After two years on the hard standing, Maximus’s batteries were completely unserviceable - it was clear that we needed new boat batteries, but which type to choose?

We’d borrowed a lead acid battery from our marine electrician, Adam McMenemy, which would get the Maxi 84 afloat and through the lock to Chichester marina. However, now she was nearing the end of the full boat rewire and we had to make a decision on new boat batteries.

I confess, science has never been my best subject, and if I could have simply gone to a shop and bought exactly the same batteries as Maximus had before, I’d have been happy. However, managing power requirements nowadays is something needing careful consideration.

With the help of Raymarine I’d be upgrading my nav gear – running a multi-function display, not just a GPS, and a tiller pilot, and VHF radio with AIS. A fridge would be nice, and enough power to run a tablet and charge phones.

But most importantly, Maximus would eventually be kept on a swinging mooring. I needed to be sure her battery could stay topped up without shore power or the engine running, so I needed a solar panel.

Before I could decide on my boat batteries, I needed to know how much power I’d be using on the boat. Like it or not, I was going to have to get to grips with this, so I started the way I always do… right at the beginning.

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Basic electrics

As Simon Jollands says in The Boatyard Book, ‘All boat owners should have a basic knowledge of electrics, both to avoid encountering electrical problems at sea and to stand a chance of solving them should they occur.’

Good advice. This meant getting my head around current, which is the flow of electrons within a circuit (measured in amps); voltage, which is the force that pushes that current through the wire; and watts, the rate of energy consumed by an appliance.

So, for example, a 2,500W electric windlass (200A) might consume electricity at a rate 100 times higher than a 25W light bulb (2A). Every piece of electrical equipment on a boat should have a label or manual that explains either its demand for amps, its output in watts or both.

The engine is started with a cranking battery. Very high loads are delivered for a few seconds and this is restored by the alternator once the engine is running. If the revs drop (as they later did!) this could be an indication the alternator isn’t working

Batteries are rated in amp-hours (Ah) to give you an idea of how long they’ll provide power. So, theoretically, a battery rated at 100Ah should give you 100 amps for one hour, 50 amps for two hours or 1 amp for 100 hours until completely discharged.

I say theoretically because, if you discharge a battery below 50% (or even 60% or 70%) of its stated capacity it won’t last very long.

Therefore, if you actually want 100 amps, you’ll be needing a battery bank at least twice that amount (200Ah), as even the best battery chargers or alternators struggle to cram the last 20% of charge back into a battery. Plus, sulfation on the battery banks reduces capacity by a further 10%.

Here’s how amps, volts and watts work together:

  • Volts x Amps = Watts (example: 12V x 6A = 72W)
  • Watts / Volts = Amps (example: 300W / 12V = 25A)
  • Amps x Time = Ah (example: 2.5A x 5h = 12.5Ah)

Battery cycle life

Batteries don’t last forever. There’s an important distinction between shelf life – how long an inactive battery can be stored before it becomes unusable (ie having only 80% of its initial capacity) and cycle life – the number of times the battery can completely charge and discharge (1 cycle) before it becomes unusable.

For a lead acid battery, lifetimes of 500 to 1,200 cycles are typical (double for lithium-ion batteries), but the ageing process results in a gradual reduction in capacity over time.

However, there are other factors affecting cycle life, such as temperature (the ideal temperature is 10°C or less, which isn’t easy to achieve on boats, especially if the battery is in an engine room), capacity, and age. All sealed lead acid batteries will eventually exceed their life expectancy.


An often hot engine room is not a great place to house batteries if you want them to last

The number of useful cycles any battery will give you over its lifespan is also governed by how low you take its charge each time, and how quickly you take the power out. This is known as Depth of Discharge (DoD).

Cold cranking amps (CCA) is the number of amps a battery can support for 30 seconds at a temperature of 0°C before the battery voltage drops to unusable levels.

When comparing starter batteries, this can be a useful consideration, especially in colder climates. Replacement batteries should equal or exceed the original battery’s CCA.


The three new Exide batteries wouldn’t fit in the engine room but we found space for them to the aft of the boat under the cockpit. A box fixed to the boat keeps them secure

DC and AC systems

Small boats like Maximus have a 12V DC (direct current) system, whereas larger boats often run off a 24V system. In addition, some boats have an alternating current (AC) generator to power domestic electrics and charge batteries instead of the engine.

Shore power is your yacht’s second electrical system, and this allows you to bring AC electricity on board using a power cord, with the male connector plugging into the power outlet on the pontoon and the other end, a female connector, plugging into the vessel.

As well as charging your batteries, this enables you to use low wattage appliances (ideally no more than 2kW) with 3-pin plugs like you get a home.

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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