs from here:
http://www.modelelectronicscorp.com/price-list.html#spt
This way, if you have a cell go bad, you can easily replace the cell without any solder. I've been using these for some time now and they work great. GP2200's, plenty of power with less weight :D