Here we present drone-specific energy sources. As mentioned in the components section, there are combustion and even jet engines, here we focus on electrical energy sources, in short, batteries. Drone batteries are connected directly to the ESCs and through them drive motors as most of the current is flowing this way.

Lithium-Polymer (in short LiPo) batteries are the base for powering both drones and ground stations. Their popularity is because of the energy density they present the best energy to weight ratio, so far. It is the most important factor in the case of aerial units.

LiPo batteries are composed of cells, that can be used as single ones, connected in serial (common) and parallel (rare). A single cell marking is “1S”. A single cell voltage is on average 3.7V, while fully charged, reaches 4.2V, and in any case, should not be discharged below 3.0V on normal use. LiPo batteries are very fragile and overcharging usually finishes with fire and explosion. Discharging below 3.3V causes increased battery wear out, 3.0V is critical, breaking its internal structure, and may cause inability to re-charge it or lead to fire and explosion while recharging. For this reason, LiPo batteries should be under instant monitoring. When treated with care, they last for years of uninterrupted power delivery. You may expect a maximum of 3-5 years' lifetime.

LiPo batteries have a known and predictable discharge curve. It means monitoring their voltage tells you how much energy is left inside. Observe discharge curve on Figure 1).

Discharged to 3.3V is considered to be a situation where immediate battery replacement or recharging is necessary, as then voltage starts to rapidly (non-linear) fall. Note, it is advisable to issue warnings earlier as there is usually some time needed for UAVs to return to the launch location and safely land that also requires energy.

LiPo batteries present increasing internal resistance for a cell, over time. It is an important factor because it helps to monitor battery aging and it affects discharge curve as observed from the external, user's point of view: the older the battery is, and the bigger the internal resistance is, the earlier the low-voltage warning should be issued (for higher voltage) to ensure safety zone. The following considerations present some typical battery parameters, and it becomes clear what is the impact of the internal resistance.

LiPo battery packs are stacks of cells interconnected inside with two major (power) cables for charging and discharging, and several smaller ones used to “balance” particular cells during charge.

Typical LiPo pack is composed of more than one cell, and they are connected in serial (rarely in parallel). Cell construction is marked and usually observable as the LiPo pack is just a stacked number of single cells, interconnected internally. Typical marking, i.e. 3S tells there are 3 cells connected in serial, thus increasing total voltage.

Nominal single cell voltage is 3.7V (4.2V max), so:

• 2S ↔ 7.2V (8.4V max);
• 3S ↔ 11.1V (12.6V max);
• 4S ↔ 14.8V (16.8V max);

and so on.

4S1P tells us there are 4 cells in serial and 1S2P tells there are 2 cells in parallel. Theoretically, any combination is possible, but parallel constructions are rare as it is problematic to charge them when there is a major difference in internal resistance.

Depending on the drone size, the number of cells (and batteries) grow: miniature drones use 1S, some 10-15cm ones use 2S, 250 class racers use 3-4S, and video filming drones use 4S-5S. There are bigger constructions, even up to some 10S and more in case of heavy lifter UAVs.

Each battery has some designed capacity. It changes over time but in any case, there are two types of markings of the designed capacity: using mAh and using Ah units. 850 means measurement is done in mAh (Figure 2), while i.e. 2.2 tells it is 2200mAh = 2.2Ah (Figure 3).

One of the major factors is the maximum current, the battery can deliver. There are usually two values: constant maximum current and burst one (burst is considered to last a couple of seconds, i.e. on take-off). The maximum current is given in the “C” number (multiplier of battery capacity). As on Figure 3, the maximum constant current provided is 35C, and burst is 45C which means, maximum constant current in A is:
2.2 (battery capacity in A) * 35 = 77A
while maximum burst current in A is:
2.2 * 45 = 99A

Battery charging requires a smart charger, that can balance the battery during charge, to ensure energy delivered via main connectors is equally distributed among all cells. This is a reason we use two sets of plugs when charging a battery: main plugs, delivering a majority of the current, and smaller connector for balancing. Sample connection schema for 5S battery charging is present in Figure 4. The high charging current is delivered to the battery via two main wires, red and black. Green wires connecting battery and balancer are to ensure equivalent voltage distribution, not to overcharge one cell because of undercharging the other: the reason for this situation to happen are differences in the internal resistance of the cells that is natural.

Universal chargers (Figure 5) can charge a variety of different types of batteries and also deliver other functions like controlled discharge, storage, internal resistance measurement, and so on.
Drone manufacturers usually deliver their charging solutions, sometimes very simplified ones, that do not provide, i.e. “storage” function, thus causes quick battery wearing out (i.e. Yuneec). Some others provide batteries with “intelligence” that discharges themselves automatically to the “storage” level, if not used for a long time (i.e. DJI).

In UAVs, there is a variety of different power connectors. It is mostly related to different origins and a wide range of currents the plugs need to handle. Each connector has some maximum current rating, and their name usually explains it, i.e. XT60 is up to 60A. For this reason, universal chargers usually come with a bunch of cables and converters, virtually enabling you to charge any battery without the need for soldering (Figure 6). Of course, manufacturers deliver battery packs with their own, usually proprietary plugs but it is common to find third-party adapters that will enable you to use universal and more advanced chargers instead of those provided by the manufacturer. Fortunately, for universal batteries, balancer connectors are standardized (so far there is one niche, different solution, used by Czech manufacturer Pelican) and it is JST standard plugs (Figure 7). Plug size is related to the number of “S” and the rule of thumb is the number of connectors is a number of “S” + 1.