How to choose a subsea battery pack

1. Energy capacity (watt-hours)

Capacity, measured in watt-hours (Wh), determines how long your system can run. Size it to your mission, not to a round number: estimate the average power your payload draws in watts and multiply by the runtime you need in hours. A pack that is too small cuts missions short; one that is needlessly large adds weight, buoyancy, and cost.

2. Depth rating and enclosure

The enclosure is what makes a battery “subsea.” Check the rated depth in metres and the housing material — aluminium is light and cost-effective for moderate depths, while titanium resists corrosion and pressure for the deepest work. Look at the connectors too: pressure-rated bulkhead connectors (such as MCBH types) matter for reliable integration.

3. Chemistry and voltage

Most modern subsea packs use lithium-ion for its high energy density and rechargeability. Confirm the nominal and maximum voltage match your system's bus, and that the cells are a known, safety-tested type. Higher voltage can reduce current and cabling losses for power-hungry tools and thrusters.

4. Power delivery: continuous and peak

Runtime is about energy; performance is about power. Check both the continuous current the pack can supply and its peak or inrush capability — thrusters, manipulators, and tools can demand large, brief surges. A pack that sags or trips under peak load will limit what your vehicle can do.

5. Charging — especially in place

How you recharge matters as much as capacity. In-application (in-place) charging lets you top up the pack without opening the enclosure or removing it from the vehicle, which is a huge time-saver on deck or at a docking station. Check the charge voltage and current, charge time, and whether the pack can charge from the supply you have available.

6. Power-path control and UPS behaviour

For tethered systems with a battery backup, power-path (UPS) behaviour keeps the low-voltage bus alive if the primary source drops — the battery takes over seamlessly, with no reboot. If uninterrupted operation matters to your mission, make this a requirement, not an afterthought.

7. Battery management and diagnostics

A good battery management system (BMS) protects against over-voltage, over-current, over-temperature, and short circuits, balances the cells, and reports state of charge and health. Telemetry over a standard interface (such as RS-485 / Modbus or CAN) lets you monitor the pack live and catch problems before they end a mission.

8. Safety and transport

Subsea lithium packs should be designed with cell-level safety, isolation, and thermal protection, and should meet transport requirements such as UN 38.3 for shipping. Ask about the certifications relevant to your operation.

9. Temperature and environment

Confirm the operating and storage temperature ranges suit cold deep water and warm deck storage alike, and that the sealing and materials suit saltwater exposure across the service life.

10. Lifecycle and total cost of ownership

Cells wear out; the enclosure, connectors, and electronics usually do not. Packs designed to be re-celled — refurbished with fresh cells rather than replaced whole — can dramatically lower lifetime cost and waste. Factor cycle life and serviceability into the price, not just the sticker.

Sizing on average power only. Ignoring peak and inrush current leads to packs that trip or sag under thrusters and tools.

Chasing capacity at the expense of integration. The biggest pack is useless if its connectors, voltage, or dimensions don't fit your vehicle.

Overlooking the charging workflow. If you can't charge in place, every turnaround costs time.

Forgetting the lifecycle. A cheaper pack that can't be re-celled can cost more over five years than a serviceable one.

Underrating safety and telemetry. Without live health data and robust protection, a subsea fault becomes a lost mission — or worse.