ROV Battery vs Surface Power: Which Power Architecture Fits Your Mission?

An ROV Battery is usually the better fit when mobility, fast deployment, and a lighter power-free tether matter, while surface power makes more sense when the vehicle must remain underwater for long periods without stopping for battery changes. Hybrid systems can combine elements of both when the mission has variable loads or needs backup energy.

The decision is therefore not simply battery capacity versus unlimited runtime. Power architecture changes tether diameter, electrical losses, vehicle mass, surface equipment, charging workflow, peak-power capability, and even how an ROV behaves in current. Those system-level tradeoffs should be defined before an OEM selects voltage, amp-hours, or a battery manufacturer.

What Changes When an ROV Battery Moves Power Onboard?

Moving the primary energy source onto the ROV separates propulsion power from the surface tether. That can reduce the amount of copper required in the tether and make the tether thinner, but the vehicle must now carry enough stored energy and discharge capability for the complete mission.

Commercial designs demonstrate why this distinction matters. Deep Trekker places lithium-ion batteries inside its ROVs specifically so propulsion power does not have to travel through the tether, reducing tether diameter and potential drag.

Runtime, Energy Capacity, and Peak Current

Battery capacity tells only part of the story. A useful ROV power specification needs at least three electrical numbers: stored energy in watt-hours, continuous current capability, and short-duration peak current capability.

Watt-hours determine the theoretical energy available over time. Amp-hours alone can be misleading because a 30Ah pack at 24V stores far less energy than a 30Ah pack at 48V.

A simple planning relationship is:

Energy required (Wh) ≈ average electrical load (W) × operating time (hours)

If an ROV averages 500W for a two-hour mission, the load alone represents roughly 1,000Wh before accounting for conversion losses, reserve energy, temperature, battery aging, or operational margin.

Peak current is a separate requirement. Thrusters accelerating together, holding position against current, or operating alongside a manipulator and high-power lighting can produce a short load well above the mission average. An ROV Battery therefore needs enough energy for endurance and enough current capability to avoid voltage sag or BMS protection trips during demanding maneuvers.

Tether Drag, Mobility, and Vehicle Mass

An onboard battery adds mass and volume to the vehicle, but removing propulsion power conductors can simplify the tether.

That tradeoff becomes more important as tether length increases. VideoRay documents the relationship directly: larger conductors improve electrical power transmission, but larger conductors also produce a thicker tether and greater drag. Deep Trekker similarly identifies reduced tether diameter as one advantage of placing batteries onboard.

This matters because tether drag is not simply an inconvenience at the surface. Water moving across a long tether creates force that the ROV must overcome. More drag can consume part of the available thruster authority that would otherwise be used for positioning, inspection, or carrying payload.

For mobile inspection work, an onboard ROV Battery can therefore affect maneuverability indirectly by changing what the tether needs to carry.

Charging, Battery Swaps, and Mission Turnaround

Battery-powered architecture changes the question from “Can the surface source keep supplying power?” to “How much productive mission time can the battery workflow support?”

For occasional inspection dives, recharge time may have little operational impact. For repeated daily missions, spare packs, charging current, changeover time, and the number of chargers can determine total system availability.

A swappable pack can separate vehicle turnaround from charging time: one pack operates while another charges. A fixed pack requires the operator to plan around recharge windows or another form of opportunity charging.

Deep Trekker, for example, uses different charging strategies across its ROV platforms, including swappable batteries on the REVOLUTION and operating-time charging support on certain other systems. The broader engineering lesson is that charging architecture should be planned with the mission schedule rather than treated as an afterthought.

What Changes When Surface Power Runs Through the Tether?

Surface power removes the onboard energy ceiling, but it moves the engineering challenge into the tether and power-distribution system. Conductor resistance, transmission voltage, conversion efficiency, insulation monitoring, surface equipment, and tether handling become central design variables.

Commercial systems illustrate this architecture. Oceanbotics converts a 100–240V AC surface input into regulated 400V DC for transmission through its Topside Power System tether, while VideoRay Mission Specialist systems also use 400V DC tether power.

Continuous Runtime vs Tether Losses

Surface power can keep an ROV operating without returning solely to exchange or recharge propulsion batteries, provided the surface power source and system remain available. It does not remove electrical losses.

A tether conductor has resistance. Power lost as heat rises with the square of current:

Power loss = I²R

That relationship is one reason low-voltage, high-current transmission becomes increasingly difficult as distance and power demand rise.

If the ROV demands more current, voltage drop across the tether rises. If engineers compensate by using substantially larger conductors, tether weight and diameter can also increase. The power architecture therefore has to balance electrical efficiency against hydrodynamic and handling requirements.

VideoRay documentation captures that engineering compromise: larger tether conductors provide greater power-transmission capacity, while thicker tethers create more drag.

Transmission Voltage, DC-DC Conversion, and Tether Design

Higher transmission voltage allows the same power to move through the tether at lower current.

For example, ignoring losses for illustration:

• 2,000W at 48V requires about 41.7A.
• 2,000W at 400V requires 5A.

Lower current dramatically changes conductor losses because of the I²R relationship. The vehicle can then use isolated DC-DC conversion to bring the high tether voltage down to the rails required by thrusters, controllers, sensors, lights, and payload equipment.

This architecture is not theoretical. VideoRay power systems use 400V DC through the tether, while a documented modular ROV power architecture uses 400V tether input and converts it to a regulated 48V onboard bus before supplying lower-voltage electronics.

Higher voltage also raises insulation, connector, monitoring, and service requirements. Transmission voltage should therefore be engineered as part of the complete ROV electrical system, not increased independently to reduce cable loss.

Topside Equipment, Electrical Protection, and Deployment Complexity

A surface-powered vehicle needs more than a cable. The system may include an AC input stage, power conversion, emergency shutdown, overcurrent protection, insulation or ground-fault monitoring, a tether reel or management system, connectors, and an ROV-side converter.

VideoRay’s Mission Specialist documentation, for example, specifies a GFCI/circuit breaker on the surface system and a Line Insulation Monitor on its 400V DC ROV circuit.

Electrical protection deserves particular attention around water. OSHA explains that ground-fault protection is designed to interrupt current when a fault creates an unintended current path; protection equipment, grounding, insulation, and system design all have distinct roles.

For an OEM, that means “continuous power” should never be interpreted as simply routing high voltage down a longer tether. Surface-power architecture requires electrical design, environmental protection, fault management, and operating procedures appropriate to the actual system.

How Should You Size an ROV Battery for the Actual Mission?

Size an ROV Battery from the measured or estimated mission power profile, not from amp-hours alone. Runtime is an energy problem, while acceleration, station-keeping, tooling, and other high-load events are also current-delivery problems.

A battery that satisfies only one side of that equation may still be unsuitable for the vehicle.

Start With the Duty Cycle, Not Amp-Hours

Begin with everything that draws power:

• horizontal and vertical thrusters;
• cameras and onboard computers;
• lights;
• sonar and navigation sensors;
• manipulator or tooling loads;
• communications electronics;
• pumps, actuators, or payload equipment.

Next, estimate how long each load operates and at what power level. Thrusters rarely remain at maximum output for an entire inspection, while lights or control electronics may run continuously.

That produces a more useful energy budget than simply adding the maximum wattage of every component.

For prototype and validation work, actual voltage and current logging is preferable. A battery manufacturer can then see normal load, idle load, sustained high-load periods, and transient peaks rather than designing from a single nameplate number.

Account for Thruster Peaks and Payload Loads

Average energy consumption determines much of the runtime, but the BMS and cells must tolerate the current demanded at the hardest moment of the mission.

Consider an inspection ROV that cruises at modest thrust most of the time but occasionally has to resist current while operating a manipulator. The battery may have enough watt-hours for several hours of average operation yet still hit a current limit when propulsion and tooling demand peak simultaneously.

The design review should therefore define:

maximum continuous current + expected peak current + peak duration

Those values affect the cells, BMS, internal busbars, connectors, cables, fusing, and thermal behavior of the complete pack.

The same logic applies to the charger. A high-capacity LiFePO4 battery does not automatically support any desired charging current; the charging system must remain inside the battery’s specified charging limits and BMS logic.

Add Usable-Energy, Temperature, and Aging Margin

Nameplate watt-hours should not be treated as guaranteed mission energy.

The usable amount can be affected by battery protection thresholds, conversion efficiency, operating temperature, cell aging, current level, and the reserve the operator chooses to maintain for recovery.

Instead of applying one universal percentage to every ROV Battery, engineers should establish margin from the mission risk and validated system behavior. A shallow-water inspection vehicle that can be recovered easily does not necessarily require the same reserve philosophy as a difficult offshore deployment.

Battery specifications should also use realistic environmental limits. If a project must operate or charge at temperatures outside a standard pack’s validated range, the requirement belongs in the battery specification before enclosure and BMS design are finalized.

How Can MANLY Battery Support a Custom ROV Battery Architecture?

MANLY Battery can use existing LiFePO4 robotic battery platforms as electrical starting points for OEM underwater-robot projects. The final ROV Battery still needs to be engineered around vehicle voltage, current, mechanical envelope, charging method, communications, operating environment, and pressure enclosure.

A standard IP-rated robot battery should not be represented as a depth-rated subsea battery. IEC 60529 defines enclosure ingress-protection classifications; it does not by itself certify an enclosure for a specified subsea operating depth or external hydrostatic pressure.

24V 30Ah LiFePO4 Robot Battery as a Compact Electrical Baseline

MANLY 24V 30Ah LiFePO4 Robot Battery

Core parameters: 25.6V nominal, 30Ah, 768Wh, 30A continuous discharge, 60A peak for 1–3 seconds, optional RS485/RS232/CAN Bus, IP65 standard configuration.

The strongest reason to consider this platform during early ROV electrical design is not simply its 30Ah capacity. The combination of 768Wh of stored energy and clearly defined continuous and short-duration current limits gives an engineering team enough information to compare the pack with a preliminary mission power trace.

At 30A continuous discharge, the electrical architecture can be checked against sustained thruster and electronics demand; the 60A short peak provides a separate reference for transient loading. MANLY also supports customizable dimensions, enclosure materials, output connections, and optional communication, allowing an OEM project to integrate battery status and control data rather than treating the pack as an isolated power box.

For an ROV project, however, IP65 is not a subsea depth specification. This platform makes sense as an electrical and BMS baseline when a project needs a compact 24V-class LiFePO4 battery, while the finished underwater enclosure, connectors, pressure tolerance, thermal path, and vehicle-level validation are engineered for the required depth.

48V 60Ah LiFePO4 Robot Battery as a Higher-Energy Baseline

MANLY 48V 60Ah LiFePO4 Robot Battery

Core parameters: 51.2V nominal, 60Ah, 3,072Wh, 60A continuous discharge, 90A peak under 1 second; ABS/IP67 and metal/IP65 configurations are listed.

At 3,072Wh, this platform provides four times the nominal stored energy of the 768Wh 24V platform while also moving the vehicle to a 48V-class electrical architecture. The practical significance is not just longer theoretical runtime: for the same electrical power, a higher system voltage can reduce current on the battery-side distribution system, subject to the actual vehicle architecture and converter design.

The published specification supports 60A continuous discharge and a 90A peak below one second. Two enclosure configurations are listed: an approximately 24kg ABS version rated IP67 and an approximately 26kg metal version rated IP65. Optional communication and configurable connectors are also available.

Those distinctions give an OEM useful starting points for packaging and power-budget studies. They are not substitutes for subsea validation. A 48V ROV application still requires a housing and electrical interface designed for the intended immersion depth, pressure, buoyancy, heat transfer, connector system, and service procedure.

BMS, Communications, and the Subsea Enclosure Must Be Engineered Together

A custom ROV pack is a system-integration project, not simply a waterproof version of a catalog battery.

MANLY Battery supports OEM customization of voltage, capacity, current, dimensions, connectors, enclosure configuration, and communication interfaces. Its robot platforms include CAN Bus, RS485, and RS232 options, allowing the battery to exchange data with a vehicle controller where the project requires it.

That data can be used to design functions around state of charge, temperature, voltage, current, alarms, and charging logic, depending on the final BMS implementation.

For an underwater platform, the battery manufacturer and ROV engineering team should define at least:

• operating and maximum voltage;
• mission watt-hours;
• continuous and transient current;
• pack dimensions and mass limits;
• BMS protection thresholds;
• communication protocol;
• charger interface;
• connector and wiring requirements;
• operating and charging temperatures;
• enclosure and pressure requirements;
• transport and product qualification requirements.

Transportation qualification and subsea qualification should remain separate. In the United States, PHMSA states that lithium cells and batteries offered for transportation must be of a design that has passed the applicable UN 38.3 tests, while pressure tolerance and underwater system validation address different hazards.

For an OEM seeking a configurable LiFePO4 battery, this separation is useful: electrical performance, transport compliance, ingress protection, and subsea pressure design can be specified and validated as distinct requirements rather than being collapsed into a single “waterproof battery” claim.

When Does a Hybrid ROV Power Architecture Make More Sense?

A hybrid architecture becomes useful when average mission load, short power peaks, runtime, and recovery needs cannot be handled efficiently by a single power path.

The exact implementation varies. Surface power may carry the base load while an onboard battery buffers transient demand, or a tether may provide limited charging power while the battery remains the primary source for propulsion.

Battery-Assisted Surface Power

One hybrid approach is to size the tether power path around a steady or predictable portion of vehicle demand while using onboard storage to support short high-load events.

The engineering value is straightforward: the tether does not necessarily need to carry every instantaneous propulsion or tooling peak if the battery and power electronics are designed to absorb the difference.

This concept also shows why peak current should not be confused with average mission power. A vehicle might average a few hundred watts over an inspection yet demand considerably more during a brief maneuver.

Commercial ROV power architectures already demonstrate that different sources can feed a common onboard electrical system. A documented VideoRay modular architecture, for example, supports long-tether power as well as redundant 48V onboard battery sources feeding the vehicle’s regulated power network.

Tether Charging During Lower-Load Periods

A second approach uses the tether to send less power than the ROV could require at full load. When vehicle consumption falls below the available tether supply, excess power can be directed toward charging the onboard battery if the charger and BMS are designed for that operating mode.

This idea appears repeatedly in ROV engineering discussions because it can extend mission duration without forcing the tether to carry maximum thruster current continuously. Users specifically ask whether an onboard battery can remain connected while receiving charge from the surface, showing that power-sharing and charge-while-operating behavior are real design concerns rather than purely theoretical fan-out topics.

The implementation is not as simple as connecting a charger lead. Engineers must account for charger output, battery charge limits, simultaneous load behavior, BMS logic, cable voltage drop, grounding or isolation strategy, and what happens when ROV demand exceeds the tether supply.

Redundancy and Safe-Recovery Power

An onboard battery can also serve a narrower role: keeping selected systems alive after loss of the main surface-power path.

The required backup load may include control electronics, communications, navigation, or another recovery-critical subsystem rather than all thrusters and payloads. That distinction can reduce the amount of energy that must be carried onboard.

Redundancy should still be engineered around defined failure cases. A battery does not automatically guarantee recovery after any tether or vehicle fault. The design must define which equipment remains powered, how the ROV detects the failure, how long backup power is required, and what recovery action is physically possible.

For MANLY Battery OEM projects, custom BMS logic and communication interfaces can be specified around the intended role of the pack, whether the design calls for primary propulsion energy, peak support, or a dedicated backup-energy function.

ROV Battery or Surface Power: A Mission-by-Mission Decision Framework

The architecture should follow the mission. Start with operating time, tether length, current conditions, payload power, mobility, support equipment, and recovery strategy; then design the ROV Battery or surface-power system around those requirements.

Short Mobile Inspections and Remote Deployments

Onboard battery power is especially attractive when the team needs to carry the ROV to a remote location, launch quickly, and avoid generators or a high-power surface console.

Typical examples include hull inspection, tank or reservoir inspection, aquaculture checks, confined-site observation, and short search tasks.

A lighter tether that carries primarily communications rather than propulsion power can reduce cable handling and drag. The tradeoff is a finite energy budget, so the mission schedule should include reserve energy and either charging or battery replacement.

For an OEM program, a compact custom LiFePO4 battery can be designed around the actual vehicle envelope, current profile, and planned turnaround procedure rather than maximizing capacity at the expense of the complete vehicle.

Long-Duration Inspection, Tooling, and Station-Keeping

Surface power becomes compelling when stopping to change batteries would interrupt the work itself.

Pipeline inspection, extended observation, long training sessions, tooling, or lengthy station-keeping can place sustained energy demands on the vehicle. Oceanbotics positions its 400V DC Topside Power System for this type of continuous operation without onboard battery swaps.

The correct engineering question is then not simply “How long can it run?” but “Can the tether and power conversion system deliver the required continuous and peak power at the deployed length?”

Tether resistance, conductor cross-section, converter capacity, surface supply, thermal limits, and electrical protection all have to stay inside their design envelope.

Deep, Long-Tether, or Mixed-Duty Missions

Longer or more complex missions should be evaluated at system level rather than assigned automatically to battery or surface power.

Long tethers increase the importance of hydrodynamic drag and power-transmission loss. High-voltage transmission can reduce current and conductor loss, but it introduces conversion, insulation, connector, and protection requirements. An onboard battery avoids propulsion-power transmission through the tether, but stored energy and vehicle packaging become limiting variables.

Mixed-duty missions can therefore justify a third path: use onboard energy, surface energy, or both in proportions that fit the actual power trace.

The final selection should answer five questions:

1. How many watt-hours does the planned mission require?
2. What are the maximum continuous and short-duration loads?
3. How much tether can the vehicle tolerate hydrodynamically?
4. What surface infrastructure can the operating team realistically deploy?
5. What happens when the primary power path is interrupted?

Once those answers are known, battery voltage, capacity, discharge current, tether conductors, converters, charging hardware, and BMS communication become engineering outputs rather than guesses.

FAQ

Can an ROV battery be charged through the tether while the ROV is operating?

Yes, a properly designed system can use tether power to support vehicle loads and charge an onboard battery when available input power exceeds current consumption. The charger, BMS, battery charge-current limit, tether voltage drop, and isolation architecture must all support simultaneous operation. This approach can extend endurance without requiring the tether to carry every propulsion peak, but it needs deliberate power-management design rather than a direct charger connection.

Does a battery-powered ROV still need a tether?

Often, yes. Moving propulsion energy onboard does not necessarily remove the need for communications, video, telemetry, physical recovery, or other tether functions. Battery-powered commercial ROVs can therefore use a thinner tether because the tether does not have to carry propulsion power. Deep Trekker specifically uses onboard batteries to reduce tether diameter and associated drag.

Why do some ROVs send hundreds of volts through the tether?

Higher voltage reduces the current required to transmit a given amount of power. Lower current reduces I²R conductor losses and can reduce the amount of copper needed in a long tether. Commercial ROV systems from Oceanbotics and VideoRay use 400V DC tether architectures and convert the power as needed onboard. High-voltage systems also require suitable insulation, connectors, monitoring, and protection.

Is an IP67 battery automatically suitable for an underwater ROV?

No. IP67 is an enclosure ingress-protection classification, not a certification that a battery can withstand a specified subsea hydrostatic pressure or operate at a particular ocean depth. IEC 60529 covers enclosure protection against access, solids, and water ingress. An ROV Battery still needs pressure, connector, sealing, thermal, and complete-system validation for its intended depth.

What information should an OEM give a battery manufacturer for a custom ROV pack?

Provide nominal and maximum voltage, mission watt-hours, continuous current, peak current and duration, charging method, available dimensions, mass target, connectors, BMS communication, temperature range, environmental exposure, pressure-housing concept, service procedure, and qualification requirements. A qualified battery manufacturer can then design voltage, cells, BMS, wiring, enclosure interfaces, and charging behavior around the ROV rather than treating capacity as the only requirement.

Conclusion

The most important ROV power decision happens before battery capacity is selected.

An onboard ROV Battery works well when the mission values portability, reduced power-tether drag, and simple remote deployment. Surface power fits operations where continuous energy delivery outweighs the added tether and topside infrastructure. Hybrid systems create another engineering path when average power, peak demand, charging, and redundancy need to be handled differently.

For OEM projects, the most useful battery specification combines energy, continuous current, peak current, mechanical integration, BMS communication, charging strategy, and environmental requirements. MANLY Battery’s 24V 30Ah and 48V 60Ah robotic LiFePO4 platforms provide two existing electrical baselines, while custom voltage, capacity, current, dimensions, communication, connectors, and BMS integration can be developed around the final vehicle.

A custom LiFePO4 battery should ultimately be designed as part of the ROV power architecture—not selected as an isolated component.

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