Since I didn't fly large scale models... I only know about them in general.
While some of the ones I've heard about have some power regulation to keep a consistent output ... it's more about power redundancy and not just for the servos but also for the ignition on the models that require that ( such as gas engine aircraft ). On models without ignitions, many just use two battery packs and connect them to the receiver on separate ports with each having their own switch. The "boxes" do about the same but often have a single kill switch incorporated so you don't have to turn off two switches and if one battery pack shorts out, they will keep it from affecting the power from the other battery ( which would drain the secondary pack ). If using the two pack method, just make certain both battery packs are of the same type and size.
Do a search for "rc power redundant box" or similar... you should get several results and can watch some vids or read up on the general concept.