Batteries and Generators
All UAVs need on-board electrical power for avionics and control; those with electric motors also need high-power supplies for propulsion. Most commonly, these needs are met by battery
Figure 5.13 SPOTTER fuel tank level sensors. One sensor lies behind the central flap in the upper wider part of the tank (just visible in the right-hand image), while the second one lies at the bottom just above the payload interface.
Figure 5.14 Engine-powered brushless generators driven directly or by toothed belt.
supplies, optionally augmented by engine-powered generators for in-flight recharging. For most purposes, we prefer the ruggedness and resilience of NiMH or LiFe batteries wherever possible. For main propulsion batteries, LiPo systems become necessary to achieve the required power densities. Great care is called for when using LiPo batteries, as they are a major fire risk if they are damaged or misused. A wide range of capacities and voltages are readily available, though the best quality high-density LiPo batteries are expensive.
With all batteries, maintenance is very important, and we keep dedicated battery use and charge logs for all flight-critical batteries. Batteries should not become discharged too far, be overcharged, or be stored in inappropriate charge states if they are to give their best. It is also important to note that batteries represent a real fire risk, and in many countries transport of battery packs can constitute a hazardous load and be subject to regulation. Ideally, charging should be carried out in dedicated environments with suitable fire control measures in place.
For long-endurance UAVs, it is important to segregate batteries to ensure that critical systems never lose power. This can also simplify wiring and reduce unwanted noise on important control lines. For example, using separate batteries for receivers/servos and ignition systems is often desirable. If a single charge of a battery will not support an entire mission, then on-board recharging becomes necessary: we use brushless motors driven directly or by toothed belts from Wet-fueled engines to provide the required power, along with suitable battery conditioning circuits to guard against overcharging, see Figure 5.14.