The first non-military commercial drones in 2006 were able to fly for no more than 10 minutes - hardly enough to travel a few hundred meters. Such a flight time might seem sufficient for drones used by hobbyists or amateur videographers, but for commercial purposes, such as for pipeline inspections or large-scale surveys, drones need to stay airborne for much longer. Run-of-the-mill batteries were not enough to sustain these requirements and alternative energy-storage options were required quickly.
So, what batteries do drones need? Ones that are capable of discharging large consistent current throughout a drone’s flight. To ensure longer flights, batteries need to have a greater charge capacity, without adding too much weight to the drone- implying higher charge densities. They need to be rechargeable, without much loss of efficiency. All batteries face the issue of lower efficiency after multiple charge cycles as their effective charge capacity decreases with usage.
Some of the longest flying drones rely on diesel-powered combustion engines. These include some military drones, which are often refueled mid-air for even longer flights. Unfortunately, these drones are extremely heavy, owing to the added weight of the engine. Vanilla Aircraft’s VA001 fixed-wing drone (which weighed more than 200kg) was able to stay airborne for almost 5 days straight, through the use of such an engine. The problems with these solutions are obvious – they are heavy, detrimental to the environment and extremely expensive.
Hydrogen fuel cells are capable of storing large quantities of energy (nearly 120MJ/kg),and are a much cleaner alternative to diesel engines. However, a lot of material is required for the housing of the hydrogen fuel itself, contributing greatly to the weight of the drone. Hence, hydrogen fuel cells are only used in heavier drones if at all. The Doosan DS30 for example, can fly for nearly two hours, but weighs close to 25 kg. These fuel cells are still in an experimental stage, as hydrogen is still difficult to extract and isolate, contributing greatly to the cost.
For smaller rotorcrafts, a lighter solution is needed. Although diesel has a high energy density of nearly 45MJ/kg, electrical propulsion systems weigh a lot lesser than combustion engines. For the use of such electrical propulsion systems, Lithium-based battery compositions are proving to be the best way ahead, specifically, through the use of Lithium Polymer (LiPo) and Lithium Ion(Li-Ion) batteries.
Lithium Polymer batteries are most commonly used in the drone industry, owing to their higher discharge rate. However, the energy stored in a LiPo battery is nearly30% lower than a Li-Ion battery of the same weight.
The real clincher between the two variants is battery life. Li-Ion batteries can go through 400 charge cycles – 4 times more than a Lithium Polymer battery. Although this would normally tip the scales in favor of Li-Ion, most drone companies prefer the higher discharge rate offered by LiPo batteries.
However, a small fraction of companies persists in using Lithium-Ion batteries in their drones, such as Urban Matrix Technologies. Identifying that Li-Ion batteries were sufficient to satiate the current requirements for their drones, they capitalized on making use of the longer flight times and lifetime that Lithium-Ion batteries could provide.
Their custom battery packs weigh just 900 grams - 20% lighter than other LiPo batteries of comparable capacities in the drone industry. As a result, Urban Matrix quadcopters are able to stay airborne for nearly 70 minutes, compared to the 40 minutes provided by other drones using LiPo. With nearly double the flight time, the batteries can still be used for over 400 flights. This contributes to a huge drop in operational costs – the cost-depreciation of the battery with every flight is almost a third of the nearest competitor, but with superior flight time.
Although the drone industry uses Lithium Polymer batteries without much hesitation, they have their downsides and are not the best option available. With the right innovation, Li-Ion batteries can prove to be a far superior alternative as demonstrated by Urban Matrix Technologies. Developments so far are just the tip of the iceberg, and Li-Ion batteries are capable of quickly broadening our aerial horizons.
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