What you want is a balanced quad, so it actually doesn't matter where you put yor
battery, as long as the centre of gravity COG is in the middle of the craft, in line with the props. Since the
battery is the only thing that is not fixed on a quad, the
placement will decide the COG. normally, you are only able to move weight backwards or forwards.
If you take a quad with
battery attached, hold it in two of the props. Then you can see if it is balanced, top heavy or bottom heavy.
Next try to balance the quad on the frame right between the motors and move the
battery untill it is balanced.
From my experience, a top mounted
battery is what gives the best COG and not necessarily a lowered top plate. A whole lot of a racing quads weight is placed under your props (bottom plate, arms, motors, PDB etc). My lisam LS210 is quite balanced (a tiny bit top heavy) with a 1300mah 4s
battery even though it doesn't look like it.
As I mentioned, I have a quad with top mounted
battery, but also got a quad with bottom mounted
battery, and another thing I have noticed is that the
battery is better protected when put on the top. when I crash, I tend to crash more with the bottom side of the quad, which makes the
battery more vulnerable.
There are a whole bunch of frames on banggood with lowered top plate:
http://www.banggood.com/Lantian-LTX-HEX4...55199.html http://www.banggood.com/Diatone-Tyrant-2...51744.html http://www.banggood.com/RC180-V2-186mm-C...21110.html http://www.banggood.com/X220-220mm-Carbo...59493.html http://www.banggood.com/Q210W-210mm-Carb...60042.html Edit: I forgot an important part here, Moment of inertia. You actually want to keep most of the mass as close to the COG. This way, the moment of inertia will be less so your motors doesn't have to work that much when doing flips and rolls