2.4 Individual cells used in the construction of the battery pack shall be rated at:-
LiPo a maximum of 3.7 volts nominal,
LiFe a maximum of 3.3 volts nominal.
Individual cells may be wired in parallel.
- For 2S packs: the maximum “In Series” is two, to give a pack voltage of maximum
7.4v nominal for Lipo packs, or maximum 6.6v nominal for LiFe packs.
- For 1S packs: the maximum “In Series” is one to give a pack voltage of maximum
3.7v nominal for Lipo packs, or maximum 3.3v nominal for LiFe packs
Cells with a nominal voltage of 3.8V may be used as from 2017 provided that a majority of
manufacturers has them available). The maximum charging cut-off voltage remains at
4.20V. per cell

This is the updated IFMAR rule, EFRA and ROAR are using similar language