Electron suggest using a single small battery for retracts and brakes, and don´t share the receiver or engine battery.
500-900 mAh will be enough for at least 25 flights, so with a very small battery you will have power enough for a full flying day
ER-30: 2S Lipo
ER-40: 2S Lipo
ER-50: 2S LiPo
ER-50 NG: 3S LiPo
For better power distribution, and safer running, is recomended use 3 pins of MPX connector, for each polarity, like next photo show: