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 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 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 some maximum 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 warning earlier as there is usually some time needed for UAV 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 ageing and it affects discharge curve as observed from 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. Following considerations present some typical battery parameters and it becomes clear what is an 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 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 220mAh = 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 be some seconds, i.e. on take-off). The maximum current is given in “C” number (multiplier of battery capacity). As on Figure 3, the maximum constant current provided is 35C and burst is 45C that 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 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 a balancer.