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.

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.

Safety at sea principles

Safety at sea is not as simple as just spending money adding shiny new emergency equipment such as liferafts, danbuoys, distress flares, EPIRBs and so on.

Sailing at the touch of a button

Easier and more controlled sail handling can also be achieved by powering up a furling mast. I came across some interesting solutions at the Southampton Boat Show this week on the Selden Mast stand, where they were running demos of their E40i electric winch and SMF furling system.

Gybing a sailing boat

Gybing is the sailing manoeuvre used to change a boat's direction through a following wind. As with the tacking manoeuvre,...

Understanding your boat’s compass

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

Medical Emergency at Sea

How to deal with a medical emergency afloat   If you are planning a boating trip, it is important to have at least one...

Jester Challenge 2022 – Sailing single handed from Plymouth UK to the Azores: Part 5 – Boat Management

Jester Challenge – A modern experiment in old-fashioned self-reliance, self sufficiency, and personal responsibility. This is the fifth 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.

Boat ownership – some fundamentals

Owning a boat is a big commitment that should bring no end of satisfaction, but the costs of maintaining and keeping a boat are significant and should never be underestimated.

How to use tides and currents to your advantage

If you are contemplating a cruise through tidal waters and strong currents, then planning your trip carefully in advance is essential to enable you to take advantage of favourable tides rather than constantly fighting against them.

Boat gas system maintenance

There are correct types of hose for marine plumbing, sewerage, exhaust, cooling and gas and all hoses should be checked regularly for wear and deterioration.

Know your Navlights & Shapes – essential for all skippers

Know your Navlights & Shapes International Regulations for Preventing Collisions at Sea (ColRegs) Anyone who is...

Boat Engine Safety Checks

  Boat Engine Safety Checks Every skipper needs to make regular essential boat engine safety checks. Below you will...

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...

Avoiding personal dangers at sea

In order to stay safe at sea, we need to know the risks we are facing and to be aware of any personal dangers we could possibly encounter. Here are six of the most common potential dangers individual crew members should be aware of.

Sail care and maintenance – Part 2

At the end of the sailing season sails should be washed and inspected carefully for damage, including small tears, stitching failure, ultraviolet damage, stains and mildew.

Boat engine fuel system

If engines are installed and serviced correctly then most marine engines are very reliable, but one of the most important parts of the engine to check and service is the fuel system.

First Aid Afloat – jellyfish stings

  Wherever you are boating in the world I am sure you will be using a pilot guide to aid your navigation. Often in the...

Rudders and steering systems – Part 2

One thing all rudders have in common is that they have three main parts that need to be checked: the rudder, or a steerable drive leg in the case of many power boats; the system that joins the rudder to the steering; the steering control itself.

Preparing for sailboat cruising

Preparing for a sailing trip entails a lot of planning. In this blog, we take a look at some of the many safety aspects that a skipper needs to consider before heading off on a cruise.

Keel design – options to consider when choosing a yacht

Keel design is constantly evolving and nowhere is this more apparent than in modern racing yachts such as the Imoca Open 60...

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...

Antifouling for leisure boats – Part 2

To prepare for antifouling, as soon as your boat has been lifted out and pressure washed, you need to check all the surfaces of the hull below the waterline, remove any remaining barnacles and check for blisters.

Leaking decks

Leaking decks are perceived as a nuisance by some boat owners, but if leaks are ignored a much more serious situation may well be developing, especially in the case of boats with balsa or plywood deck cores. So deck leaks do need to be investigated and dealt with.

Essential boat engine checklist

Boat engine checklist Engine oil level check Even if you have checked it previously, confirming the engine oil level is up...

Safe Skipper – crew management tips

Effective crew briefings are a vital part of the good on-board communication that helps everything to run smoothly on a sailing vessel at sea, whether it is cruising or racing.