MGN 550 Explained  — What Every Boat Owner Should Know

 

 

 

 

Lithium-ion batteries are becoming increasingly popular on yachts and small craft. They offer lighter weight, higher power, better efficiency and quicker charging compared to traditional lead-acid batteries. But they also behave very differently and can be far more dangerous if they are not installed correctly.

 

 

 

 

 

To address this, the UK Maritime and Coastguard Agency (MCA) introduced MGN 550 (M+F). This guidance sets out how lithium-ion batteries should be designed, installed and operated safely on small craft. The aim of this article is to break down the guidance into simple, understandable terms for boat owners, marinas and anyone considering switching to lithium power.

 

 

 

 

 

MGN 550 applies to a large range of vessels including sailing yachts, motorboats, workboats, inland craft and any boat using lithium batteries for propulsion or house systems. In other words, if your boat has lithium batteries on board, the guidance almost certainly applies to you.

 

 

 

 

 

Lithium-ion batteries are powerful but sensitive. If a lithium cell is overheated, damaged, charged incorrectly or short-circuited, it can enter a condition known as thermal runaway. This is a chain reaction where the battery rapidly overheats, produces toxic smoke and, in severe cases, can cause fire or explosion. MGN 550 exists to reduce the risk of this happening.

 

 

 

 

 

One of the most important parts of the guidance involves where the battery is located. The battery must be positioned in a safe, protected area away from heat sources, fuel lines and anything that might damage it. It should not be placed where it could get crushed, soaked or overheated. The space must also be properly ventilated so that heat and gases do not build up. MGN 550 recommends at least six air changes per hour in the battery space.

 

 

 

 

 

Another major requirement is correct cabling and electrical protection. Lithium systems can deliver very high currents. Because of this, the wiring must be marine-grade, correctly sized and well-secured. A fuse or circuit breaker must be installed within 200 mm of the battery’s positive terminal to protect the system. This prevents an electrical fire if the cable is damaged.

 

 

 

 

 

MGN 550 also requires every lithium battery to have a Battery Management System, or BMS. This is an electronic safety device built into the battery that monitors temperature, voltage and current. If something goes wrong — such as overheating, overcharging or a short circuit — the BMS automatically disconnects the battery to prevent damage or thermal runaway. Without a functioning BMS, a lithium battery installation is unsafe.

 

 

 

 

 

The guidance also requires a clearly-labelled emergency isolation switch. This must be easy for the crew to reach and must instantly disconnect power in an emergency. Fire detection is another key part of the regulation. The battery compartment should have appropriate detection, such as smoke or heat sensors, and should be protected by a clean-agent extinguisher system. MGN 550 specifically advises against CO₂ extinguishers for lithium battery spaces.

 

 

 

 

 

MGN 550 applies not only to new installations but also to upgrades, retrofits and existing boats where the lithium battery system may not have been professionally installed. Insurance companies are increasingly requesting evidence of compliance with MGN 550 before offering or renewing cover. In some cases they may ask for a professional safety inspection to confirm that the installation is suitable and meets the guidance.

 

 

 

 

 

Marine Battery Safety provides independent inspections, compliance checks and documentation reviews based on MGN 550, MGN 505, MGN 681 and relevant ISO standards. This includes checking battery location, ventilation, cabling, protection devices, emergency isolation, BMS functionality and general system design. All work is advisory-only and non-intrusive, providing boat owners with peace of mind and insurers with confidence in the installation.

 

 

 

 

 

If you are installing new lithium batteries, converting to electric propulsion, upgrading an older installation or purchasing a boat with lithium power you know little about, a safety inspection is one of the best ways to ensure everything is safe and compliant.

 

 

 

 

 

For advice or to arrange a safety assessment, contact Marine Battery Safety. We serve the North West of England, wider UK, and Europe on request.