ROV Battery Sizing Guide: How Many Watt-Hours Does Your Underwater Drone Need?

The watt-hours an underwater drone needs start with a simple calculation: multiply its average operating power in watts by the required mission time in hours. A vehicle averaging 300W for two hours therefore needs 600Wh of energy in theory, before accounting for usable capacity, electrical losses, reserve, and changing underwater loads.

That number is only the starting point for an ROV battery specification. Thruster peaks can demand far more current than the mission average, while cameras, lights, sonar, computers, manipulators, water currents, and tether drag all affect actual consumption. Battery voltage, discharge capability, dimensions, weight, BMS limits, and the pressure-rated enclosure also have to work with the vehicle as one system.

This guide shows how to move from a mission profile to a realistic watt-hour target, then turn that figure into a battery specification that can be validated in the water.

How Many Watt-Hours Does an ROV Battery Need?

An ROV battery should contain enough usable watt-hours to support the vehicle’s average total power demand for the required operating time, with additional capacity allocated for losses and an engineering reserve. Amp-hours alone are not enough because the same Ah rating stores different amounts of energy at different voltages.

The Basic Formula: Average Watts × Mission Hours

The simplest preliminary calculation is:

Required energy (Wh) = Average power (W) × Mission time (h)

For example:

300W × 2 hours = 600Wh

That means an underwater drone averaging 300W requires 600Wh of delivered energy for a two-hour mission under the assumed operating profile.

The important word is average. An ROV rarely holds every thruster at maximum output from launch to recovery. It may cruise at moderate power, hover during inspection, make short high-thrust corrections, hold position against a current, then operate tools or lighting intermittently.

A battery sized only from maximum thruster power can therefore become unnecessarily large. At the other extreme, sizing from a low cruising load can leave too little energy for realistic operations. The useful figure is the average power of a representative mission profile.

Historic data from Woods Hole Oceanographic Institution illustrates the distinction. Its ABE autonomous underwater vehicle carried a 5kWh lithium-ion battery system and consumed about 210–300W depending on the mission, with a separate continuous sensor or “hotel” load below 50W. The documented operating duration was approximately 14–20 hours. Although an AUV and an ROV have different architectures, the example shows why mission-level energy budgeting has to include more than propulsion alone.

Converting Amp-Hours to Watt-Hours

When capacity is specified in amp-hours, convert it to watt-hours with:

Energy (Wh) = Nominal voltage (V) × Capacity (Ah)

A 25.6V, 30Ah battery therefore stores:

25.6V × 30Ah = 768Wh

This conversion is especially important when comparing batteries built around different voltage platforms. A 30Ah battery at 12.8V and a 30Ah battery at 25.6V have the same amp-hour rating but not the same stored energy.

For ROV battery sizing, watt-hours answer the energy question more clearly:

How much energy is available for the mission?

Amp-hours still matter for electrical design, but they should always be considered together with nominal voltage.

Allowing for Usable Energy and Mission Reserve

The nominal Wh rating should not automatically be treated as energy available for mission planning.

A more useful preliminary engineering expression is:

Required battery energy = (Average power × Mission time) ÷ (Usable energy fraction × System efficiency)

An additional project-specific reserve can then be included for recovery, unexpected currents, longer inspection time, battery aging, or other operating uncertainty.

For example, a theoretical 600Wh requirement would exceed 600Wh once the design accounts for an allowable discharge window and system losses. The correct allowance depends on the chemistry, BMS thresholds, expected battery condition, temperature, operating profile, and the vehicle’s recovery strategy rather than a universal percentage.

That distinction matters in underwater work because the consequence of reaching a battery cutoff is not simply “shorter runtime.” It can affect propulsion, vehicle electronics, recovery, and the ability to complete the task safely.

Build a Realistic ROV Power Budget Before Choosing a Battery

A useful ROV power budget separates average mission energy from maximum electrical demand. Average watts determine how many watt-hours the mission consumes; continuous and peak current determine whether the battery can actually support the propulsion system and other loads without excessive voltage drop or protection shutdown.

Thrusters: Average Demand vs Full-Throttle Peaks

Thrusters are often the largest variable load, but adding the maximum power rating of every thruster and multiplying by mission duration rarely represents a normal dive.

Instead, estimate the time the vehicle is likely to spend:

• cruising;
• hovering or holding position;
• ascending and descending;
• maneuvering;
• working against current;
• operating at high thrust.

The resulting duty cycle produces a more realistic average propulsion load.

Peak demand should still be calculated separately. An ROV may briefly command several thrusters at high output while changing direction, accelerating, countering a current, or stabilizing the vehicle. The battery, BMS, wiring, connectors, and motor controllers must all tolerate that demand.

Blue Robotics demonstrates this separation clearly in its BlueROV2 battery requirements. Compatible alternatives are specified not only by energy-related capacity and voltage, but also by at least 60A continuous discharge and 120A burst discharge. The recommended minimum capacity is 10Ah, while higher capacity is associated with longer runtime.

In practical terms:

Average power helps size Wh. Peak current helps size the electrical path.

Neither figure replaces the other.

Cameras, Lights, Sonar and Manipulators

Propulsion is only one part of the ROV power budget.

Add every load that draws energy from the onboard battery, including:

• onboard computers and control electronics;
• cameras and video systems;
• LED lighting;
• sonar;
• navigation sensors;
• communications hardware;
• manipulators and grippers;
• pumps or sampling equipment;
• scientific sensors and other payloads.

Some loads remain nearly constant while the ROV is operating. Others are intermittent. A manipulator may run for seconds at a time, while a computer, camera, or navigation system may remain active throughout the dive.

This is where a component-by-component spreadsheet becomes useful. Record the operating wattage and estimated duty cycle of each load, then calculate its contribution to the mission average rather than simply adding every device’s maximum rating.

The result should answer two different questions:

1. What is the realistic average power during the planned mission?
2. What is the highest simultaneous electrical load the battery may experience?

Duty Cycle, Water Current and Tether Drag

The same ROV can consume very different amounts of energy on two missions.

A slow visual inspection in calm water may keep propulsion demand relatively low. Holding position in a strong current can require sustained thrust. A long tether can add hydrodynamic drag, and extra payload can change both mass and vehicle hydrodynamics.

This is one reason a published battery capacity cannot be translated directly into a universal runtime figure.

Commercial underwater drones show the relationship. Blueye specifies an 86.4Wh battery at about 2.5 hours of normal runtime and a 216Wh battery at about five hours, while explicitly noting that runtime depends on usage and equipment configuration. The figures are useful examples of complete vehicle performance, not universal Wh-to-hours conversion factors.

For a custom ROV, mission data is more useful than another vehicle’s advertised runtime.

MANLY Battery Capacity Options for Custom ROV Power Systems

MANLY Battery offers several 24V-class LiFePO4 platforms that can serve as starting points when engineering a custom battery-powered robotic system. The final ROV battery configuration still has to match the vehicle’s operating voltage, continuous and peak current, physical envelope, connector architecture, BMS requirements, and pressure-rated enclosure.

MANLY 24V 20Ah LiFePO4 Battery — 512Wh

25.6V nominal | 20Ah | 512Wh | 20A continuous discharge | 5.7kg | IP67

At 512Wh, this MANLY lithium battery provides a useful reference point for ROV designs whose measured mission energy fits the roughly half-kilowatt-hour class. The 20A continuous discharge rating is just as important as the 512Wh figure: at 25.6V nominal voltage, the designer must confirm that the complete vehicle’s sustained current stays within the required electrical limits and that short-duration loads are handled by the finalized battery specification.

The 229 × 138 × 212mm enclosure and approximately 5.7kg weight also make the mechanical calculation visible early in the project. Those numbers affect battery-compartment space, buoyancy, trim, and overall vehicle mass instead of being secondary details after capacity has been chosen.

The battery uses LiFePO4 chemistry and an integrated BMS with overcharge, over-discharge, over-current, over-voltage, short-circuit, and over-temperature protection. MANLY also offers battery customization, which is useful when an ROV program needs a different current rating, enclosure arrangement, connector, or packaging strategy rather than an off-the-shelf installation.

MANLY 24V 30Ah Robot Battery — 768Wh

25.6V nominal | 30Ah | 768Wh | 30A continuous discharge | 60A peak for 1–3 sec | optional RS485/RS232/CANBus

The 768Wh robot battery is a particularly relevant engineering platform when the ROV requires more stored energy while also needing a defined short-duration current capability. Its 30A continuous rating and 60A peak rating for one to three seconds let the electrical designer evaluate sustained operation and transient propulsion demand separately.

Communication is another useful integration point. Optional RS485, RS232, and CANBus interfaces can support robotic systems that need battery information to interact with vehicle controls or monitoring hardware. Voltage, capacity, current, dimensions, housing, and output arrangements can also be customized.

Those features make the 24V 30Ah platform a logical starting point for an OEM or engineering team that already knows its target Wh, current envelope, and available battery volume but still needs the pack architecture adapted around the robot. The 768Wh rating should not, however, be treated as a promise of a specific number of dive hours; actual endurance remains a function of the complete mission load.

MANLY 24V 50Ah LiFePO4 Battery — 1,280Wh

25.6V nominal | 50Ah | 1,280Wh | 50A continuous discharge | 12.3kg | IP67

The 1,280Wh platform moves the energy budget into a substantially larger class while increasing continuous discharge capability to 50A. That combination can suit projects with longer operating windows, higher average loads, or additional onboard equipment when the ROV’s mechanical design can accommodate the corresponding battery mass and enclosure volume.

Its approximately 12.3kg weight and 330 × 173 × 221mm dimensions illustrate why simply selecting the highest available Wh rating is not an engineering shortcut. A larger battery has to be considered alongside displacement, buoyancy material, center of gravity, pressure-housing dimensions, and service access.

As with the smaller MANLY packs, the integrated BMS protects against major electrical fault conditions, and custom battery engineering is available when the production system requires a different physical or electrical configuration. For an ROV developer working with a battery manufacturer, that ability to coordinate energy, current, packaging, and BMS requirements is often more useful than choosing capacity in isolation.

Why Watt-Hours Alone Do Not Define the Right ROV Battery

Watt-hours tell you how much nominal energy a battery stores. They do not tell you whether the battery can operate at the vehicle’s voltage, supply a high-current maneuver, fit inside the available enclosure, or remain above the electronics’ minimum voltage during a transient load.

Two packs with similar stored energy can therefore behave very differently in the same underwater robot.

Match Nominal Voltage and the Full Operating Voltage Window

Start with the vehicle’s electrical architecture, not simply a “24V” or “48V” label.

Check:

• nominal battery voltage;
• fully charged voltage;
• discharge cutoff voltage;
• ESC input range;
• DC/DC converter input range;
• onboard electronics limits;
• charger compatibility;
• BMS operating window.

A 25.6V nominal LiFePO4 pack, for example, does not remain at exactly 25.6V throughout the discharge cycle. The complete operating voltage range has to remain compatible with the propulsion and electronics systems.

Voltage also affects current. Because electrical power is:

P = V × I

a higher-voltage architecture can deliver the same power at a lower current, but that does not make higher voltage automatically suitable. ESCs, BMS hardware, insulation, connectors, chargers, and other components must all be designed for the selected voltage.

Check Continuous Current, Peak Current and Voltage Sag

A battery can contain enough watt-hours for the planned mission and still be electrically unsuitable.

Three ratings should be considered independently:

• average current, which reflects typical mission consumption;
• continuous current, which defines sustained load capability;
• peak current and peak duration, which matter during short high-power events.

Internal resistance also matters. A sudden current increase can pull terminal voltage downward. Excessive voltage sag can reduce the voltage available to thrusters or electronics and, depending on system design, may activate protection thresholds.

That is why an ROV battery specification should follow the entire current path: cells, internal conductors, BMS, fuse, cables, connectors, ESCs, and motors.

Blue Robotics’ BlueROV2 requirements provide a practical industry example. Its alternative battery guidance defines voltage, continuous current, burst current, capacity, connector type, and physical dimensions rather than treating capacity as the only compatibility parameter.

IP Rating Is Not the Same as a Pressure-Rated Battery Enclosure

An IP67 or IP68 rating should not be interpreted as a subsea depth rating.

An IP rating addresses defined ingress-protection test conditions. A submerged ROV battery system may also have to withstand hydrostatic pressure, repeated pressure cycling, seal compression, corrosion, temperature changes, mechanical loads, and the pressure created at the intended operating depth.

For that reason, an IP67 MANLY battery should not automatically be described as a battery that can operate exposed at a specified ROV depth.

One common engineering architecture places the battery inside a dedicated pressure-rated enclosure. Blue Robotics, for example, installs the BlueROV2 battery inside a separate battery enclosure with sealed end caps, penetrators, O-rings, and a pressure-relief arrangement.

For custom underwater equipment, the electrical battery pack and the subsea pressure system should therefore be specified and validated as related but distinct design layers.

Turn Watt-Hours into a Battery Pack That Fits the ROV

Once the target energy is known, the next question is whether that energy can be integrated into the vehicle without disrupting the mechanical design.

More Wh can extend the available energy budget, but the pack also occupies space and adds mass. Those changes affect buoyancy, trim, pressure-housing requirements, maintenance access, and sometimes hydrodynamic drag.

Battery Mass and Enclosure Volume

A practical ROV battery selection table should include more than capacity:

This is also where custom manufacturing can be valuable. A standard rectangular lithium battery may satisfy the electrical requirement but use the available pressure-housing volume poorly. Conversely, a pack that fits perfectly can still be unsuitable if it cannot supply the required current.

The battery manufacturer therefore needs the electrical and mechanical requirements together.

Buoyancy, Trim and Center of Gravity

Battery weight does not disappear underwater.

Changing battery mass or position can move the ROV’s center of gravity and alter the relationship between the center of gravity and center of buoyancy. The result can affect pitch, roll, stability, and the amount or position of buoyancy material required to achieve the intended trim.

Pressure housing can compound the tradeoff. The source material supplied for this project correctly highlights that battery capacity, housing utilization, vehicle weight, and buoyancy are linked design variables rather than independent specifications.

The useful design question is therefore not simply:

“Can we add another 500Wh?”

It is:

“Can we add the required energy while keeping the vehicle’s electrical limits, enclosure volume, buoyancy, trim, and payload targets within specification?”

One Large Pack or Swappable Battery Modules?

More mission time does not always require one physically larger pack.

Depending on the ROV architecture, an engineering team may consider:

• a larger onboard battery;
• multiple battery modules;
• field-swappable packs;
• parallel battery architecture designed for that purpose;
• or surface-supplied power for very long operations.

The appropriate arrangement depends on mission duration, turnaround requirements, vehicle access, pressure housing, charging logistics, fault isolation, current sharing, and the way the ROV is deployed.

Swappable batteries can be especially useful when fast redeployment matters. Blueye, for example, uses field-swappable batteries across its underwater drone platform, while Blue Robotics offers both battery power and a topside surface-power architecture for operations where continuous runtime is required.

For a custom design, multiple packs should not simply be connected together without engineering the BMS behavior, current sharing, fusing, connectors, charging strategy, and pack matching.

How to Validate Your ROV Battery Size Before Deployment

A calculated ROV battery capacity is an engineering estimate. Final sizing should be validated with voltage, current, energy, temperature, and mission-time data collected under operating conditions that resemble the intended underwater work.

The most reliable design loop is:

Estimate → integrate → measure → validate → refine

Log Voltage, Current and Energy Consumption

Battery monitoring converts assumptions into usable mission data.

At minimum, record:

• battery voltage;
• current;
• peak current;
• consumed Ah;
• consumed Wh;
• mission duration;
• battery or enclosure temperature where relevant.

An autopilot or dedicated power monitor can help collect these values. ArduPilot Sub, for example, supports battery monitors that provide real-time voltage and current monitoring and can support low-battery failsafe functions.

The resulting logs reveal whether the original average-power estimate is realistic and whether high-thrust events create problematic current or voltage behavior.

For a development program, this is significantly more useful than assuming that a battery’s rated Wh will translate directly into a fixed number of underwater hours.

Test a Representative In-Water Mission Profile

A bench test is useful for validating electrical function, but it does not recreate the entire underwater operating load.

A representative in-water test should include the activities the vehicle is actually expected to perform, such as:

• cruising;
• hovering;
• vertical movement;
• rapid maneuvering;
• position holding;
• operation in realistic currents;
• normal lighting levels;
• sonar or sensor use;
• manipulator or tool operation;
• the intended payload.

If a vehicle spends much of its real mission holding position in current, a test dominated by low-power cruising will underestimate energy use.

Similarly, an inspection ROV carrying lights and sonar should be tested with those loads active rather than using propulsion-only measurements.

Re-Size the Pack from Measured Mission Data

Once real mission data is available, recalculate the target energy using measured average watts rather than initial estimates.

Suppose the development estimate was 300W but in-water logging shows a 380W average under the representative mission profile. A two-hour theoretical requirement changes from 600Wh to:

380W × 2h = 760Wh

Usable-energy allowance, system efficiency, battery condition, temperature, and engineering reserve must still be considered after that calculation.

The same data should also be used to review:

• maximum observed current;
• duration of current peaks;
• minimum loaded voltage;
• temperature behavior;
• remaining capacity at recovery.

This feedback loop turns battery sizing from a catalog decision into a validated vehicle requirement. It also gives a battery manufacturer much better information for configuring capacity, BMS current limits, communication, connectors, wiring, and packaging for subsequent prototypes or production.

ROV Battery Sizing FAQs

How Long Should an ROV Battery Last?

There is no universal ROV battery runtime. Endurance depends on usable Wh, average propulsion load, water current, payloads, lights, sonar, electronics, tether drag, temperature, and the operating profile. Complete commercial systems illustrate the variation: Blueye rates its 86.4Wh and 216Wh batteries for different normal runtimes while noting that equipment and usage affect the result. For a custom ROV, measured average watts provide the more reliable basis for estimating endurance.

Is a Higher-Ah Battery Always Better for an Underwater Drone?

No. Amp-hours do not show total stored energy unless voltage is also known, so compare watt-hours first. A higher-capacity pack can also add mass and occupy more enclosure volume. The battery still needs the correct voltage window, continuous and peak current capability, BMS configuration, connector rating, and mechanical fit. Select the capacity that supports the mission while remaining compatible with the complete ROV architecture.

Can a Marine LiFePO4 Battery Be Used in an ROV?

A marine LiFePO4 battery can provide suitable voltage, energy, and current for some ROV architectures, but those electrical specifications alone do not establish subsea compatibility. An IP-rated marine battery should not automatically be treated as pressure-rated for a particular operating depth. The lithium battery pack, electrical interfaces, pressure enclosure, seals, corrosion protection, thermal behavior, and complete vehicle design all require verification for the intended underwater environment.

Conclusion

The right ROV battery starts with the mission, not with the largest Ah figure that fits the budget.

Calculate theoretical energy from average watts × mission hours, then account for usable capacity, efficiency, and an appropriate engineering reserve. Separately verify nominal and maximum voltage, continuous current, short-duration peak current, loaded voltage behavior, BMS limits, connector ratings, battery dimensions, weight, buoyancy effects, and the ROV’s pressure-rated enclosure.

MANLY Battery’s 512Wh, 768Wh, and 1,280Wh 24V-class LiFePO4 platforms provide practical starting points for different energy requirements, while custom voltage, capacity, current, packaging, BMS, communication, and connector configurations can support OEM projects that need a pack designed around the vehicle rather than the other way around.

For a custom ROV battery evaluation, define the target voltage, measured or estimated average power, continuous and peak current, required mission time, available battery-space dimensions, target battery mass, operating temperature, connector requirements, communication interface, and pressure-enclosure architecture. Those inputs provide a much stronger basis for engineering the battery system than capacity alone.

Learn More About Battery

ROV Battery Sizing Guide: How Many Watt-Hours Does Your Underwater Drone Need?

How Do Underwater Drones Compare in Terms of Battery Life?

How Long Do Underwater Drone Batteries Last? ROV Runtime Explained by Mission Type

Server Rack Battery Safety in the U.S.: UL 1973, UL 9540A, BMS and System Protection

12...17