I agree with your general idea, more than half an hour is not posible with the current technology. But I think this conversion is wrong:

> let's just round that to 10 m/s^2. 10 m/s^2 100g is 1 N, which means it requires 1 W of power to keep that battery aloft*

To transform some force into power, you need to multiply it by a speed (m/s). My favorite example is that a brick resting on a table can "hover" without spending any power (for a weird definition of "hovering").

I think the correct calculation involves the density of the air, to estimate the speed of the air that must be pushed down.