4. Additional Technologies for Improved Performance
Several additional technologies can be added to overcome the inherent limitations of batteries, albeit often at the cost of extra weight, complexity and cost.
4.1. Battery Management Systems
The Battery Management System (BMS) ensures that the battery is operated within the Safe Operating Area (SoA) and helps prevent accelerated degradation of the battery
[33]. It can therefore be considered a safety-critical component. The main functions of the BMS are (1) monitoring the voltage, current and temperature; (2) diagnosis of the battery by estimating the state of the battery; (3) cell balancing by keeping the cell voltage at the same level; (4) management of the battery system, both electrical and thermal; and (5) communications. Advanced BMS with wireless communication systems and advanced sensor technologies could improve battery monitoring
[34][35]. Hence, next-generation models for battery performance and aging (SoH and RUL) monitoring can upgrade BMS functionalities
[36][37][38][39][40].
BMS units are commercially available, and off-the-shelf devices often present the most convenient solution. However, for robotic applications, these may end up being less cheap, less safe, and less sustainable in the longer term compared to customized solutions. Customized BMS have been developed for rescue robots
[41] and underwater robots
[42].
4.2. Thermal Management Systems
Achieving high battery performance under safe conditions is a challenge that is addressed by the design of appropriate battery thermal management systems (BTMS). Accurate 1D–3D thermal models are indispensable for predicting the thermal behavior of the battery to improve its design and shorten the development process
[43]. Robotic applications may require a tailor-made BTMS to fit their specific needs, the development of which may be driven by space limitations and the movement of the robot. There is a need for scientific research in this domain.
In addition to the thermal management of the BMS, active cooling (based on air or liquid
[44]) and passive cooling (phase change materials and heat pipes
[45]) can be introduced to keep the battery’s temperature in check. Thermal management is, of course, not limited to the batteries: the performance of actuators and power electronics is also directly affected by temperature, which is why they often have their own thermal management systems. An overview of thermal management systems for robotic systems can be found in
[46].
4.3. Recharging and Battery Swapping
The need for high energy densities can be reduced by frequent recharging. Various charging methods have been developed, often optimized for the working conditions of the robot. In many cases, the robot is idle or has limited capabilities while it is being charged; therefore, a high charging speed is desired. Here, the BMS plays an important role in controlling the charging speed, resisting overcharging if safety is compromised
[47], and optimizing the charging strategy, which supports the energy management of the battery
[48][49]. In terms of battery selection, the fast-charging capabilities of battery technologies such as LFP, LTO, NCA and LiPo puts them ahead of others when charging speed is a priority.
To enable easy charging, docking stations are now utilized for many commercial robots such as vacuum cleaners, grass-mowing robots and mobile social robots
[26]. Moreover, for electric vehicles, especially for the upcoming driverless cars, robotic EV chargers, both stationary
[50] and mobile
[51], are under development. These approaches are being transferred to untethered robots
[52][53], although they present some application-specific challenges. Docking stations for underwater robots that utilize contact-based wet-mate connector technology require high-precision docking and are prone to corrosion and electrical safety issues
[54]. To overcome these limitations, wireless recharging techniques are being investigated
[55]. Inductive and capacitive power transfers are also possible at the cost of lower charging speed and higher losses
[56][57].
An advantage of wireless technologies is that they can be deployed for continuous charging
[58][59], which is an interesting option if the downtime associated with recharging is considered prohibitively long. However, the cost of continuous wireless charging infrastructure can turn out to be high, and the resonating coils that are typically used may produce high values of stray magnetic fields
[60][61]. Power transfer through sliding contacts (so-called “powered floors”) can be a more cost-effective alternative
[62]. For mobile robots that are not in contact with the ground, recharging during the mission is made possible by photovoltaic cells
[63] and electromagnetic field (EMF)-based alternatives such as charging from high-voltage power lines
[64]. Alternative approaches include laser beam and battery dumping.
Another method to reduce the downtime is battery exchange or battery swapping. In
[65], a change/recharge station is presented that swaps the depleted battery of a UAV with a fully charged one. A queue of batteries, which are continuously being recharged, is available at all times. Barrett et al. developed a mobile ground robot for the battery exchange of small-scale UAVs
[66].
4.4. Hybrid Architectures
High charge and discharge currents at very short pulse lengths are challenging for many battery chemistries. Hybrid architectures (battery + battery or battery + capacitor) could present a solution for robots that perform cyclic tasks, but require additional attention to factors such as cost, self-discharge and temperature effects
[67][68][69]. Capacitors, for example, generally exhibit higher self-discharge rates than batteries. A capacitor will thus drain energy from the battery in a battery–capacitor hybrid, decreasing the autonomy of the robot. Temperature effects are another important consideration for hybrid battery structures. They affect some battery technologies more than others, which is why they should be considered when designing a hybrid battery structure for a robot expected to operate at a wide range of temperatures. This is the case for, e.g., field robots and wearable robots. The hybrid use of battery technologies would also require advanced control strategies to efficiently optimize the power and energy demand, for example, shown in
[70]. Finally, the mixed-use of coupled batteries may challenge the sizing requirements in robotic applications
[71][72].
4.5. Packaging
The performance and safety of a battery can be affected by the working conditions, in particular, moisture, vibrations and shock loads. Many untethered robots are deployed in such harsh conditions. A robust packaging protects against these conditions, but adds around 15% to the volume and mass of the battery
[73]. Typical shapes of Lithium-ion batteries (cylindrical, pouch and prismatic) provide benefits in packaging density, but this comes at the cost of more complex thermal management
[74].
The impact of shock and vibration on battery performance is, however, a poorly investigated topic
[75]. Standard shock and vibration tests found in the literature report no effect on the battery capacity
[76]. However, an external shock due to accidents, improper packaging, pressure and operational thrust can have a detrimental effect on the battery characteristics triggering sudden failure
[76][77]. A robust mechanical design considering the thermal protection, egressing outlet, vibration isolation, crashworthiness, packaging material, and concept not only improves the battery reliability but also positively impacts performance
[74][78].
On a robot level, shock-absorbing materials, airbags
[79], compliant actuators
[80], fall and impact detection techniques
[81] with prevention strategies and constraint handling control techniques
[82] can be used to mitigate the damaging effects of shock loads. A robust battery packaging can further protect against these conditions, improving mechanical stability but also enhancing safety
[78]. There is however a packaging penalty in terms of the mass and volume of the battery packs. A good rule of thumb is to deduct 15 percent from the cell performance figures
[7][75]. Typical shapes of Lithium-ion batteries (cylindrical, pouch and prismatic) provide benefits in packaging density, but this comes at the cost of more complex thermal management
[74].
Batteries are usually subject to standards and passed through quality control before being used in an application that already certifies safe handling during regular and irregular shocks
[76]. Errors in the system design can, however, also trigger mechanical failure or damage and, consequently, battery performance. In this case, the LiPo battery is often considered a better choice due to its flexible shape and size.