Boats usually have two batteries (or banks of batteries) to ensure one is for starting the engine only, and the other to run the domestic systems.

Sometimes (though not on Maximus) there’s a third set of batteries for high power items such as a bow-thruster or electric windlass.

Wet lead acid

Batteries that have liquid electrolyte sloshing around in them are sometimes referred to as ‘wet’, ‘flooded’ or ‘wet lead acid’ batteries.

To start the engine, boats have a starting or ‘cranking’ battery, like those used in cars, which delivers very high loads for a few seconds.

Only a small portion of the battery’s capacity is used and this is restored by the alternator once the engine is running.

For powering everything else (i.e. chartplotter, lights, fridge, etc.) boats use a domestic, or ‘house’ deep-cycle (or deep-discharge/traction) battery bank which draws a much smaller amount of power over a longer period of time.

It’s important that wet lead acid batteries are stored in a ventilated area. A disadvantage is that they can ‘gas off’ (release hydrogen) if faulty, overcharged or getting old.

Pros: cheap, readily available, lots of choice, starter batteries can be bought in automotive stores.
Cons: heavy, can leak, need regular charging, useless once flat, can gas off.

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Wet lead acid – dual/leisure battery

An ‘in-between’ option is the ‘dual’ or leisure battery. A compromise between starting and deep-cycle batteries, these are popular on boats that carry only a single battery, or prefer to have the same type of battery for simpler charging.

Maximus had a leisure battery for starting the engine (an 85Ah Green Power) and a 100Ah deep-cycle Lifeline Battery for powering the rest of electrical equipment.

Both were flat after being left on board for two years and so could not be revived.

Pros: cheap, readily available, lots of choice, dual-purpose starter and domestic.
Cons: heavy, can leak, need regular charging, useless once run flat, can gas off.


Rolls sealed VRLA deep cycle batteries on the Ecobat stand at Southampton Boat Show

Sealed lead acid batteries (gel & AGM)

Nobody wants battery acid leaking into the bilge, but there’s a price to pay if you want more durable batteries.

Sealed lead acid batteries are sometimes referred to as VRLA (valve regulated lead acid) and there are two main types – absorbed glass mat (AGM), where the battery plates are protected by fine-stranded glass mats – and gel, where the liquid electrolyte has been converted into a gel.

AGM batteries can be used for both starter and house applications whereas gel batteries are better suited for house applications only.

Both types can withstand many more charge/discharge cycles than conventional deep-discharge batteries – for example, a 7-10 year lifespan rather than a 4-5.

Pros: leak-proof, longer lifespan than wet lead acid, AGM can be dual-purpose starter and domestic.
Cons: twice the price of wet lead acid, up to 25% heavier, need regular charging, useless once flat.

The extensive lead carbon battery bank for electric narrowboat Old Nick

Lead carbon batteries

Lead carbon batteries are not widely known. Our electrician had never fitted them, and indeed I hadn’t considered these until I spoke to Victron Energy.

However, they are highly regarded by PBO contributor Paul Sumpner, who fitted Leoch lead carbon batteries to his electric-propulsion narrowboat Old Nick.

Paul worked out his requirements, which were a regular DoD of 50%, year-round usage, a minimum of 48V, 600Ah, zero maintenance, a life cycle of 3,000+ discharges, non-vertical mounting and to be able to survive a regular partial state of discharge. Plus, they couldn’t “completely blow the budget”.


Old Nick, Paul Sumpner’s narrowboat, is electrically propelled and runs off lead carbon batteries.

“It became clear that only lead carbon or lithium batteries would be suitable,” he said, and later confirmed he couldn’t be happier with the equipment he chose.

Lead carbon batteries are an advanced type of VRLA battery, which has a positive plate (anode) of lead, but a negative (cathode) plate made of carbon composite.

According to Victron, the advantages are less sulfation, and a lower charge voltage, meaning higher efficiency and less corrosion of the positive plate. Overall, the result should be an improved cycle life.

Pros: leak proof, longer life, less sulfation.
Cons: more expensive, can’t be used for starter motor, larger and heavier than AGM or lead acid.


AMPS 100Ah deep cycle lithium-ion battery

Lithium batteries

Lithium-ion is a broad chemistry of batteries, the most common being LiFePO4 (Lithium Iron Phosphate). They can withstand four or five times the number of cycles compared to most deep-cycle lead acid batteries (4,000+), can go down to a much lower state of charge, and can keep a reasonable level of charge for up to 12 months.

While lead acid batteries, in practice, only allow 30% of rated capacity, the best lithium batteries can be discharged to 70-80% of the rated capacity.

So really, a 100Ah lithium battery is the equivalent to having a 200Ah lead acid battery, only it will usually charge much quicker, is half the weight and a lot smaller.

A word of warning, though. Although lithium batteries can accept fluctuating voltage (13.6V-14.6V) like lead acid batteries, they should not be directly charged from an alternator because voltage spikes could damage them. A dedicated charger and battery management system is a necessity.

Pros: long-lasting, can use 80% of capacity, easier to manage, small, lightweight.
Cons: expensive (up to 4 x cost of wet lead acid), boat adaptations required.

*Batteries chosen to show a range of brands and prices, not because PBO recommends these over others. Prices include VAT and UK delivery, and are taken from online retailers

Next steps

Having had a brief look at batteries, I called marine surveyor Ben Sutcliffe-Davies. I wondered if there was any possibility of reviving flat batteries.

“I’ll be honest with you, start with new batteries,” he said. “You’re relying on this battery to start the boat. Yes it might pick up, but I guarantee it’ll be like a dying swan when you need it.”


The new batteries were fitted in a ventilated space below the cockpit

For domestic systems, Ben likes Numax batteries, which he says a lot of the industry use. For the cranker, he said, any automotive battery would do.

“I went down to my local tyre and exhaust place, and just got four heavy-duty deep cycle leisure batteries and one tidy cranker for the engine,” he told me.

Thanks to our Project Boat Supporters

Dell Quay Marine, Osculati, Raymarine, Shakespeare Marine, TruDesign, Screwfix, Coleman Marine Insurance, MDL Marinas, Premier Marinas, seajet, Marine & Industrial, Clean to Gleam, Dometic, West System, Farécla, Navigators Marine, Lewmar, RYA, Aqua Marine, Ecobat, Victron Energy, Scanstrut, T Sails and XP Rigging.

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