Electricity On Board

The basic units

• Volt (V): electrical pressure. Most small boats use 12 V or 24 V DC, plus AC shore power or inverter power.
• Ampere (A): current flowing in the cable.
• Watt (W): power used by a load. Watts are volts multiplied by amps.
• Amp-hour (Ah): a common battery capacity number, but it only makes sense together with voltage and usable depth of discharge.
• Watt-hour (Wh) or kWh: a better way to compare total stored energy across different voltages.

Small numbers can hide big loads

A 120 W load at 230 V AC draws only about 0.5 A on the AC side. The same 120 W taken from a 12 V battery through an inverter draws more than 10 A before losses. That is why AC equipment can empty a DC battery bank faster than expected.

Current matters because cable heating, fuse size, voltage drop and connector stress all follow the current path.

The common bad installation

The most common bad pattern is a new cable added because a new device was installed: chartplotter, pump, charger, USB outlet, fridge, inverter, relay or sensor. It is connected to a nearby positive point, sometimes straight to the battery, with a fuse that is too large or too far away.

A fuse belongs close to the source of energy: the battery, busbar, charger output or other feed point. A small fuse near the plotter or other equipment may protect that unit, but it does not protect the long cable run before it. The source-side fuse is the fire protection for the circuit. If the cable is overloaded or shorts before the equipment, the fuse near the equipment may never see the fault.

• The cable may be sized for the normal load, but not protected against a short.
• The fuse may protect the device, but not the full cable run from the battery or busbar.
• The return cable may be smaller, longer, corroded or hidden in an old bundle.
• Several ring terminals may be stacked on a battery post with no proper busbar.
• The installation may work until vibration, water, heat or a heavy load exposes the shortcut.

Battery connections deserve respect

Battery terminals are not a convenient place to build a whole boat distribution system. They are high-current source points. Every accidental short near them has energy behind it.

• Use proper main fuses and main switches close to the battery bank.
• Move distribution to rated busbars instead of stacking many small cables on battery posts.
• Cover positive terminals so a tool, spare part or loose cable cannot bridge them.
• Support heavy cables so their weight and vibration do not work the terminals loose.
• Keep battery spaces clean, dry, ventilated and protected from loose metal objects.

The dangerous shortcut: an unfused cable

One of the worst mistakes on a boat is a cable connected to a battery or busbar with no correctly sized protection near the source. The equipment at the far end may be small, but the battery is not small. If the cable shorts to metal, the battery can try to feed the fault until something melts, burns or explodes.

• Protect positive feeds close to the source, unless the circuit has a specific marine-standard exception and is installed accordingly.
• Do not add a temporary direct battery cable and leave it in place.
• Do not assume a switch protects the cable. A switch is not automatically overcurrent protection.
• High-current circuits such as starter motors, windlass and thrusters need deliberate design, not copied small-load habits.

A blown fuse is a warning

Always carry spare fuses on board, in the correct types and ratings. But if a fuse blows, there is a reason. It may be the first and only warning before a serious fault becomes heat, smoke or fire.

Do not only replace the fuse and continue. Find out why it blew: wrong load, wet equipment, damaged cable, bad connection, motor stalled, chafe, corrosion or too many devices on the same feed.

Never fix a blowing fuse by installing a larger fuse unless the whole circuit has been checked and is designed for that rating. A larger fuse may simply allow the cable or equipment to burn instead.

Bad connections are also loads

A loose or corroded connection can behave like a small heater. It may work for a while, then become hot under load, then damage insulation or start a fire. This is why visual checks alone are not enough.

• Look for darkened insulation, melted holders, green corrosion, loose screws and warm terminals.
• Check high-load circuits after they have been running, using safe methods and avoiding exposed live parts.
• Use proper crimping tools, terminals, strain relief and marine-rated parts.
• Do not rely on a soldered joint where vibration and flexing can break the cable at the hard edge of the solder.
• Label both ends of important cables so the fault can be traced later.

Battery systems: the quiet danger

A 12 V or 24 V battery system looks harmless compared with 230 V AC, but the current can be enormous. A dropped tool across a battery, a chafed positive cable, or a wrong fuse can create a serious fire risk very quickly.

• Keep battery terminals covered and protected from falling tools.
• Use main fuses, main switches and busbars that are rated for the system.
• Make sure cables cannot rub against sharp edges, engine parts or moving gear.
• Do not use the sea, hull, rig or bonding system as a casual DC return path.
• Keep battery spaces ventilated and secured according to the battery type.

Battery capacity is not all usable

A battery may be sold as 100 Ah, but that does not mean the skipper should plan to use 100 Ah every night. Lead-acid batteries are normally much happier when they are not deeply discharged. Lithium can use more of its capacity, but then the BMS, charging limits and emergency reserve become even more important.

• Write down the real usable capacity, not only the label capacity.
• Plan for bad-weather loads: autopilot, radar, instruments, navigation lights, bilge pumps and communications.
• Keep a reserve that is not spent on comfort loads.
• Know what happens if one battery bank, one switch, one BMS or one charger fails.

Lithium needs different thinking

Lithium batteries are useful, but they change the system. The BMS can disconnect suddenly, charge sources may need protection, and the available fault current can be very high. A lithium conversion is not just replacing one box with another box.

• Alternator protection and charge control must be part of the design.
• Charging from solar, shore charger, inverter/charger and DC/DC sources must be compatible with the battery and BMS.
• Low-temperature charging limits must be respected where relevant.
• Critical loads, bilge pumps and emergency communication should not depend on one hidden automatic disconnect without thought.
• Set alarms and load-shedding limits before the BMS shuts the system down.

Charging sources must not fight each other

A modern boat may have alternator, shore charger, inverter/charger, solar, wind generator, generator and DC/DC chargers. Each one can be correct alone and still create problems together if the whole system is not designed.

• Know which sources charge which bank.
• Know what stops charging when the battery is full, hot, cold or disconnected.
• Protect alternators from sudden battery disconnects.
• Label charger settings and keep manuals where they can be found.

AC, shore power and inverters

AC wiring should be treated with far more respect than boat gadget wiring. Shore power, generator and inverter systems can kill people. They also interact with bonding, earth leakage protection and metal parts on board.

• Use proper RCD/GFCI protection where required by the system and local rules.
• Do not mix shore input, inverter output and generator output without a proper transfer arrangement.
• Neutral-earth bonding must be handled by the correct equipment design. A wrong bond can create a dangerous fault or make protection unreliable.
• Clearly label what the inverter feeds and what it does not feed.
• Make sure anyone on board knows how to shut down AC power quickly.

Solar and PV strings

Solar is easy to like because it is silent and automatic. But PV cables can stay live whenever there is light. Higher-voltage strings deserve the same careful routing and disconnect thinking as other serious power systems.

• Use PV cable, connectors, fuses and disconnects suited to the actual voltage and current.
• Route PV DC separately from AC unless the installation method and rules explicitly allow otherwise.
• Protect cables from deck movement, sharp edges, UV, heat and water entry.
• Label PV disconnects so a tired skipper or helper knows what is still live.
• Use surge protection where the installation and local lightning risk justify it.

Testing and fault finding

Many electrical faults are invisible until the circuit is under load. A cable can show voltage on a meter with no load and still fail when a pump, radio or autopilot draws current.

• Measure voltage at the battery and at the load while the load is running.
• Look for voltage drop across switches, fuses, breakers and connectors.
• Check warm terminals only with safe methods and no exposed live parts.
• Use the correct meter range and know whether you are measuring AC or DC.
• After fixing a fault, run the load long enough to prove the repair.

After bad weather or a wet fault

Heavy weather shakes cables, fills bilges, works terminals loose and exposes weak routing. A good electrical check after a hard period can prevent the next failure.

• Check bilge pump wiring, float switches and high-water alarms.
• Look at alternator belts, charger status and battery monitor behavior.
• Inspect deck glands, mast wiring, solar cable entries and wet lockers.
• Open only what is safe to open, and isolate power before touching terminals.

Documentation is safety equipment

A tidy diagram is not only for the next owner. It is for the night when the cabin smells warm, the pump has stopped, or the charger alarm is flashing and the skipper is tired.

• Keep a one-page system overview: batteries, main switches, main fuses, busbars, chargers, inverter and shore power.
• Keep a fuse and breaker list with rating, cable size, load and location.
• Label both ends of important cables.
• Write down charger settings, battery type, BMS limits and inverter modes.
• Log weak points, chafe repairs, overheated terminals and repeated faults.

How this links to Passage Companion

Electrical systems are only useful if the skipper can find the right information at the right time. The Where Is? inventory and logbook notes in KSF Passage Companion for Mac can be used for spare fuses, breakers, cable labels, charger manuals, inverter settings, battery notes and repair history.

The goal is simple: when something fails, the boat should not depend on one person remembering where everything is hidden.