This is step by step how I'm soldering my Li-ion Battery for my F3F planes. I use 2 cells of 2900mAh, and a servo connector as balancer blug, the signal being the middle point. I'm also now connect the 2 cell with a wire no rigid connection, it is more secure from my point of view. When charging about 1A, it is not really necessary to use the balancer all the time, once or twice a year for maintenance is enough. This battery type is giving me entire satisfaction, they are now cheap, they are reliable, no self-discharge, and provide enough power for digital servo for an afternoon of intensive practice. There are many references of 18650 format cell. I usually don't go higher than 3100mAh because of the higher internal resistance, so the lower capability to draw high current.