I a use 2 x rx batteries, either life 2s or liIon 2s , depends if HV system used.

Then I usually also use large 2S battery for the engine and route power for retracts and lights from it. In one case I took power for lights and retracts fron ARXL smoothflite unit which regulates V to 7.4V, and I used 5200mAh 2s LiIons then.