Select Page

It is important for all boat skippers to have a basic knowledge of boat electrics and this includes understanding how batteries work. Under-sized battery banks are one of the key factors behind power failure at sea, as well as the premature failure of batteries. It’s therefore worth analysing the set up on your boat when it’s ashore in the boatyard to see whether it measures up to the use you put it to.

boat battery bank

An automotive-type battery, of a similar specification to those used in cars, can be used to supply the starter motor with the very high loads for the few seconds it takes to start the engine. This type of battery, however, is not suitable for powering the boat’s other systems, which will typically draw a relatively small amount of power for many hours, or even days, at a stretch.

Deep-discharge (or traction) batteries are designed for slow discharge over a period of time, before being recharged when the engine is running or via shore power chargers, or solar or wind generators. This type of use would quickly destroy an automotive battery, but a good leisure battery will withstand several hundred such cycles. However, discharging even the best deep discharge batteries below 50 per cent of their rated capacity will dramatically shorten their life.

There are various grades of such battery – the cheaper ones are not sealed and will need topping up with distilled water from time to time and are likely to have a shorter life span than more expensive models. Gel and AGM type batteries cannot spill battery acid and don’t produce potentially explosive hydrogen gas when charging. They can withstand many more charge-discharge cycles than conventional deep discharge batteries and so have many advantages for use on boats, despite their higher initial purchase price. Some are also capable of being used for both starting and deep cycling applications.

All batteries loose some of their charge over time. When storing them over the winter, ensure they are fully charged at the start of the storage period and, if possible, charge once a month to maintain the charge level. Modern three-stage and four-stage mains powered chargers may be left connected permanently.

Determining battery capacity

Deep discharge batteries are rated in Amp-hours – a fully-charged 100Ah battery, for instance, will deliver 5A for 20 hours before becoming completely discharged. However, discharging even the best deep discharge batteries below 50 per cent of their rated capacity will dramatically shorten their life.

boat battery connection

Calculating your estimated daily power usage, by multiplying the current in Amps of each device by the length of time for which you expect to use each it, will help determine the size of the batteries needed for the boat. If you aim to charge once a day and don’t plan to discharge the batteries to more than 50 per cent of their total capacity, your battery bank should in theory be at least twice the size of your estimated daily power usage.

However, even with good battery charging technologies it becomes increasingly difficult to cram the last 20 per cent of charge into a battery. It’s therefore best to size battery banks at around three times the expected daily power usage. Even then, this gives little scope for adding new power hungry devices such as a fridge or electric autopilot and even with correctly sized batteries it’s important to maintain a watch over the battery state throughout a voyage.

 

Examples of power usage  
Navigation light 1A (boats up to 12m)
Navigation light 2.5A (boats over 12m)
Interior lights (each) 1A
LED lights (each) 0.2A
Laptop 3-6A
Instruments 1A
Chart plotter 1-2A
Stereo 1-3A
Auto pilot 3-6A
Fridge 4A
   

Tip:

  • Some boat owners always keep a fully charged reserve battery for engine starting.

Boat Improvements

My Boat - practical improvements Author - Mike Rossiter Most boat owners who have had their craft for any length of time will have made what they...

Marine engine electrical system

The typical basic electrical system associated with a marine engine includes a dedicated engine starting battery, a starter motor, a charger in the form of an alternator, a solenoid and some engine sensors and instruments.

Understanding your boat’s compass

Article submitted by Mike Rossiter, Certificated Compass Adjuster. Since the magnetic compass was first used by the Chinese...

2023 Rolex Fastnet Race

Safe Skipper’s Simon Jollands took part in the 2023 Rolex Fastnet Race – a classic offshore sailing event. This is the first of several articles on the race and focuses on the start.

Boating emergency – how to broadcast a MAYDAY emergency call

How to broadcast a MAYDAY emergency call   How to broadcast a MAYDAY emergency call if a vessel or person is in grave...

Always have an emergency grab bag to hand when at sea…

  Grab bag: In the event of having to abandon ship, it is recommended to have a designated waterproof bag to carry...

Keel maintenance and Repair – Part 2

If you have ever witnessed a boat colliding with a rock or other submerged obstacle you will know that there is an almighty thump and the whole boat shakes and judders. While such hard groundings seldom result in catastrophic keel failure, something has to give and even the sturdiest keels can easily be damaged by such an impact.

Seacock maintenance

If seacocks are always left open and neglected they can eventually seize which will prove a serious threat to boat safety should a connecting hose fail and the seacock refuses to close. There are three main types of seacock – ball valves, cone valves and gate valves.

Fire safety advice at sea from the Marine & Coastguard Agency

Fire safety advice for boaters Top fire safety advice at sea: 1. Fit smoke alarms, carbon monoxide and gas detectors 2. Turn...

An explanation of the IALA maritime buoyage systems – IALA A and IALA B

What are the differences between the two IALA buoyage systems, IALA Region A and IALA Region B, and where are they used?   As recently as the 1970s...

Saildrive maintenance

There are less maintenance tasks to carry out on a saildrive transmission than on a traditional inboard shaft drive system with its associated stern gear. However, there are a few critical things that require maintenance, as recommended in detail by the engine manufacturers, and should be adhered to.

Top five windvane self steering installation questions

Top five windvane self steering installation questions answered by Sarah Curry of Hydrovane International Marine, courtesy of Viki Moore from Island Cruising NZ

Stern gear maintenance

The stern gear of a boat needs to be checked carefully when the boat is ashore as this is something that can only be done when it is out of the water. The same applies for any maintenance and repairs that may need doing, so it is best to check it all over as soon after an end of season lift out as possible.

A five day sailing cruise of the Solent, UK

Welcome to our virtual Solent sailing cruise – a five day sail in the south of England from Bosham Quay in Chichester...

Keel maintenance and repair – Part 1

Keels are designed to act as underwater foils that generate lift as the boat moves through the water, counteracting the leeward force of the wind and enabling the boat to sail closer to the wind. Keel maintenance and repair is essential for the performance of your boat.

Navigation safety: a quick-reference mobile app to learn the ColRegs NavLights and Shapes

 Safety at SeaSafety at sea will always remains a topical and important subject that will no doubt dominate the syllabuses of nautical...

Cutless bearing replacement

Cutless bearings can last for many years but if the propeller shaft is out of alignment they will wear through more quickly. If you have noticed a clunking sound when motoring then it could be a worn cutless bearing that is causing the problem.

Essential Knots: Reef knot

Essential Knots: Reef knot Use: Tying two ends of rope together, often used for tying up a bundle of loose sail around the boom. Step...

Boating Rules of the Road – International ColRegs

    International ColRegs Rule 7: Risk of Collision Anyone who is responsible for a vessel at sea, from the...

Sailing into fog – being prepared and staying safe

Most skippers will sensibly delay their departure, if fog is forecast. However, if fog begins to form when you are at sea it is important to be prepared, and know what precautions to take, to help make your vessel detectable or visible in fog and keep the crew safe.

Jester Challenge 2022 – Sailing single handed from Plymouth UK to the Azores: Part 2 – Weather

Jester Challenge – A modern experiment in old-fashioned self-reliance, self sufficiency, and personal responsibility. This is the second of a 10-part post where solo sailor, Bernie Branfield, shares his first hand account of his single-handed, 2022 Jester Challenge, from Plymouth, UK to the Azores, in his 26′ Invicta Mk2, Louisa.

Jester Challenge 2022 – Sailing single handed from Plymouth UK to the Azores: Getting to the Start

Jester Challenge – A modern experiment in old-fashioned self-reliance, self sufficiency, and personal responsibility. This is the start of a 10-part post where solo sailor, Bernie Branfield, shares his first hand account of his single-handed, 2022 Jester Challenge, from Plymouth, UK to the Azores, in his 26′ Invicta Mk2, Louisa.

Stress cracks on GRP boats

It is quite common to find cracks in the gelcoat when inspecting the deck and superstructure of a GRP boat. It is important to differentiate between a gelcoat crack and a scratch.

Hull inspection – the annual checks

With the boat ashore for the winter it is time to do a hull inspection - the annual checks. Are there any scratches and chips in the...

Fixing position at sea using traditional methods

This post looks at some traditional methods used for fixing a vessel’s position at sea, within sight of land. Electronic fixes using chart plotters are very straightforward to record, but if for some reason a vessel’s electronics are faulty it is essential that a skipper knows how to use traditional methods.