Bartlett Electrical

Lithium batteries on your boat: what's needed, what it means for you, and why

The short version

Fitting lithium batteries is not the same as swapping old batteries for new ones. Several parts have to work together as one system: the batteries, the electronics that look after them (the battery management system, or BMS), everything that charges them, and the cables and fuses around them.

This page explains what that involves before we start, and why each part matters.

Use lithium iron phosphate batteries (LFP) that come with proper safety information from the maker.
They need a space of their own: dry, secure and away from where you live. Never in the engine bay, and never near anything that gets hot or makes sparks.
Cables, fuses and switches must be strong enough to cope with a fault. Lithium batteries can release a lot of power very quickly if something goes wrong.
Everything that charges the batteries has to be set up for lithium, including the engine's alternator.
You need working smoke and carbon monoxide alarms, and a plan for getting off the boat.
Tell your insurer before the work starts, and keep their reply in writing.

Requirements checklist

Every lithium installation I do should tick all of these. The grey tag on each line shows where it comes from: ISO is ISO 23625:2025, BSS is the Boat Safety Scheme advice, and CA/IWA is the Cruising Association and Inland Waterways Association guidance note.

Where they go

Their own dry, secure space, separate from living areas, like a gas lockerCA/IWA 3.2
Not in the engine bay, and away from heat, sparks and flameMy requirement, based on ISO 6.1 and Annex A, and CA/IWA 3.2
Vented to the outside, with the vents kept clearCA/IWA 3.2 and Appendix 1 · ISO 7.4
Fixed down firmly. Watertight (IP67) if the space could flood, weatherproof (IP55) if notISO 6.3 and 6.4.1 / 6.4.2
Easy to reach to inspect and switch offCA/IWA 3.2

Mixing with lead-acid (hybrid systems)

Lithium is never wired in parallel or in series with lead-acid or any other type of batteryBSS advice (stricter than ISO 4.10, which only bans mixed types in series)
A lead-acid starter battery stays separate, linked only through equipment designed for the jobCA/IWA 3.4 and Appendix 1
Existing split-charge diodes, relays and "1-2-both" switches can't be assumed to be suitableCA/IWA 3.4 · ISO 6.10
Every charger, including the alternator, set up for lithiumISO 4.8 and 6.12 · CA/IWA 3.3

Standards and guidance I work to

Most of this is industry standards and guidance rather than law. It still matters, because it's what installers are expected to follow, and what insurers and surveyors may ask about.

Document	What it covers
ISO 23625:2025 Small craft, lithium-ion batteries	The main standard for choosing and fitting lithium batteries on boats. It applies to battery systems over 500 Wh (watt-hours, a measure of how much energy a battery holds). The Boat Safety Scheme says installations should follow it. Systems running at 60 volts or more have extra requirements, which the standard leaves to the battery maker.
ISO 13297:2020 Small craft, AC and DC electrical installations	The standard for the rest of the boat's electrics, both mains and battery power. The lithium batteries connect into this.
Boat Safety Scheme (BSS) advice Installed lithium-ion batteries	Use a competent installer. Use batteries that suit boats and meet two industry safety standards (IEC 62619 and IEC 62620). Never mix different types of battery. Fit a suitable BMS. Tell your insurer.
CA / IWA Guidance Note Installing Lithium Batteries for Domestic Power on Leisure Craft (Issue V1, 5 August 2026)	Joint guidance from the Cruising Association and the Inland Waterways Association, with a checklist for owners.
The battery maker's instructions	These always apply on top of everything else. They cover how the battery is mounted, how much space it needs, how it vents, the temperatures it can work in and which chargers are allowed.

This page is about leisure boats such as narrowboats and liveaboards. Commercial and passenger vessels have extra requirements from the Maritime and Coastguard Agency (MCA), which I'll go through with you separately.

What this means on your boat

Each part below says what's needed, then why it matters.

1. The batteries themselves

What's needed

LFP is the only type of lithium battery the CA/IWA group currently recommends for boats.
Each battery comes with a data sheet from the maker. It shows the capacity, how fast the battery can be charged and used, the temperatures it can work in, and whether it can be joined to others.
The batteries meet two industry safety standards (IEC 62619 and IEC 62620).
All the batteries in one bank come from the same maker.
Lithium is never joined to old lead-acid batteries.

Why it matters

Labels like "drop-in" or "marine grade" don't tell you what a battery does when it's overcharged, too cold or short-circuited. A proper data sheet and certification do. Lithium and lead-acid batteries also behave very differently, so mixing them causes problems.

2. Where they go and how they're fixed

What's needed

A space of their own that is dry, secure and kept apart from your living areas.
Not the engine bay.
Nowhere near anything that gets hot or can make a spark or flame. That includes the engine, starter motor, alternator, exhaust and fuel system.
Protected from water, corrosion, knocks, direct sun and freezing.
If the space could flood or briefly go under water, the batteries and their parts must be fully watertight (rated IP67). Anywhere else they must be weatherproof (rated IP55). An IP rating is a standard way of showing how well something keeps water out.
Fixed down firmly on a strong base, so they can't move in rough weather.
Easy enough to reach that I can inspect them and switch them off without taking half the boat apart.
Any vents the maker asks for are kept clear. Any fan must be the non-sparking type.

Why it matters

If a lithium battery fails badly, it can give off gases that burn. The standard warns that any spark or flame is likely to set them alight. An engine bay has heat, vibration, sparks from the starter and alternator, and often fuel, so it's the wrong place.

The standard also says batteries must not sit anywhere that heat or cold pushes them outside the maker's limits. That includes spaces warmed by other things, because heat from outside can start a failure too.

Lithium batteries are also much lighter than lead-acid ones, so I check whether the change in weight affects how the boat sits.

Where does that leave us?

The batteries need something like a gas locker: a space of their own, separate from the rest of the boat. If a battery fails, the gases it gives off can then vent to the outside, away from your living areas and away from anything that could set them alight.

That's the aim. The exact design depends on your boat and on what the battery maker requires. Some of the gases are lighter than air and some are heavier, which is one reason there's no one-size-fits-all design, so I work it out for each boat. If your boat has no suitable space, we'll need to build one.

3. Cables, fuses and switches

What's needed

Cables, fuses, circuit breakers, switches and busbars (the metal bars that join cables together) are all rated to cope with a fault, not just normal use.
Fuses are fitted as close to the batteries as practical, and can safely cut the power from a lithium bank in a fault.
Cable ends are crimped properly with the right tools, and tightened to the correct tightness (known as torque).
The switches that cut the power are clearly labelled and easy to reach.
Cables are supported, protected from rubbing, and kept away from fuel pipes, gas pipes, sharp edges and hot surfaces.

Why it matters

A lithium bank can push far more power into a fault than lead-acid batteries can. Weak or cheap parts, or a loose connection, can overheat. Good parts and careful workmanship matter as much as the batteries do.

4. The battery management system (BMS)

The BMS is the electronics that look after the batteries.

What's needed

It protects against overcharging, draining too far, getting too hot or too cold, drawing too much power, and individual cells drifting out of step with each other.
It is matched to your batteries and chargers.
It gives a warning before it switches the batteries off, wherever that's practical.
It is never bypassed, unless the maker says that's allowed.
I work out what happens when it does switch off, for example with the engine running or on shore power.
If things like navigation lights, engine starting or your radio could be affected, they may need their own separate power supply.

Why it matters

The BMS keeps the batteries inside their safe limits. If it cuts the power suddenly, the chargers and everything on the boat need to cope with that, without damage and without leaving you without essential equipment.

5. Charging, including the alternator

The alternator is the part of your engine that charges the batteries while it runs.

What's needed

Everything that charges the batteries is checked and set up for lithium. That means the alternator, the mains (shore power) charger, the solar controller, any other chargers, and wind or water generators.
Anything that can't be set up properly is replaced.
Alternators usually need extra protection, such as a temperature sensor, a limit on the current, or a separate charger between the alternator and the batteries.
A lead-acid starter battery (the one that starts the engine) stays separate from the lithium bank. They're only linked through equipment designed for that.
Older charging gear can't be assumed to be suitable. That includes split-charge diodes, voltage-sensitive relays and "1-2-both" switches.
Charging is avoided below the maker's minimum temperature.

Why it matters

Lithium batteries accept a lot of charging current for a long time. That can overheat an alternator built for lead-acid batteries. Every charger also has to follow the maker's limits, or the batteries are being charged outside their safe range.

6. Monitoring and settings

What's needed

You can see how full the batteries are, their voltage, the current flowing, the battery temperature and any alarms.
Old lead-acid battery monitors often can't read a lithium battery's charge level reliably, so the monitor may need replacing or resetting.
Settings that affect safety, like the charging voltage, are protected so they can't be changed by accident.

Why it matters

You can only deal with a problem you can see, and one changed setting can quietly push a battery outside its safe limits.

7. Smoke, gas and getting out

What's needed

Certified smoke and carbon monoxide alarms, tested monthly and loud enough to hear from where you sleep and steer.
Emergency switch-off points that are labelled and easy to reach.
A simple written plan that everyone aboard knows, including the way off the boat.
The battery details written down somewhere easy to find (type, number, voltage, capacity and location), so you can tell the fire service.

Why it matters

A lithium battery fire can spread fast, give off toxic smoke and start again after it looks out. The BSS and the CA/IWA guidance both say not to try to fight one. Raise the alarm, get off the boat, stay off and call 999.

Ordinary smoke alarms may not warn you early that a battery is giving off gas. Take hissing, a strange smell, heat or swelling seriously.

Insurance

What's needed

Tell your insurer before the work starts. Don't assume your current policy covers lithium. Have these details ready:

the battery make, model and type
where it's fitted and how it's fixed down
which BMS is fitted
how charging is controlled
what fire detection you have
who installed it, and which standards were followed

Keep their reply, and any conditions, in writing.

Why it matters

If you fit lithium without telling your insurer, you could be uninsured if something goes wrong. Some insurer conditions are hard to meet, such as only charging in daylight, and some small extinguishers they ask for won't cope with a battery bank fire. If a condition doesn't work for your boat, raise it with them rather than ignoring it.

What to expect from me

Before the work

We go through how you use power: daily use, big loads like an inverter, and anything you might add later, like an electric hob or a bigger fridge. The size of the battery bank comes from that, not from the size of your old lead-acid batteries.
I look at what you have now: the charging sources, the starter battery, the switching, and where the batteries could go. That space can't be the engine bay, so on some boats we'll need to find or build somewhere suitable.
I put the design on paper: battery bank, BMS, fuses, switches, cable sizes, chargers, alternator protection and monitoring.
You tell your insurer. I can help with the technical details they ask for.

After the work

I check the finished system against the design. The main parts are labelled, and I check the cable routes, crimps and how tight the connections are.
I record the BMS and charger settings, and test the alarms and monitoring.
I explain how the system behaves, including what each alarm means and what happens if the BMS switches the batteries off.
You get a handover pack: wiring diagram, manuals and data sheets, fuse ratings, settings, photos and my contact details.
Keep one copy of that pack on the boat and one at home. It helps with surveys, insurance claims, selling the boat and fault finding.

Looking after it

For the first few weeks, watch for unusual heat, smells, alarms or unexpected shutdowns.
Regularly check the connections for heat or corrosion, and check the vents, the mounting, the alarms and the charger settings.
Follow the maker's storage advice if the boat is left for a long time.
If a battery is swollen, very hot, leaking or keeps tripping its BMS, switch it off if that's safe, and call me or the supplier. Don't carry on using it.
Never put old or damaged batteries in household rubbish or overboard. Take them to a proper battery recycling point.

The BSS advice is clear that lithium batteries shouldn't be fitted DIY, and the CA/IWA group doesn't recommend self-installation for most owners. If you still decide to do it yourself, make sure you have the right tools and the knowledge to do it safely.

Glossary

Lithium iron phosphate (LFP, LiFePO4): The type of lithium battery most commonly used on boats.
Battery management system (BMS): Electronics, built into the battery or fitted alongside it, that stop it being overcharged, drained too far, or getting too hot or too cold.
Alternator: The part of the engine that charges the batteries while it runs.
Starter battery: The battery that starts the engine.
Fuse and circuit breaker: Safety devices that cut the power if too much current flows.
Busbar: A metal bar that joins several cables together.
Shore power: Mains electricity from a mooring hook-up.
Inverter: A unit that turns battery power into mains-style (230 volt) power for your sockets.
Watt-hours (Wh): A measure of how much energy a battery holds.

Sources

Boat Safety Scheme, Installed Lithium-ion Batteries and Systems
ISO 23625:2025, Small craft, Lithium-ion batteries
Cruising Association and Inland Waterways Association, Guidance Note: Installing Lithium Batteries for Domestic Power on Leisure Craft, Issue V1, 5 August 2026

This page is general guidance. It doesn't replace the battery maker's instructions or a survey of your boat, and requirements vary with the boat, the battery system and your insurer.