Underwater robots need power to operate, and batteries are the key. Different battery types offer varying energy and power densities, affecting how long and hard these robots can work. Choosing the right battery is crucial for mission success.

Safety, pressure, and temperature all play a role in battery performance underwater. Engineers must consider these factors when designing power systems for aquatic robots. The right battery can mean the difference between a successful mission and a costly failure.

Battery chemistries for underwater applications

Common battery chemistries and their characteristics

• Lead-acid batteries have low energy density but high power density, making them suitable for short-duration, high-power applications (starter batteries in combustion engine vehicles)
  • Relatively inexpensive and have a long cycle life
• Nickel-cadmium (NiCd) batteries have higher energy density than lead-acid but lower power density
  • More expensive but have a longer cycle life and can operate in a wider temperature range (power tools, emergency lighting)
• Nickel-metal hydride (NiMH) batteries have higher energy density than NiCd but lower power density
  • More expensive than NiCd but have a longer cycle life and are more environmentally friendly (hybrid electric vehicles)
• Lithium-ion (Li-ion) batteries have the highest energy density and power density of all the common battery chemistries
  • Most expensive and have safety concerns due to their high reactivity (smartphones, laptops, electric vehicles)

Performance metrics and suitability for underwater applications

• The specific energy (Wh/kg) and specific power (W/kg) of each battery chemistry determine its suitability for different underwater applications based on the power and energy requirements of the vehicle or device
  • Higher specific energy allows for longer mission durations or smaller battery packs
  • Higher specific power enables faster response times and higher peak power demands
• The charge and discharge characteristics, including the voltage profile, capacity, and rate capability, vary between battery chemistries and impact their performance in underwater applications
  • Voltage profile determines the operating voltage range of the vehicle's electrical system
  • Capacity determines the total energy storage of the battery pack
  • Rate capability determines the maximum discharge current and power output of the battery

Energy density vs power density in battery selection

Energy density and power density trade-offs

• Energy density refers to the amount of energy stored per unit volume (Wh/L) or weight (Wh/kg) of the battery
  • Higher energy density allows for longer mission durations or smaller battery packs (long-range autonomous underwater vehicles)
• Power density refers to the amount of power that can be delivered per unit volume (W/L) or weight (W/kg) of the battery
  • Higher power density allows for faster response times and higher peak power demands (high-speed underwater vehicles, torpedoes)
• The optimal battery chemistry for a given underwater application depends on the specific power and energy requirements, mission duration, and safety constraints of the vehicle or device
• Trade-offs must be made between maximizing energy and power density while ensuring adequate safety margins and redundancy in the battery system design

Safety considerations in battery selection

• Safety is a critical consideration in battery selection for underwater applications due to the potential for short circuits, overheating, and explosion in the event of damage or misuse
• Li-ion batteries have the highest energy and power density but also the highest safety risk due to their high reactivity and potential for thermal runaway (underwater drones)
  • Require advanced battery management systems and protective measures to mitigate safety risks
• Lead-acid and NiCd batteries have lower energy and power density but are generally considered safer due to their lower reactivity and more stable chemistry (backup power systems for underwater sensors)
  • May require larger battery packs or more frequent replacements to achieve the same performance as Li-ion batteries

Battery pack design for underwater vehicles

Power and energy requirements

• Battery pack design for underwater vehicles involves selecting the appropriate battery chemistry, cell configuration, and packaging to meet the power and energy requirements of the vehicle
• The power requirements of the vehicle, including the peak power demand and average power consumption, determine the required power density and discharge rate of the battery pack (propulsion motors, sensors, communication systems)
• The mission duration and energy requirements of the vehicle determine the required energy density and capacity of the battery pack
  • Longer mission durations require higher energy density batteries or larger battery packs to provide sufficient energy storage (long-range autonomous underwater vehicles)
  • Shorter mission durations with high power demands may prioritize power density over energy density in the battery selection (underwater vehicles for high-speed maneuvers)

Cell configuration and packaging

• The cell configuration, including the number of cells in series and parallel, determines the voltage and current output of the battery pack
  • Cells connected in series increase the voltage output, while cells connected in parallel increase the current output and capacity of the pack (battery packs for high-voltage propulsion systems)
  • The optimal cell configuration depends on the voltage and current requirements of the vehicle's electrical system and propulsion motors
• The battery pack packaging must be designed to withstand the high-pressure underwater environment and protect the cells from mechanical damage and water ingress
  • Pressure-compensated battery housings equalize the pressure between the interior and exterior of the housing, reducing the risk of implosion or leakage (deep-sea exploration vehicles)
  • Reinforced battery casings and potting materials provide structural support and insulation for the cells, improving their resistance to shock and vibration (underwater vehicles operating in harsh environments)

Battery management systems

• Battery management systems (BMS) are essential for monitoring and controlling the charging and discharging of the battery pack, ensuring safe and efficient operation
  • The BMS monitors cell voltages, temperatures, and currents to prevent overcharging, over-discharging, and thermal runaway (lithium-ion battery packs)
  • Balancing circuits in the BMS ensure that all cells in the pack are charged and discharged evenly, maximizing the overall capacity and cycle life of the pack (large battery packs with multiple cells in series)
• Advanced BMS features, such as state-of-charge estimation, state-of-health monitoring, and fault detection, provide valuable information for optimizing battery performance and maintenance in underwater applications
  • State-of-charge estimation helps to prevent over-discharging and ensures that the vehicle has sufficient energy to complete its mission
  • State-of-health monitoring enables predictive maintenance and timely replacement of degraded battery packs
  • Fault detection and isolation mechanisms protect the battery pack and vehicle from damage in the event of a cell failure or short circuit

Environmental factors affecting battery performance

Pressure effects on batteries

• Underwater environments expose batteries to high pressures that can significantly impact their performance and safety
• Pressure effects on batteries include the compression of internal components, deformation of the cell casing, and changes in the electrolyte volume and concentration
  • High pressures can cause mechanical damage to the cells, leading to short circuits, leakage, or explosion (deep-sea exploration vehicles)
  • Pressure-induced changes in the electrolyte can alter the electrochemical reactions and reduce the capacity and cycle life of the battery (underwater energy storage systems)
• Pressure-compensated battery housings and reinforced casings help to mitigate the effects of high pressures on the cells
  • Pressure-compensated housings maintain a constant pressure differential between the interior and exterior of the housing, preventing deformation and leakage (underwater vehicles operating at varying depths)
  • Reinforced casings provide structural support and protect the cells from mechanical stress and damage (underwater vehicles operating in high-pressure environments)

Temperature effects on batteries

• Temperature effects on batteries include changes in the reaction kinetics, electrolyte conductivity, and material properties of the electrodes and separators
• Low temperatures reduce the ionic conductivity of the electrolyte, slowing down the electrochemical reactions and reducing the available capacity and power output of the battery (underwater vehicles operating in cold environments)
  • Low-temperature electrolytes and improved cell designs can help to mitigate the effects of low temperatures on battery performance
• High temperatures accelerate the degradation of battery materials, leading to increased self-discharge, capacity fade, and risk of thermal runaway (underwater vehicles operating in warm environments)
  • Thermal management systems, such as active cooling or insulation, can help to maintain the optimal operating temperature range for the batteries
  • Advanced battery chemistries with improved thermal stability and high-temperature performance can be used in applications with elevated temperature requirements

Other environmental factors and mitigation strategies

• Other environmental factors that can impact battery performance include humidity, salinity, and the presence of contaminants or corrosive agents in the water
• High humidity levels can lead to condensation and water ingress into the battery pack, causing short circuits and corrosion of the electrical connections (underwater vehicles operating in humid environments)
  • Sealed connectors, gaskets, and potting materials provide barriers against water ingress and corrosion, protecting the electrical connections and sensitive components of the battery pack
• Saltwater environments are particularly challenging due to the high conductivity and corrosiveness of seawater, which can accelerate the degradation of battery materials and seals (underwater vehicles operating in marine environments)
  • Specialized coatings, corrosion-resistant materials, and cathodic protection systems can be used to mitigate the effects of saltwater on battery performance and longevity
• Thorough testing and characterization of battery performance under simulated underwater conditions is essential for validating the design and ensuring reliable operation in the field
  • Accelerated life testing, pressure cycling, and environmental exposure tests help to identify potential failure modes and optimize the battery pack design for underwater applications
  • In-situ monitoring and diagnostic techniques, such as electrochemical impedance spectroscopy and acoustic emission sensing, can provide real-time information on battery health and performance in underwater environments