Use two batteries and two switches. I had been running a big model on two 4-cell 2400mah nimh AA packs wired together to make one big pack. Bad idea! there are issues with charging them at the same time. I found I was running dangeriously low voltage after some aerobatics. Could have lost the plane. Nottice how ehn you move the servos the voltage dances all around on the volt watcher with a 4-cell pack, bad.

Run two five cell packs with seperate heavy duity switches using built in charging ports. This give you more headspace on the voltage and if one switch glitches you won't crash. Cermark is top notch on battery stuff. I had them make me a couple of 5-cell sub-c packs. I have never seen the voltage drop any significant amount even under load. I suggest you call them and get theri input then buy the right batteries.