1.4 or 2.6 volts to start. 4 cell Rx pack is 4.8 volts. 5 cell pack is 6 volts.

2S Lipo is 7.4 volts. Many Rx and servos are not made to take the extra voltage and need regulators to bring the juice down to 6 volts or less.

Also you need a charger that is enabled to charge Lipo.