Paolo,


The Vlach ignition system that comes with all MVVS spark ignition engines, is designed to use 4.8-6V nominal voltage.
This means you can use either a 4-cell or a 5-cell Ni-Cd, or Ni-MH pack.

Using the 7.4V two-cell Li-Po, directly would shorten the life of the system, as well as voiding its warranty.

If you use Ni-MH, a 4 cell pack of AAA batteries would be light enough and offer sufficient flying time.
Don't risk using Li-Po directly.