Well, as a starter, balancing happens with multiple cell packs. When you charge the pack, the cells charge at different rates. Balancing insures that they are all charged to the same level and the discharge rate is equal among the cells in the pack.

As far as the NiCd packs as secondary packs, well, some do that some don't. It comes down to a personal choice. As for me, well, I don't do that, or better worded, have not yet done that. There are advantages to powering the receiver and servos with a secondary battery pack. You just have to weigh the differences and see if there is an advantage to your setup. But, always keep in mind, with electric power, weight is always a condideration and adding a secondary battery pack adds that much more weight and raises the required power to fly that particular plane.

CGr.