Select Page

Boat batteries need to be kept properly charged, which means never allowing the batteries to discharge below 50 per cent of their total charge.

As well as the batteries themselves, keeping a boat’s charging systems in good shape will also help to keep batteries topped up to a higher level of charge. The output of standard alternators can drop to as little as one-third of the alternator’s rated output after only 15 minutes or so of engine running. However, a smart charging regulator will keep the charging rate close to the initial figure, thus re-charging the batteries in minimum time. These have a further advantage in that they charge batteries to as much as 95 per cent of capacity – compared to less than 80 per cent for a standard alternator regulator.

In addition to improving charging systems, minimising the power drawn from batteries will also extend their lifespan. This need not involve major inconveniences – changing to LED lighting, for instance, will significantly reduce daily current drain, and costs significantly less than replacing a battery bank that prematurely runs out of puff. It’s also worth ensuring the fridge has at least four inches of insulation all round. Unfortunately, many are lacking in this respect, which can put a huge strain on batteries.

Avoiding and identifying electrical problems

The diagnosis of many boating electrical problems is within the grasp of anyone with basic knowledge of electrical systems and a few key skills.

Battery state

At a basic level, one of the most useful diagnostic tools is a simple digital voltmeter. The voltage produced by a battery, when no load is being drawn from it, is a good guide to its state of charge. When fully charged, a 12V battery can theoretically hold up to 13.2V, although in practice 12.8V or 12.9V is a more likely maximum. At 12.5V, the battery has 75 per cent of its maximum charge remaining, and at 12.2V there’s 50 per cent of the battery’s total capacity left – the point at which the battery should be recharged.

boat voltmeter

If the reading drops to 12.0V, there’s only 20 per cent of the battery’s capacity remaining, and the battery will be (almost) fully discharged at 11.8V.  A battery that’s nearing the end of its life may still give reasonable voltage readings when fully charged, but only if no load is being drawn. Switching a couple of lights on will create a large voltage drop in a weak battery, whereas an example in good condition will show a drop of only 0.1 or 0.2V in these conditions.

The voltmeter can also be used to check that the alternator is producing charge. Standard alternators have their output capped at around 14.2-14.4V, although there may be a voltage drop of 0.5V, and occasionally more, by the time the charge reaches the batteries. If there is little or no increase in voltage across the battery when the engine is running, then it’s very likely that there’s a problem with the charging system that will need further investigation.

Battery monitors

Unfortunately, it takes a long time for battery voltages to settle when load is removed, or after charging, and while a reasonably stable voltage may be seen after a few minutes, it takes several hours for the voltage to completely stabilise. This is one of the benefits of more sophisticated battery monitors that give a precise indication at any time of how much charge there is left in the batteries.

Boat battery monitor

When properly calibrated, battery monitors can account for all factors that affect battery state, including calculating the total charge delivered to the batteries, and subtracting the total charged used by the boat’s systems. In addition to battery voltage, these monitors can display battery charge/discharge current; state of charge of the battery in Amp-hours (Ah) or in percentage of total capacity; and time to go until the battery needs to be recharged.

Additional information available includes the average depth of discharge, depth of deepest discharge; number of charge/discharge cycles and number of occasions on which an under-voltage alarm has been triggered.

Alternative charging inputs

With many boats running numerous electrical devices, keeping the batteries topped up on a long passage, or during a period at anchor, can be a challenge. However there are now many options that mean there should be no need to rely on the engine for this. In the past wind generators were the mainstay for the power needs of many long-term cruisers and still have their fans today. However, they also have a number of drawbacks – when sailing downwind, for instance, the reduction in the apparent wind speed means their output is quite low.

The cost of solar panels has fallen rapidly and they are increasingly becoming the primary means of charging on many boats. On passage this can be supplemented by further inputs from a towed generator, or from the type of hydro-generator that’s increasingly common to see on long distance racing yachts.

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...

Propeller care and maintenance

Propellers are complicated and repairs should be done by specialists but owners can carry out checks and some routine maintenance themselves when the boat is in the boatyard. A propeller is critical to a boat’s performance, fuel consumption and ride, so it makes sense to keep a propeller in good working order.

Boat interior varnishing

Most boat interiors have a combination of varnished and painted surfaces including solid wooden joinery, plywood laminates with thin hardwood veneers and glass reinforced plastic. When making your assessment of what you are going to do, bear in mind that the varnishing process consumes a lot of time, especially if the existing surfaces are in poor shape.

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.

Wooden Hulls – Part 2

It is important to ensure the essential hull maintenance of a wooden boat is done, even if you are paying others to look after your boat for you. The priority is to prevent rot from taking hold. The protective layers of paint and varnish over wood are far more critical than on GRP boats, where the topsides are painted more for cosmetic reasons.

ColRegs – avoiding collisions at sea

ColRegs - avoiding collisions at sea ColRegs Rule 8: Action to avoid collision (a) Any action taken to avoid collision shall...

Marine toilets – care and maintenance

There are a number of different types of marine toilet, or heads. They fall into one of three categories – manual, electric and vacuum, the most common being the manual, hand pumped type. These have double acting piston-pumps which both discharge the waste and flush the toilet with sea water.

Essential Knots: Sheet bend

Essential Knots: Sheet bend Use: Joining two ropes together. A sheet bend is particularly useful for joining two ropes of different...

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.

How to tackle osmosis

Many owners of old GRP boats live in fear of osmosis, but what exactly is osmosis and what can be done about it? Osmosis comes about...

Capsize – understanding the risks

A skipper should know how their boat will cope with rough seas. By working within known limits and understanding the risks,...

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...

Boatyard Health and Safety

Boat storage facilities are potentially hazardous environments and it is the responsibility of both boat owners and boatyards to ensure that the...

Rudders and steering systems – Part 3

In the third of our three blog articles on rudders and steering systems, we look at how to replace rudder bearings and repair a water-saturated core.

Boat Handling – anchoring

Anchoring your yacht or motorboat Anchoring is one of the most important boat handling skills. If you can set an anchor...

ColRegs when sailing single handed

  Don’t neglect the Colregs when sailing single handed Sailing single-handed represents several challenges for skippers, not least how to...

Essential Boat Spares for Safety

  Boats Spares Tool kit What you carry in the boats tool kit will be useful for many boat repairs, but you might want...

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.

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...

Narrowboating on the Kennet and Avon Canal

A recently cancelled sailing event I was due to take part in left us with a free weekend in the diary. Given that my wife and I were celebrating a bumper wedding anniversary and the weather forecast was for fine weather, we decided to hunt around for a last minute canal holiday.

Understanding boat engines

Irrespective of what kind of engine a boat is equipped with and who does the work, the regular care and maintenance of a marine engine is essential. The most common cause of marine engine failure is widely known to be lack of maintenance.

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...

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.

Feeling anxious at sea

  Some people feel anxious at sea. Will they be seasick? What if they get caught in a violent storm? Could the boat...

Distress flares – which flare, how & when to use?

How to use distress flares at sea Flares should be kept in a waterproof container in an easily accessible location such as a...