RE: Flyzone Sky Fly
Sorry the correct battery is 2/3 A 900 mah.
Based on the following formula power is the product of voltage and current.
P= V X I
8.4V X 0.9A= 7.56 watt
7.2V X 0.9A= 6.48 watt
If your 380 motor consumes 4 A in average then it uses 4 A X 8.4 V= 33.6 watt. 8.4 v battery provides 7.56 watt for one hour. Thus, your flight duration would be 7.56 X 60min / 33.6 = 13.5 min.
With your 7.2 v battery your flight duration would be 6.48 X 60 / 33.6= 11.57 min.
I hope it might help to figure out what is the effect of voltage on the power.
Good luck,
Fred