Let's work out how that would work, in a simplified model of the world. Let's say you have a 1000mAh battery at 10V that weighs 100g (these numbers have all be

Let's work out how that would work, in a simplified model of the world. Let's say you have a 1000mAh battery at 10V that weighs 100g (these numbers have all been made round for purposes of easier arithmetic). Everything else on your drone is weightless, and your motors have perfect efficiency. To keep this aloft, you need to overcome 9.8 m/s^2 that is applied by Earth's gravity; but since we're simplifying things, 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 (resist the force of gravity). 1Ah * 10 V = 10 Wh, so that means you could stay aloft for 10 h (remember, these numbers are unrealistic, we have ignored any kind of efficiency, or the weight of the rest of the drone, etc). So, lets use two batteries. Now we have 20 Wh of energy available, but we're holding up 200g of battery, which requires 2 W of power to stay aloft, so again, we can stay aloft for just 10 h. And remember, all of this was ignoring any notion of efficiency or the weight of any other part of the craft to support this. As you increase the weight, motors will need to get more powerful to be able to actually keep it aloft, which means they need to get bigger, which is extra weight that you need to support that isn't giving you any extra energy. So the real world is actually even worse than our idealized model; as you add on more battery, only a fraction of the increase in weight adds more energy, so you will actually see shorter flight times. Now, of course, there are designs for which increasing the amount of battery can lead to increased flight times. If the power supplied by the motors is sufficient to sustain the extra weight, so they don't need to be scaled up, then increasing the amount of battery relative to the rest of the weight of the craft can lead to higher efficiency as there is less relative "dead weight" (weight besides the batteries, that is not carrying energy). But that can only take you so far, which is why the flight times of drones plateau at a certain point and you can't fix it by just adding more battery. This is why you don't see electric helicopters carrying people. The energy to weight ratio for batteries is too low; only fossil fuels have a sufficient energy density.