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Marine electrics: How to solve any problem onboard

Jan 15, 2024

After engine breakdowns, electrical failure is the second most common reason for a yacht becoming stranded. Duncan Kent explains how to troubleshoot problems with your marine electrics

If you have problems with your marine electrics, start with the batteries.

Marine electrics: How to solve any problem onboard

Should you ever suffer total loss of power, the obvious place to start troubleshooting is at the batteries.

Older boats with several previous owners will have had many extras added over the years and, inevitably, some won't have been installed to a proper standard, which can cause all sorts of problems when trying to track down a fault.

If your battery terminations are corroded or covered with acidic residue (often caused by lack of ventilation), loosen and remove all connected cables, taking care to mark their designation first.

The key to easy troubleshooting is to label every cable indelibly, ideally adding this information to the boat's wiring diagram.

Check your alternator belt regularly. Heat and friction will eventually wear it down

Then, wipe clean the inside and outside of the battery terminals with a damp cloth, before lightly abrading with fine Emery paper (not wire wool) and smearing them with silicone grease before re-tightening each connection.

As you do so, note that battery clamps are usually tapered, so completely loosen the terminal clamping bolts before putting them back onto the terminals, ensuring they mate together well.

If there are too many connections made directly to the battery terminals, then complications can arise.

A better method is to use positive and negative bus bars, taking a single cable to the bus bar from each battery terminal, with the appropriate large fuse in the positive feed.

This will allow you to use proper crimped and insulated ring terminals, preferably the heat-shrink sealed type.

If you are suffering from consistent low voltage, disconnect and separate the batteries, leaving each one to settle for a few hours before measuring the voltage across its terminals, then charge each battery separately using a simple single-stage mains charger until it is 100% charged.

The best way is to read the current going into the battery and when it drops to around 2% of the battery's capacity (ie: 2A for a 100Ah battery), with the voltage around 14.4V+, it can be judged to be fully charged.

An external regulator will help control a yacht's alternator more accurately and efficiently

Leave the batteries overnight to settle, and in the morning, if one is still showing a particularly low voltage (say, <12.5V) try putting a known load across it (a spotlight or car headlamp filament bulb is ideal) and seeing how long it takes for the battery to discharge to 50% (12.15V).

If it's a matter of minutes, then the battery is unlikely to be any good.

You could take them to a garage and have them load-tested, but this is only really useful for starter types and maybe some budget ‘leisure’ batteries, not proper deep-cycle traction batteries and AGMs.

Identifying an electrical fault can be a daunting task

Quite often, in a multiple battery bank, one cell can short out in a single battery, which then causes the other good ones to fail.

Obvious clues are if one battery is getting hot, drying out, swelling up or gassing excessively while drawing an unprecedented amount of current.

Removing the faulty one as soon as possible might well allow the others to recover, but not always.

If your batteries are charged and there's power at the switch panel, but it's not reaching your device, then you have no choice but to ‘buzz’ the cable through with a multimeter or a small battery and bulb.

Check that the terminal at the switch is clean and securely connected, before following the cable along the boat.

If there's power at one end and not at the other then there's a good chance there's a fuse, circuit breaker or dodgy junction somewhere along the line.

It's not unusual to come across cabling that is too small for the job, often caused by owners adding extra items to the circuit without upgrading the cable size.

Cleating cables makes troubleshooting problems with marine electrics easier

There are plenty of tables online showing the correct cable cross-section size for a known circuit distance, load current and voltage drop.

If you come across a blown or burned-out fuse or breaker along the line then check the maximum load against the cable's carrying capacity.

If it has become too small for the increased load, then you’ll either have to remove some of the load or upgrade the cable, making sure you replace both the positive and negative wires.

Undersized cabling can lead to problems with your marine electrics

The most likely causes of a poor connection are cheap, unsuitable terminal blocks such as DIY store ‘chocolate block’ strips with steel blocks and screws.

These hate moisture and will corrode within a few weeks unless slathered with grease and sealed in a watertight box.

If you find these during your troubleshooting, replace them with decent marine-grade blocks.

While you’re at it, cut back, trim and re-terminate the wires with crimped ferrules or ring terminals.

Do not use solder as this creates a hard spot, which can cause the wire to break.

Another common cause of failure is using cheap, cloned circuit breakers, which have been found to either trip out well below their stated capacity or slowly melt under continuous load.

Never save money on circuit protection or one day you’ll regret it.

The single most useful tool for troubleshooting problems with your marine electrics is a multimeter.

Simple ones are cheap and often adequate for basic tasks, although I’d recommend you spend a little more to get one with a DC clamp ammeter that will enable you to measure current flow through a cable without having to disconnect any wires.

It also helps a great deal if you have a battery monitor fitted, which will indicate at a glance (some even remotely on a smartphone) what current is going in and out of the battery bank and its approximate State of Charge (SoC).

There are a few important things to remember about batteries. A lead-acid battery of any type should never be discharged below 50% of its capacity or it will die prematurely.

A battery monitor lets you see output at glance, helping you to identify problems with your marine electrics

A rough estimate for those with only a voltmeter is 12.15v rested (no charge or load for at least four hours).

Open cell, flooded batteries need checking and topping up with distilled water at least monthly, or weekly if you draw heavy loads and regularly cycle them.

Lead-acid batteries also like to be fully recharged on a regular basis. Partial recharging is one of the most common causes of premature battery failure.

Battery monitor State of Charge (SoC) meters aren't always accurate as to keep them so would require frequent parameter resets.

The heart of a boat's marine electrics system

But there are other ways to check a bank's SoC when recharging it, as mentioned above.

Once rested after charging, the voltage should remain at around 12.7V-12.8V with no load.

There's no point in measuring a battery's voltage when it's under load unless you’re checking for voltage drop or specifically testing to see how it is holding up under duress.

None of this advice is relevant to a Lithium-ion battery bank of any sort as these have totally different charge characteristics and parameters to lead-acid batteries.

Battery voltages below 13.5v when the engine is running can be due to numerous problems.

Cruising yachts usually have a means of splitting the charge between the start battery and domestic bank, while keeping them isolated when the engine is off.

But these devices fail, fooling one into thinking the alternator isn't working.

Some alternators, particularly on bluewater yachts, have an external regulator fitted.

An alternator is essentially an electrical generator. Credit: Graham Snook/Yachting Monthly

This bypasses the built-in regulator and controls the alternator much more accurately and efficiently, often taking into account the temperatures of both the alternator and batteries.

These can fail, making it look like the alternator has failed.

The only way to determine if that is the case is to disconnect it and revert to the original internal regulator, a process which might involve resoldering some wires inside.

At sea, or a long way from professional assistance, there will nearly always be ways of overcoming charging problems, but they can be complex, so you’ll need a little inside knowledge.

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Put simply, an alternator's output depends on the voltage level at its sensing (field) terminal.

At a full 12v it will output its maximum charge at the B+ terminal.

So, provided its rectifier is working and you’ve disconnected any external regulation, you can feed battery voltage directly to the field terminal via a switch, to get it charging, after which most alternators self-excite, so you can switch it off again.

Should your rectifier diodes blow, they are relatively inexpensive to replace, so it's always worth a try before you ditch the whole alternator.

Many cruising yachts are fitted with battery isolating relays, the most popular being the Voltage Sensing Relay (VSR) or Automatic Charging Relay (ACR), which sense a charge source at either of the terminals and then switch on if that voltage reaches the relay's upper switching voltage threshold (usually 13.3v+).

This connects the start and leisure batteries together to charge both simultaneously. Some early models only sensed on one terminal, but this quickly changed to dual sensing.

Although usually reliable, a VSR can sometimes fail or cause problems with contact chatter (switching on and off rapidly), usually only apparent when an additional charging source is present, such as solar direct to the domestic bank, in which case it's better to use a simpler, ignition sensing relay or replace the relay altogether with a battery to battery (DC-DC) charger.

A DC-DC battery to battery charger

The latter behaves like a ‘smart’ mains charger, except it runs off the start battery and feeds charge to the domestic bank in a controlled multi-stage manner – essential if you have a Li-ion domestic bank as the different charged voltages will force a VSR/ACR to remain on permanently.

A common problem with a relay is when too high a current has been fed through it.

If you drain your house batteries very low, when you start your engine and the VSR senses the charge in the start battery, it will switch on, immediately sending a very high current rush from the alternator.

A Voltage Sensing Relay

For this reason, the relay must be rated the same as your alternator's maximum output current, with the wire sizes matched and fused accordingly.

I’ve seen cases where the relay contacts have welded together, keeping the batteries permanently connected, thereby draining the start battery along with the domestics.

To check for this, disconnect the relay with the engine off and meter through its two large battery terminals with an ohmmeter (multimeter continuity tester).

A dead short indicates the relay is permanently activated.

To overcome this problem temporarily you can fit a heavy-duty isolation switch or simple relay (rated at alternator current) between the two for charging, but remember to switch it off to disconnect the start battery when the engine isn't running.

More and more owners wanting their boats to be more akin to their homes are installing inverters to power their numerous 220v devices.

However, many have little idea of just how much power a loaded inverter can pull from their domestic battery bank.

Not only do inverters consume power, even when idle, they also pull massive DC currents.

For instance, a 1kW hair dryer will pull around 100A, a microwave 150A and a fan heater 200A+, requiring thick cabling with high quality fuses and terminations to prevent a poor connection dragging the whole lot down.

Low voltage can cause anchor windlass problems. Credit: Graham Snook/Yachting Monthly

Lead-acid batteries don't take kindly to a 200A draw and will seriously reduce your bank's capacity.

A new, fully charged 200Ah AGM battery bank would be at 50% SoC after around 15 minutes of such a heavy load, even less as the battery ages.

I hear so many stories of inverters going into alarm mode, or simply switching off after a few seconds, nearly always caused by flat batteries, poor connections, inadequately sized cables or burned-out circuit breakers and fuses.

The first, easy test is to put a voltmeter across the inverter's DC terminals and take a reading when you switch the AC load on.

Many AC devices require an initial in-rush of current, dropping back to a continuous lower level after a few seconds.

For this reason, when specifying your inverter, you need to allow for at least a 50% initial power surge.

Always carry a quality digital multimeter to sort out problems with your marine electrics

Other heavy loads that commonly cause headaches are bow thrusters and anchor windlasses.

Some boats wire them to the engine start battery, but this usually requires a very long length of large-diameter cable from stern to bow.

Scrimping on the size causes voltage drop, which in turn raises the current drawn and often burns out cables and breakers, causing fires.

Others have separate batteries forward to take the load over shorter cables, but this presents problems charging them.

To take charge from the alternator forward to the bow battery requires pretty large cables too, particularly when the engine is running and your bow battery is low, forcing the alternator and start battery to provide much of the current required.

The safest way is to install a current-limiting device such as a Current-limiting Voltage Sensing Relay (CVSR), which will allow the bow battery to charge automatically, but disconnect the link if the load gets close to the circuit's limits.

Once again, low voltage problems are easily detected using a voltmeter over the windlass's terminals when operating.

Solar power brings a new set of complications, the most common being low charge, almost always due to panel shading, undersized wiring or poor connections.

Most solar panels come with MC4-type waterproof connectors, but if you need to extend them do source the proper extension cables with matching plugs and sockets.

Multiway MC4 connectors are available for connecting panels in parallel.

A charge controller helps identify low solar panel voltage

When connecting up a controller with screw-type terminations, it's better to crimp a ferrule to the wire to provide a solid connection against the bare screw.

Other controllers are specifically designed for bare wire connections and usually have clamp plates in their terminals.

However, these often go unnoticed, resulting in the installer placing the cable on the wrong side of the plate when tightening, causing it to fall out later or make a high-resistance connection.

Always tug at the cable when you’ve finished to ensure it is properly secured.

Shading or poor connections could be the reason your solar panels are not working

Another common mistake is connecting the panels to the controller before wiring the controller to the batteries.

Solar controllers need to be connected to the battery first so they can sense the system voltage and set themselves up with the correct parameters.

Doing it in the wrong order often means the controller doesn't fire up and can damage the less-protected models.

A digital voltmeter (DVM) is invaluable for troubleshooting DC or AC electrics.

They’re cheap to buy and most offer a Volts, Amps and Ohms (resistance) range, plus a test buzzer.

To measure voltage, plug the +ve (Red) test lead into the VΩmA socket and the -ve (Black) into the common (Comm).

Then set the range, usually 20vdc for 12v wiring. For AC volts leave the leads in the same holes but switch to the AC range.

Cheap DVMs only measure up to 10Adc and not AC. For measuring .milliamps (mA) leave the leads in the Volts sockets but set the DVM to the mA range.

Above the mA range move the +ve test lead into the 10Adc socket.

Then break into the circuit by taking a wire off a device, putting one probe on the wire and the other back on the device.

It will read negative if you have the polarity wrong, but no harm will be done.

If you leave the +ve lead in the Volts socket (very common), the internal fuse will blow, as it will if you go over 10A.

For higher currents buy a ‘clamp-style’ DVM, which costs a bit more but allows you to measure current without disconnecting any wires.

Measuring resistance can help track down poor connections or damaged wires.

Put the leads in the same sockets as for testing volts but set the DVM range to Ohms (Ω) or KiloOhms (KΩ).

To check for a break in a wire, put the leads in the VΩmA sockets and switch to the continuity setting (speaker symbol).

Touching both probes together will sound a buzzer. For long cables you might need to extend the probe leads.

The best method of terminating a wire in a boat is with a crimped terminal or ferrule.

Try to find those that have a tinned copper barrel and a self-sealing heat shrink cover that you heat to shrink-wrap the joint.

These give optimum security in both physical strength and moisture resistance.

If you’re doing a lot of rewiring then do buy a good quality, ratchet-style crimper.

On a yacht, wires should be terminated with a crimped ring for increased security and moisture resistance. Credit: Duncan Kent

Note that terminals and ferrules come with different colour covers depicting their recommended wire size.

Remember, don't assume anything. Start at the power source and work your way methodically towards the faulty item, not the other way around.

Take clear notes and photographs before disconnecting any wires.

Draw up any non-standard circuit modifications and annotate them to help with future troubleshooting.

A subscription to Yachting Monthly magazine costs around 40% less than the cover price.

Print and digital editions are available through Magazines Direct – where you can also find the latest deals.

YM is packed with information to help you get the most from your time on the water.

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After engine breakdowns, electrical failure is the second most common reason for a yacht becoming stranded. Duncan Kent explains how to troubleshoot problems with your marine electrics Marine electrics: How to solve any problem onboard costs around 40% less than the cover price can also find the latest deals Facebook Twitter Instagram.