Many unmanned systems and autonomous vehicles (UAVs, UGVs, AUVs, USVs and robotic platforms) are powered by batteries. Larger unmanned vehicles may be powered by internal combustion engines which contain batteries, providing a reliable backup power system that will enable the recovery system to operate in the event of a main engine failure, allowing the craft or vessel to navigate to a safe landing or extraction point.
The most commonly-used battery technologies in unmanned systems are lithium-based, as they have a higher energy density than the older nickel-based technologies so they can provide more useful power per unit weight. The two most popular lithium chemistries are lithium polymer (LiPo) and lithium ion. Lithium sulphur technologies are currently being developed which promise new breakthroughs in battery performance.
Lithium polymer battery technologies are a popular choice for unmanned systems, as the cells can be enclosed in thin, flexible aluminium pouches. This saves weight compared to the thick metal cans required with other battery chemistry manufacture and also allows for a wider variety of design configurations.
The main parameters to look for when choosing a battery are voltage level, capacity (how much power the battery can hold, measured in milliamp hours) and discharge rate (how much current can be discharged from the battery at once). Other factors that affect battery selection include activation time, charging time, lifespan and cost.
In a typical unmanned system, many different components, such as propulsion systems, processors and sensors, may all have different voltage and load requirements. Different parts of the system’s mission – takeoff, landing, hovering, information gathering – will also have different power profiles. This means that the battery will be undergoing continuously varying loads and have a wide variety of demands placed on it.
In UAVs, the flight time is critically dependent on the battery life, and a “dead stick” condition in which the battery becomes completely drained during flight can be disastrous. In order to avoid this, a battery management system (BMS) is vital. A battery management system will continuously monitor important battery parameters, while dealing with the varying power demands of the many aspects of the UAV’s operation and optimizing the usage of the battery.
The battery management system may monitor battery voltage, current, temperature, state of charge, state of health and other parameters, and may calculate additional information based on these. Some systems can report this data to an external device via a communications link.
In addition to managing the battery usage, the battery management system can also protect the battery during charging, safeguarding against conditions such as over-current or over-voltage.