Lithium on boats. The standards, explained.
Which standards apply to a lithium battery on a boat, what each one asks for, and what a surveyor, examiner or insurer will want to see. Written for UK boats, with the EU and US equivalents where they differ.
Five things to know first.
- 01
ISO 23625:2025 is the standard for lithium on small craft. It applies to installed lithium-ion battery systems above 500Wh, for house loads or electric propulsion, and covers selection, BMS protection, location, fire protection and the safety information the manufacturer must provide.
- 02
It works with ISO 13297, not instead of it. ISO 23625 requires the installation to follow ISO 13297:2020, the small craft electrical standard that sets the rules for battery location, securing, the battery isolator and fusing.
- 03
The battery must meet IEC 62619 and IEC 62620. ISO 23625 asks for both: IEC 62619 for safety and IEC 62620 for performance.
- 04
The Boat Safety Scheme has no lithium-specific checks. Its general battery and electrical checks still apply to a lithium battery, and its advice points installers to ISO 23625.
- 05
Commercial and coded vessels follow the MCA. MGN 550 is the Maritime and Coastguard Agency's best-practice guidance on lithium-ion, and the vessel's Certifying Authority has the final say.
Every document, in one table.
| Document | What it covers | Applies to | Status |
|---|---|---|---|
| ISO 23625:2025 Small craft, lithium-ion batteries | Selection and installation of lithium-ion batteries: BMS protection, location, fire protection, manufacturer safety information | Installed systems above 500Wh, for house loads or propulsion | International standard, published March 2025. Replaced ISO/TS 23625:2021 |
| ISO 13297:2020 Small craft, electrical systems | AC and DC installations: battery location and securing, the battery-disconnect switch, overcurrent protection and cabling | Craft up to 24m hull length; DC up to 50V, AC up to 250V | International standard. EN ISO 13297:2021 is a designated standard under the UK Recreational Craft Regulations 2017 and harmonised under the EU Recreational Craft Directive |
| ISO 16315:2026 Small craft, electric propulsion | Design and installation of electric and hybrid propulsion systems | Craft up to 24m with electric propulsion | International standard, used alongside ISO 23625 for the battery |
| IEC 62619:2022 | Safety of lithium cells and batteries in industrial applications, including marine | Cells and battery packs | International standard. Required by ISO 23625 clause 4.6 |
| IEC 62620 | Performance of lithium cells and batteries in industrial applications | Cells and battery packs | International standard. Required by ISO 23625 clause 4.6 |
| UN 38.3 | Transport tests for lithium batteries | Every lithium battery that is shipped | UN Manual of Tests and Criteria. Needed to ship a battery; not part of ISO 23625 |
| Boat Safety Scheme | Safety examination of boats on UK inland waterways | Boats licensed or registered with the Canal & River Trust, the Environment Agency or the Broads Authority, among others | Examination needed for the licence. Its lithium advice is guidance, not an examination check |
| MGN 550 Amendment 1 | MCA best practice for the design, installation and operation of lithium-ion batteries on vessels | UK vessels, with most weight for commercial and coded vessels | Official guidance, January 2024 |
| MGN 681 Amendment 1 | Stowage and charging of lithium-powered tenders and water toys on yachts | Yachts carrying electric small craft; not house or propulsion batteries | Official guidance, December 2025 |
| Workboat Code Edition 3 | Small workboats and pilot boats. Annex 1 covers lithium-ion propulsion and references ISO 23625 | New UK workboats and pilot boats up to 24m | MCA code, in force since December 2023 |
| ABYC E-13 | US standard for lithium-ion batteries on boats | Boats built for the US market; systems above 500Wh in the 2025 edition | US voluntary industry standard |
ISO 23625 is not yet listed as a designated standard under the UK Recreational Craft Regulations 2017, or as a harmonised standard under the EU Recreational Craft Directive (lists checked October 2026). It is still the standard that the Boat Safety Scheme's lithium advice, the Workboat Code and the industry point to.
What applies to your boat.
| Your boat | What applies | Who checks it |
|---|---|---|
| A new boat, 2.5m to 24m, sold in the UK or EU | The UK Recreational Craft Regulations 2017 or the EU Recreational Craft Directive. EN ISO 13297 for the electrical system, ISO 23625 for the lithium installation | The builder, through UKCA or CE conformity assessment |
| A lithium upgrade on a private sea-going boat | No statutory approval for a private pleasure boat. ISO 23625 and ISO 13297 are the standards to install to | Your insurer, and any surveyor they appoint |
| A narrowboat or other boat on UK inland waterways | A Boat Safety Scheme examination for the licence. Its general battery and electrical checks apply to lithium | A BSS examiner |
| A coded or commercial vessel | The MCA code for the vessel, MGN 550, and Annex 1 of the Workboat Code Edition 3 for lithium propulsion | The vessel's Certifying Authority |
| Electric or hybrid propulsion | ISO 16315 for the propulsion system and ISO 23625 for the battery. Systems at 60V or more carry extra safety requirements | The builder or installer, and the Certifying Authority on a commercial vessel |
| A boat built for the US market | ABYC E-13, with ABYC E-11 for the electrical system | The builder and the surveyor |
What the standard actually asks.
The main requirements, with clause numbers so you can find them in the standard.
Install to ISO 13297
Every lithium installation must also meet ISO 13297. The standard notes that lithium-ion batteries do not release gas in routine use, so the lead-acid gas ventilation rules do not apply.
A BMS on every battery
A battery management system must control all installed lithium-ion batteries and cut the battery off if hazardous conditions exist.
IEC 62619 and IEC 62620
The batteries must meet both: IEC 62619 for safety and IEC 62620 for performance.
An alarm before disconnect
Where the battery powers propulsion or equipment vital to safety, the operator must get a visual or audible alarm at the main helm before the battery disconnects.
Safe operating limits
The manufacturer sets the voltage, current and temperature limits, and they must be kept to in design, storage, installation and operation.
Temperature
The battery must not be installed where temperatures outside its acceptable range can occur.
Water
IP67 where the battery could be flooded or submerged, and IP55 everywhere else.
No bypassing the BMS
No connection may be made directly to the battery that would bypass the BMS.
Fusing
Overcurrent protection must not exceed the current the BMS can interrupt, and a bank whose fault current would exceed the fuse's interrupting rating must be split.
Clauses 7, 8 and 9 cover fire protection and cell venting, BMS testing, and the safety information and operator's manual the manufacturer must supply.
HVC, LVC, HTC, LTC. In plain English.
ISO 23625 defines four events and the BMS response to each. Surveyors and assessors ask about them by these names.
| Cut-off | The event | What the BMS does |
|---|---|---|
| High voltage (HVE, HVC) | A cell is charged above the cell maker's maximum voltage | Stops charging, to protect the battery from overcharge |
| Low voltage (LVE, LVC) | A cell is discharged below the cell maker's low voltage limit | Stops discharge, to protect the battery from over-discharge |
| High temperature (HTE, HTC) | A cell goes above its maximum temperature | Stops the current, to protect the cells from heat |
| Low temperature (LTE, LTC) | A cell goes below its minimum temperature | Stops charging, the first thing cold cells cannot safely take |
Every TITAN BMS has all four, with a warning stage ahead of each cut-off. Charge and discharge switch separately, so after a high-voltage cut-off the loads stay powered, and after a low-voltage cut-off the battery still accepts charge. A built-in heater warms the cells so the battery can take charge in the cold. Models with the comms package send charge and discharge limits over CAN, so a Victron system with DVCC can stop chargers and loads before a cut-off is reached.
The full limit tables are available to installers, surveyors and assessors on request.
The electrical rules still apply.
- Location. Dry, ventilated and above the anticipated bilge water level (clause 8.1).
- Securing. The battery must not move more than 10mm in any direction (clause 8.2). The Boat Safety Scheme checks the same 10mm limit on inland boats.
- Isolation. A battery-disconnect switch in the positive conductor, readily accessible and as close as practical to the battery (clause 9.1). A BMS cut-off is electronic and protects the battery; it is not an isolator, so the switch is still needed.
- Fusing. Clause 12 covers DC overcurrent protection. For lithium, the battery fuse also needs an interrupting rating above the battery's short-circuit current. We recommend an MRBF fuse on the battery positive within 175mm, or Class T on our 460Ah and 520Ah or with an inverter of 1000W or more.
BSS, MCA and your insurer.
Boat Safety Scheme
The BSS examination has no lithium-specific checks. The general ones still apply: the battery must be secured, its terminals protected, and the electrical system able to be disconnected safely and quickly. BSS advice says lithium should be installed by a competent installer to suitable industry standards, naming ISO 23625, using batteries that meet IEC 62619 and IEC 62620. You need a valid BSS certificate to license a boat on Canal & River Trust and Environment Agency waters, and to register one on the Broads.
Maritime and Coastguard Agency
MGN 550 Amendment 1 (January 2024) is the MCA's best-practice guidance for lithium-ion on vessels, covering design, the BMS, ventilation, firefighting and isolation, and it references ISO 13297, IEC 62619 and IEC 62620. MGN 681 Amendment 1 covers stowing and charging lithium-powered tenders and water toys on yachts, not house or propulsion batteries. For new workboats and pilot boats, Annex 1 of the Workboat Code Edition 3 sets the rules for lithium propulsion and references ISO 23625.
Your insurer
Tell your insurer when you fit lithium. UK insurer guidance commonly asks for professional installation and use to the manufacturer's recommendations, and some policies also rule out unattended charging or require a lithium-specific extinguisher on board.
Our boat insurance checklistVentilation, venting and fire.
- Ventilation. LiFePO4 does not release gas in normal charging and discharging, so it needs none of the gas ventilation a flooded lead-acid battery does. It needs a location inside its temperature limits, with air around the case and away from engine heat.
- Cell venting. Each cell has a pressure relief vent that opens only under abuse or a fault. The gas is flammable and harmful, so the installation should let it disperse away from the people on board rather than collect in the accommodation.
- Detection. Fit a heat or smoke detector in or beside the battery compartment, with an alarm that can be heard at the helm.
- Extinguishers. For lithium battery fires, carry an aqueous vermiculite dispersion (AVD) extinguisher such as Lith-Ex, which its maker states holds marine equipment approval in its larger sizes.
Bilge water and immersion.
IP55 means protected against water jets from any direction. IP67 means protected against temporary immersion under set conditions. Neither makes a battery suitable for use under water.
A TITAN case is IP67 with the comms ports closed and IP32 while a comms cable is fitted, so mount the battery in a dry place above the bilge water line, as ISO 13297 asks, or in an enclosure that gives the protection.
Salt water conducts. Across the terminals it drives electrolysis, which gives off hydrogen and chlorine and corrodes the connections fast. Fresh water does the same more slowly. After any immersion, isolate the battery, do not charge or use it, and contact us.
What to have ready for a surveyor.
- IEC 62619 pack type test certificate
- IEC 62620 cell certificate
- UN 38.3 test summary
- Safety data sheet
- BMS limits for HVC, LVC, HTC and LTC
- Battery short-circuit current
- Fuse type and interrupting rating
- Isolator position
- Wiring diagram showing how chargers and loads are controlled
TITAN supplies the battery-side documents together: the Certificate of Compliance, the IEC test certificates and the safety data sheet, with the BMS limits on request.
Common questions
Is ISO 23625 a legal requirement in the UK?
Does the Boat Safety Scheme allow lithium batteries?
Does a lithium battery on a boat need ventilation?
What IP rating does a marine lithium battery need?
Do I still need a battery isolator if the battery has a BMS?
What fire extinguisher should I carry for a lithium battery on a boat?
What do HVC, LVC, HTC and LTC mean?
Do I need to tell my insurer about a lithium battery?
Built for the boat, and the paperwork.
Type tested to IEC 62619:2022 in our own name, IEC 62620 cells, a custom BMS with all four cut-offs and an IP67 case, backed by a lifetime, fully transferable warranty.
This guide summarises the standards and guidance as published at 15 October 2026 and does not replace the documents themselves. Clause numbers refer to ISO 23625:2025 and ISO 13297:2020.
