The low cost, simplicity, and easy use of battery-powered multirotor aircraft has led to their adoption in commercial, industrial, agricultural, and military applications. These aircraft, however, have limited payloads and shorter endurance and range than fuel-powered conventional aircraft. While individual use cases of multirotor craft vary drastically, one of the most important variables in performance is aircraft mass. Lithium batteries offer an energy density high enough for most small multirotor flight applications, however these batteries often constitute a significant portion of the aircraft’s mass. The mass fraction of the multirotor that consists of batteries often inhibits flight characteristics similarly to an increased payload. This research proposes the use of lithium-ion batteries in a multifunctional configuration, providing both energy for flight operations and structural load-bearing capability. This implementation is proposed to decrease the structural frame mass and thus increase multirotor performance characteristics such as payload capacity and flight time. Multifunctional lithium-based batteries have been previously proposed for the efficient use of space and mass in electric vehicles [1].

Standard 18650 cylindrical battery cells (Panasonic NCR18650B) were used in this research as they are commercially available, have a circular cross-section, and have high energy density relative to prismatic or pouch lithium polymer cells often used in this application. The proposed configuration consists of a thin tube filled with 18650 cells to provide a multifunctional member with a high bending stiffness. The method of reinforcement is similar to that of the jellyroll electrodes placed within a thin cylindrical battery shell being substantially stronger than either alone [2]. The proposed configuration is compared with the non-reinforced tubing in three-point bending and shown to increase stiffness due to the load-bearing capabilities of the battery cells.

The mechanical abuse tolerance of bare battery cells has been extensively tested, however, little data is available for reinforced cells or multifunctional cell configurations [3]. It has been noted that high strain rate and reduced state of charge negatively affect the integrity of Li-ion batteries [4]. Additionally, the mechanical properties of the jellyroll electrodes have been found to be anisotropic, contributing to non-uniform mechanical response of the cell as a whole [5]. In this study, testing was performed at a zero state of charge and a high strain rate to simulate the worst-case relative performance of the multifunctional members. In addition, multiple sizes of reinforced members were tested to investigate the scalability of the multifunctional configuration.

The cylindrical lithium-ion structural battery presented here can provide both a power source and a load-bearing member for multirotor aircraft. Battery reinforcement is shown to provide up to 1100% stiffness and 750% yield strength improvements. Substitution of the proposed structural battery for aluminum tubes in a notional quadcopter design showed a 41% improvement in theoretical maximum hover time. This research motivates future multifunctional battery configurations with topologically optimized designs for various loading scenarios.

[1] Zhang YC, Ma J, Singh AK, et al. (2017) Multifunctional structural lithium-ion battery for electric vehicles. Journal of Intelligent Material Systems and Structures 28: 1603-1613.
[2] Zhang XW and Wierzbicki T. (2015) Characterization of plasticity and fracture of shell casing of lithium-ion cylindrical battery. Journal of Power Sources 280: 47-56.
[3] Zhu J, Zhang XW, Sahraei E, et al. (2016) Deformation and failure mechanisms of 18650 battery cells under axial compression. Journal of Power Sources 336: 332-340.
[4] Xu J, Liu BH, Wang XY, et al. (2016) Computational model of 18650 lithium-ion battery with coupled strain rate and SOC dependencies. Applied Energy 172: 180-189.
[5] Sahraei E, Campbell J and Wierzbicki T. (2012a) Modeling and short circuit detection of 18650 Li-ion cells under mechanical abuse conditions. Journal of Power Sources 220: 360-372.