This is a common question, yet the answer is complex. I normally break it down into two categories: sport and commercial. For most first-person view drones and other sport models, you do have some flexibility regarding different-capacity packs without worrying about damaging your unit. In fact, most of the modern racing drones actually can handle batteries of not only different capacity but also different voltages. If your power system can handle it, you can learn to fly your drone using a 3S pack, for example, but as you get better at flying, you may be able to push it up to a 4S pack to increase your power.
All things come at a price, however, so the bigger the capacity, the heavier the battery; at some point, you hit the “law of diminishing returns,” which means the battery may have more capacity yet the weight of the battery limits the performance and flight time so much that you really would have been better off with a smaller battery. It’ll take some time to find that balance.
In general, most commercial unmanned systems are designed to operate with a specific battery voltage, capacity, and footprint. So you want to make sure that you’re using a battery that is at least compatible with your specific unit. If you fly a system that uses multiple batteries—for example, two 22.2V 6S 16000mAh batteries—you will always want to keep those packs are as closely matched as possible. This would mean the same battery brand, the same capacity, and as close to the amount of cycle use as possible. Using a battery in a dual-battery system that does not match can cause the weaker of the two batteries to experience fatigue under heavy load, which ultimately leads to premature failure of that pack.—Keith Wallace, CEO, Venom Power.