i have a DC motor that requires 2 Amp power at 8.4 volts.....what would be the best possible battery(and cheap too)...i need it to power my hovercraft.i use a 7.4 volt,2000mAh battery...but still it does not work....
You can get get both cheap and very good, but unfortunately not at the same time.
If your "still does not work", is about lift (or rather the lack of it), you need more than a single Volt extra.
Since you haven't got enough lift (still assumed), you don't want to add weight, so you need to go for lithiums (see, cheap and good don't go well together).
You don't need
2000mAh cells for a 2A current draw. The "h" (meaning hours) in the former makes it a capacity spec. not a current spec like the latter.
a 600mAh battery can drive a 2000mA motor for (500mAh/2000mA) around a quarter of an hour (not a mistype/miscalculation, but the capacity falls with large current draws).
All that apart... If you have trouble getting lift, you might wanna reconsider the entire construction, its materials, the motor (type, speed and torque ratings), the skirting material etc. etc.
Best type of motor here is a brushless, considering weight to power relation.
You could post some snaps of what you have build, list its weight, RPM of the motor you have, what do you use for propellor, what building material etc. and you would surely get some advice on what works and what needs replacement.