Almost every problem we hear about at the counter — flat batteries on a Monday morning, a fridge that quits at 3am, nav lights that dim when the bilge pump runs, an alternator that cooks itself — comes back to the same thing: a power system that was assembled one piece at a time, without anyone ever adding up the numbers.
This guide walks through every element of a boat's electrical system, from the battery bank to the last crimp terminal. For each part you'll get two things: what it actually does, and how to size it. It applies whether you're powering a fishing punt on Galway Bay, a 34ft cruiser heading for the Aran Islands, or a liveaboard that never sees a shore lead.
You don't need to read it end to end. Start with the energy budget below, then dip into whichever component you're buying. There's a full system calculator at the foot of the page that pulls it all together and lets you send the results straight to us for a quote.
1. How a Boat Power System Fits Together
Two principles hold the whole thing together. The first is that the bank must be big enough to ride out the gaps between charges — overnight, or a weekend on a mooring in Connemara with the engine off. The second is that whatever goes out has to go back in: if you draw 100 amp-hours a day, something has to put 100 amp-hours a day back, or you're on a slow slide to a flat bank and a ruined set of batteries.
Almost every "my batteries are no good" call is really one of those two failing — either the bank is too small for the way the boat is actually used, or nothing aboard is putting back what comes out. Everything that follows is aimed at those two problems.
2. Step One: Work Out Your Energy Budget
What's a watt-hour? A watt-hour (Wh) is just a unit of energy — how much power a device uses multiplied by how long it runs. The sum is simple: watts × hours = watt-hours. A 5-watt LED cabin light left on for 4 hours uses 20Wh. Do that for everything aboard, add it up, and you have your daily energy budget.
Working in amps? Most boat and van gear is rated in amps at 12 volts. To turn amps into watts, multiply by the voltage: watts = amps × volts. So a water pump pulling 5A at 12V draws 60W — run it for half an hour and that's 30Wh.
And amp-hours? Batteries are often rated in amp-hours (Ah). At 12V, multiply Ah by 12 to get watt-hours — so a 100Ah battery holds roughly 1,200Wh. That's the bridge between how batteries are sold and how your usage is measured.
🔌 Daily Usage Estimator — tap to open or close
Set how many of each item you have and roughly how many hours a day you use it. We've pre-filled typical values for a small cruiser — adjust to suit. High-power 230V items only run for minutes, so enter a small fraction of an hour (0.1h ≈ 6 minutes).
Two things people consistently get wrong. The fridge is almost always the biggest item — it runs day and night, and while its compressor might pull 45W, it only cycles for part of each hour, so we average it to around 20W continuous. And 230V appliances are brutal. One kettle boil is roughly 100Wh, about the same as running every light on the boat for an evening.
Hold onto that Wh/day figure — every section that follows is sized from it, and the system calculator turns it into a full specification you can send to us.
3. The Battery Bank — The Heart of the System
The first thing to understand is that a boat needs two separate batteries doing two very different jobs. An engine-start battery delivers a huge current for a couple of seconds and is then immediately recharged — it has thin plates and hates being deeply discharged. A service (or leisure/deep-cycle) battery delivers a modest current for hours on end and is built with thicker plates to survive being run down and filled again, hundreds of times.
Running both jobs off one battery is the single most common mistake on small craft. It works right up until the evening you flatten it running the lights, and then you're on a mooring with no engine. Keep them separate, and keep the start battery ring-fenced.
The second thing is usable capacity. A battery's Ah rating is not what you can take out of it. A traditional lead-acid battery should not routinely go below 50% state of charge — take it to 20% regularly and you'll be buying a new set inside two seasons. Lithium (LiFePO₄) can safely give up 80–90%. So a 200Ah lead-acid bank and a 110Ah lithium bank deliver roughly the same real energy.
📐 How to size the bank
Usable %: 0.5 for lead-acid, AGM or gel; 0.8 for lithium.
Worked example: 1,500Wh/day, 2 days between charges, 12V, AGM → (1500 × 2) ÷ (12 × 0.5) = 500Ah. The same boat on lithium → (1500 × 2) ÷ (12 × 0.8) = 313Ah, so a 300Ah bank and a bit of discipline.
Two practical notes. Match batteries within a bank — same chemistry, same capacity, same age. Adding one new battery to three tired ones drags the new one down to the level of the worst. And allow for the cold: a lead-acid battery at 5°C, which is a normal winter reading in an Irish bilge, delivers noticeably less than its rated capacity.
4. Battery Types: Which Chemistry Suits Your Boat?
There is no single best battery — there's the one that suits how you use the boat. A trailer-sailer that lives on shore power wants something cheap and forgiving. A boat that spends a fortnight at anchor wants something that cycles hard and charges fast.
| Type | Usable | Typical cycles | Best for | Watch for |
|---|---|---|---|---|
| Flooded lead-acid | ~50% | 300–500 | Budget builds, engine starting, boats on shore power | Needs topping up and a vented, upright locker |
| AGM | ~50% | 400–700 | The sensible default for most cruisers | Unforgiving if left flat over winter |
| Gel | ~50% | 500–800 | Deep, slow cycling; hot engine spaces | Lower charge voltage — the charger must have a gel setting |
| Lithium (LiFePO₄) | 80–90% | 2,000–5,000 | Liveaboards, heavy anchoring, weight-critical boats | Needs a BMS, alternator protection, and no charging below 0°C |
The honest lithium summary: it costs several times more per Ah up front, but you get roughly twice the usable energy from the same nameplate capacity, a third of the weight, five to ten times the cycle life, and it will accept charge as fast as you can supply it — which is what really shortens engine hours at anchor.
The catch is that lithium changes the rest of the system. Because it accepts current so greedily, a standard alternator wired straight to a lithium bank will run flat out and can cook itself. It also needs a battery management system, chargers with a LiFePO₄ profile, and a fuse rated to interrupt the very high fault current a lithium bank can deliver — a Class T, as covered under circuit protection further down. Budget for the system, not just the battery.
5. 12V or 24V? Choosing Your System Voltage
12V is the default and the right answer for the overwhelming majority of Irish boats. Everything is available in it, from nav lights to VHF sets, and the engine already runs at 12V. Below about 2kW of total demand there is no reason to look further.
24V earns its place on larger vessels, on anything with a bow thruster or a windlass, on boats with long cable runs fore and aft, and on any system running a 3kW-plus inverter. A 3,000W inverter at 12V pulls close to 300A — that's 95mm² cable and a serious fuse. The same inverter at 24V pulls 150A and the whole installation becomes half the cost and twice as tidy.
📐 How to choose
- Total continuous demand under ~1.5kW, no big motors → 12V
- Inverter over 2kW, thruster or windlass fitted, or runs over about 8 metres → consider 24V
- Already have a 12V engine and want 24V house? Perfectly normal — a DC-DC converter feeds the 12V start side, or a step-down converter runs your legacy 12V gear
Whichever you pick, decide before you buy anything. Changing voltage later means new batteries, new charger, new MPPT, new inverter and, usually, new cable.
6. Charging Source 1: The Engine Alternator
The awkward truth about alternators is that the rating on the case is not what you get. A "90 amp" alternator makes 90A cold, at high revs, into a very flat battery. Hot, at cruising revs, into a half-full bank, real output is often half that. And a standard internal regulator drops to a float voltage long before a lead-acid bank is actually full — which is why boats that motor for two hours a day still end up chronically undercharged and sulphated.
There are three ways to get charge from the engine to the service bank:
- Split-charge relay (VSR). Cheap and simple — it connects the two banks once the start battery reaches a set voltage, and disconnects when the engine stops. Fine for lead-acid. Not suitable in front of a lithium bank.
- DC-DC charger. Takes whatever the alternator gives and delivers a proper multi-stage charge at a controlled current. This is the modern answer, and it's the only sensible way to charge lithium from an engine, because it caps the current the alternator has to produce.
- External smart regulator. Replaces the alternator's internal regulator so the alternator itself holds a full bulk/absorption profile. Best output-per-euro on boats with a large lead-acid bank and a decent alternator.
📐 How to size it
Engine hours to replace a day = (daily Wh ÷ volts) ÷ (charger A × 0.9)
Worked example: 1,500Wh/day at 12V is 125Ah. With a 30A DC-DC charger that's about 4.5 hours of engine running — fine if you're motoring between Kilronan and Rossaveal, painful if you're on a mooring for a week.
Never fit a DC-DC charger rated above about half the alternator's output. The alternator has to run the engine's own loads as well, and it is air-cooled by a fan that isn't designed for hours at full belt.
That engine-hours figure is usually what pushes people toward solar or a bigger bank. It's worth calculating before you spend anything.
7. Charging Source 2: Shore Power and Mains Chargers
A shore installation has three parts: the inlet and shore lead (a 16A blue commando connector is the standard on Irish pontoons), the AC distribution with an RCD and breakers, and the battery charger itself. The AC side is not a DIY job — 230V aboard a wet aluminium or GRP boat is unforgiving, and on a coded vessel it forms part of survey. Get a qualified marine electrician to install and test it.
The charger matters more than people expect. A cheap single-stage unit will hold the bank at a fixed voltage indefinitely, which slowly boils a flooded battery dry and shortens the life of an AGM. What you want is a multi-stage charger — bulk, absorption, then float or storage — with a chemistry setting that matches your batteries and, ideally, temperature compensation.
📐 How to size it
Lithium: charger ≈ 20–50% of bank Ah
Worked example: a 300Ah AGM bank wants a 30–60A charger. Go much smaller and a flat bank takes all weekend to recover; go much bigger and you're paying for capacity a lead-acid battery can't absorb anyway.
If your boat lives on a swinging mooring rather than a pontoon, a portable inverter generator feeding the same charger does the same job — just size the generator to the charger's AC input, plus a margin.
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8. Charging Source 3: Solar, Wind and Hydro
These are the sources that work while you're asleep, ashore or away from the boat — and that's their real value. Even a small panel that only offsets the fridge changes how often you have to start the engine.
Solar
Solar is silent, has no moving parts and is by far the most popular addition. The honest caveat for Ireland is seasonality: a panel that comfortably runs your fridge in June will contribute very little in December. Size for the season you actually use the boat, and treat winter solar as trickle-charging rather than a power source.
📐 How to size an array
The 0.7 covers heat, cable and controller losses. For peak sun hours on the west coast, use 2.5 as a conservative full-season figure — roughly 4 to 4.5 in high summer, well under 1 in midwinter.
Worked example: 1,500Wh/day ÷ (2.5 × 0.7) = 857W, so around 900W of panels. You can safely oversize the array — the MPPT simply caps at its rated output — which is exactly what buys you useful shoulder-season performance.
Mounting matters as much as wattage. Shade is the enemy: a boom, a backstay or a furled sail across one corner of a rigid panel can knock out most of its output, because cells in series drag each other down. Panels on a stern arch or bimini generally out-perform larger deck panels for that reason. Flexible panels bonded to a coachroof are neat but run hotter and typically have a shorter life than framed rigid panels.
Wind
A wind generator is one of the few sources that genuinely suits the Atlantic coast, because it produces at night, in winter and in exactly the conditions that keep you aboard. Output climbs steeply with wind speed, so a turbine that makes a trickle in 10 knots can be making real amps in 25. The trade-offs are noise, vibration through the pushpit, and the cost of a proper mounting pole. On a boat that lies to a mooring in Connemara through the winter, one often does more useful work than twice its cost in panels.
Hydro
A towed or transom-mounted hydro generator produces serious current under sail — several amps at 5 knots, plenty to run everything on a passage. It's a passage-maker's tool rather than a coastal cruiser's, and it costs you a fraction of a knot. If you're heading offshore, it's the most reliable source there is.
9. Charge Controllers: MPPT vs PWM
A PWM controller is essentially a fast switch. It's cheap, but it drags the panel down to battery voltage, so a panel producing 18V into a 13V battery loses the difference as wasted potential. A MPPT controller is a DC-DC converter that holds the panel at its most efficient operating point and converts the surplus voltage into extra current — typically 20–30% more harvest from the same panels, and considerably more in cold, bright weather.
MPPT is not just about efficiency, though. It's what lets you use ordinary high-voltage domestic panels, which cost far less per watt than "12V" marine panels, and it lets you wire panels in series and run thinner cable from deck to locker.
📐 How to size it
MPPT controllers are named for two numbers — maximum PV voltage and maximum output current. A "100/30" handles up to 100V from the array and delivers up to 30A to the battery.
Max PV voltage ≥ array open-circuit voltage × 1.25
Worked example: 600W of panels on a 12V bank = 600 ÷ 12 = 50A, so a 100/50 or 150/50. The 1.25 factor on voltage covers cold mornings — panel voltage rises as temperature falls, and exceeding the controller's PV limit destroys it instantly.
For an exact cold-weather check on the panels you've chosen, use Victron's MPPT calculator, or send us the panel model and we'll do it for you.
10. Inverters: Running 230V Kit Aboard
First, the waveform. Pure sine wave inverters produce a clean output identical to mains. Modified sine wave units are cheaper but produce a stepped approximation that makes motors run hot, upsets some battery chargers and laptop supplies, and can put a buzz through audio and instruments. For anything beyond a drill or a work light, buy pure sine.
Second, the current. This is where people get caught out. Power is power: a 2,000W kettle drawing 8.7A from the mains draws around 195A from a 12V battery. That is a very serious current — it needs heavy cable, a proper fuse, and a bank that can actually deliver it without the voltage collapsing.
📐 How to size it
DC current draw (A) = AC watts ÷ (system volts × 0.85)
Size to your biggest simultaneous load, not the sum of everything aboard. Then check surge: motors, compressors and pumps can pull three to five times their running current for a second at startup, so the inverter's peak rating must cover it.
Rule of thumb for the bank: lead-acid struggles to supply more than about 0.5C, so a 2,000W inverter at 12V (≈195A) really wants at least a 400Ah lead-acid bank behind it — or lithium, which handles that current happily.
Don't leave it switched on. Most inverters draw half an amp to an amp just sitting there idling. Over 24 hours that's 12–25Ah gone for nothing — often more than the fridge. Wire it through its own switch, or use a model with a search/eco mode.
If you also need a shore charger, look at an inverter/charger instead of two separate boxes. It combines both, adds an automatic transfer switch that swaps between shore power and inverter without you touching anything, and on better units can supplement a small shore supply from the batteries when a load exceeds the pontoon's 16A limit.
11. Distribution: Isolators, Busbars and Switch Panels
The chain runs like this: battery → main fuse → battery isolator switch → positive busbar → distribution panel → individual circuits, with every negative returning to a single negative busbar. That last part matters. A common negative bus, close to the battery, is what stops the mysterious voltage drops and interference that plague boats wired with negatives chained from one appliance to the next.
- Battery isolator switch. One per bank, as close to the battery as practical, so the whole system can be killed instantly. Many boats fit a 1-2-Both-Off selector; a cleaner modern approach is a separate isolator per bank plus an automatic charging relay or DC-DC charger.
- Busbars. A positive and a negative distribution point with proper studs. Far safer and tidier than stacking six ring terminals on a battery post — and it means the main cables only have to be made off once.
- Switch panel. Rocker switches with integral breakers, or switches plus a fuse block. Label every circuit. Leave two spare ways — you will need them.
- Emergency parallel switch. Worth fitting: it links the service bank to the start bank if the start battery ever lets you down.
📐 How to size it
Isolators and busbars: rate for the largest current that could flow — usually the inverter, the windlass or the starter, whichever is biggest. A boat with a 2kW inverter at 12V needs a 300A-plus switch and busbar, not the 100A one that looks adequate. Panel ways: count your circuits and add 25%. Mount everything as high and dry as possible; a corroded busbar in a damp locker is a hot joint waiting to happen.
Browse our marine electrical range for panels, switches, connectors and busbars.
12. Circuit Protection: Fuses and Breakers
Every positive conductor leaving a battery needs a fuse, and it needs to be as close to the battery terminal as physically possible — a few centimetres, not a few metres. The unfused length between the post and the fuse is the one piece of cable on the boat that nothing protects.
Never fuse the negative. If a negative fuse blows, the circuit looks dead but the positive side is still live — and the current will find another path home, usually through something expensive.
| Device | Typical range | Use it for |
|---|---|---|
| Blade fuse | 1–40A | Branch circuits — lights, pumps, electronics |
| Thermal breaker | 5–50A | Panel circuits you want to reset without spares |
| MRBF terminal fuse | 30–300A | Neat, bolts straight to the battery post |
| MEGA / ANL | 100–500A | Main feeds, inverters, windlasses, thrusters |
| Class T | 100–400A | Lithium banks — very high interrupting capacity |
📐 How to size it
The fuse must be big enough not to nuisance-blow, and small enough that the cable can survive it. If you can't satisfy both, the cable is too thin — fix that, don't fit a bigger fuse. And keep a labelled box of spares aboard; a blown fuse at 2am in a Galway Bay swell is not the time to be improvising.
13. Cable: The Bit Everyone Undersizes
Cable is chosen for two independent reasons, and you must satisfy whichever gives the bigger answer. The first is current-carrying capacity — the cable must not overheat. The second is volt drop — the cable must deliver useful voltage at the far end. On boats, volt drop almost always wins, because the runs are long and 12V has very little to give away.
Aim for 3% volt drop on critical circuits (navigation lights, VHF, instruments, bilge pumps) and no more than 10% on non-critical ones. At 12V, 3% is just 0.36V — which is why a "small" cable over a long run is such a common cause of dim lights and pumps that struggle.
| Current | 3m run | 6m run | 10m run |
|---|---|---|---|
| 5A (lights, VHF) | 1.5 mm² | 4 mm² | 6 mm² |
| 10A (pump, fridge) | 4 mm² | 6 mm² | 10 mm² |
| 20A (sub-panel) | 6 mm² | 16 mm² | 25 mm² |
| 40A (charger feed) | 16 mm² | 25 mm² | 50 mm² |
Indicative sizes for tinned copper at 12V, 3% volt drop. "Run" is the one-way distance — the calculation already accounts for the return leg. At 24V the current halves for the same power, so cable sizes drop dramatically.
📐 The formula, if you'd rather calculate
Use marine-grade tinned cable. Not automotive, not house wire. Salt air wicks up untinned strands and turns them green and resistive from the inside out, long before anything looks wrong at the terminal. Solid-core cable has no place on a boat — vibration work-hardens it until it snaps.
Terminate with properly crimped lugs and heat-shrink, using a ratchet crimper sized to the terminal. Soldered joints go brittle where the solder wicks into the strands and fail at exactly the point the cable flexes. Support cable runs every 450mm or so, keep them out of the bilge, and never let a run chafe on a bulkhead edge.
14. Monitoring: Knowing What's Actually in the Bank
A voltmeter is not a fuel gauge. Voltage on a lead-acid battery is only meaningful after it has rested for hours with no load and no charge — under load it sags, and just after charging it holds a misleading surface charge. On lithium it's worse still: the discharge curve is so flat that the voltage barely moves between 90% and 20% full, then falls off a cliff.
A shunt-based battery monitor solves it. A precision resistor goes in the main negative cable, and the monitor measures every amp passing through it in both directions. You get true state of charge as a percentage, current draw in real time, and time-remaining at the present rate — which is exactly the number you want at 11pm deciding whether to run the heater.
📐 How to size and fit it
- 500A shunt suits almost every leisure installation, including a 2kW inverter
- The shunt goes in the battery negative, and every negative on the boat must connect on the load side of it — one stray negative straight to the battery post and the readings quietly lie to you
- Programme it with your actual bank capacity and chemistry, or the percentage means nothing
This is the single best-value upgrade on most boats. It costs a fraction of a battery bank and it stops you destroying one through chronic undercharging — which is what quietly kills the majority of leisure batteries we see replaced.
15. Earthing, Bonding and Galvanic Corrosion
Three different things get called "earth" on a boat and they are not interchangeable. The DC negative is the return path for your 12V system. The AC protective earth is a safety conductor that must connect to the shore earth so a fault trips the RCD. Bonding is a separate green wire linking underwater metals — skin fittings, shaft, keel bolts — so they corrode as one controlled system protected by anodes, rather than individually.
The trouble starts on the pontoon. Connecting the shore lead ties your underwater metals, through the earth conductor, to those of every other boat on the same supply. Dissimilar metals in salt water make a battery, and a small DC current starts flowing — usually out of your anodes, and if they're gone, out of your prop, shaft or skin fittings.
📐 What to fit and how to size it
- Galvanic isolator in the shore earth — blocks low-voltage galvanic current while still passing fault current. Rate it at or above your shore supply (16A on most Irish pontoons)
- Isolation transformer — the gold-standard alternative, fully separating boat from shore. Heavier and dearer, but it also cures shore-supply polarity problems
- Anodes — sized to hull and shaft, and replaced when roughly half consumed. Check every lift-out
If your anodes are disappearing in a single season on a marina berth, that's the diagnosis. Browse electrical fittings or talk to us — and if you're operating a coded vessel, our Fishing Vessels Code of Practice guide covers where electrical installations sit in the survey.
16. Size Your Whole System
Put your daily figure from the estimator in here, tick the charging sources you have or want, and this pulls together every number from the sections above — bank, charger, array, inverter, main fuse and cable. It's indicative rather than a specification, but it's the right starting point for a conversation.
⚡ Power System Sizing Calculator
Recommends a buildable specification from standard components we can supply. Indicative only — contact us to confirm a final specification.
Solar figures assume around 2.5 peak-sun-hours, conservative for the west of Ireland, and typical system losses. Cable and fuse figures assume a short main run — use the cable sizing table further up the page for longer runs, and remember that the bigger of the ampacity and volt-drop answers always wins.
17. Three Worked Examples
Numbers make more sense against a real boat. Here are three that cover most of what we see coming through the shop.
Day boat / small fishing punt — ~150Wh/day
Nav lights, VHF, a chartplotter and a bilge pump. One 110Ah AGM leisure battery plus a separate start battery, split with a voltage-sensitive relay, and a 10A shore charger for the winter. A 50W panel on the console keeps everything topped up between trips. Total investment: modest. Total worry: none.
32ft cruiser, weekends and a two-week summer cruise — ~900Wh/day
Fridge, instruments, autopilot, lights, phone charging. Around 300Ah of AGM on 12V gives two days at anchor. A 30A DC-DC charger from the alternator refills it in roughly two and a half hours of motoring; a 30A shore charger handles marina nights. Add 300W of solar and you'll rarely start the engine to charge in July. A 500A shunt monitor and a 150A main fuse finish it.
Liveaboard, mostly on a mooring — ~2,800Wh/day
Fridge and freezer, heating fan, laptops, a washing machine, occasional induction hob. This is a 24V lithium boat: around 300Ah at 24V, a 3kVA inverter/charger, 900W of solar with a 150/45 MPPT, a 50A DC-DC from the engine, a Class T main fuse and 70mm² cable to the inverter. Every element of this guide, in one installation.
Notice the pattern: the further you get from a pontoon, the more the money moves out of the batteries and into the charging. A big bank with nothing to fill it is just a heavy way to be flat by Sunday.
18. Ten Common Mistakes
- One battery for starting and domestics. Sooner or later you'll flatten it, and it'll be the day the weather turns.
- No main fuse at the battery. The most dangerous omission on any boat, and the cheapest to fix.
- Cable sized by eye. If it looks about right, it's usually one or two sizes too small.
- Automotive or house cable instead of tinned marine cable. It corrodes invisibly from inside the insulation.
- Soldered joints instead of proper crimps. They fail exactly where the cable flexes.
- Charging lithium through an unregulated alternator. A quick route to a cooked alternator and a very expensive morning.
- Sizing solar for July. On the west coast, midwinter output is a small fraction of summer. Plan for the season you actually use.
- Leaving the inverter switched on. Idle draw quietly eats more than the fridge.
- Adding one new battery to an old bank. The oldest battery sets the performance of the whole set.
- Judging state of charge by voltage. Fit a shunt monitor and stop guessing.
Every one of these is cheaper to avoid than to repair. If you're unsure about any of them on your own boat, get in touch — we'd far rather talk it through before you buy.
19. Looking After It: Seasonal Checks
Every few weeks in season
- Check terminals are tight and clean — a loose terminal gets hot, and heat is the first sign of a joint about to fail
- Look for green powder or white crust on lugs and busbars; clean and re-protect with a terminal spray
- Top up flooded batteries with distilled water if the plates are showing
- Glance at the monitor: is the bank actually reaching 100%, or stalling at 85% every time?
Laying up for winter
- Lead-acid and AGM: charge fully, then keep them charged. A lead-acid battery left flat over an Irish winter sulphates and may never recover. Either leave a smart charger connected, or fully charge and disconnect and recharge every couple of months
- Lithium: the opposite — store around 50% charge, in a cool place, with the BMS isolated. Do not attempt to charge it below 0°C unless it has integral heating
- Isolate everything that doesn't need to run, and check what's still drawing — bilge alarms and trackers add up over five months
- Inspect the shore lead for cuts, and the inlet for corrosion and heat marks
- Check anodes and replace anything more than half gone
Spring recommissioning is a good moment to load-test the bank rather than just measuring voltage. A battery that reads 12.7V and collapses under load has already failed — better to find out alongside than three miles off Black Head.
20. Frequently Asked Questions
How many batteries do I need for a weekend aboard?
Work out your daily watt-hours, multiply by two nights, and divide by usable capacity. A typical small cruiser using 900Wh/day needs about 300Ah of AGM or 190Ah of lithium at 12V to cover two nights comfortably.
Can I just swap my AGM batteries for lithium?
Not as a straight drop-in. Lithium needs a battery management system, chargers set to a LiFePO₄ profile, alternator protection (usually a DC-DC charger), and a fuse with a suitably high interrupting rating. It's a system change, not a battery change — but it's a very worthwhile one on a boat that cycles hard.
Will solar keep my batteries charged over the winter in Ireland?
A modest panel will usually offset self-discharge and small parasitic loads like a bilge alarm, which is genuinely useful. It will not run a fridge or recover a flat bank in December — daylight and sun angle are simply too low. Treat winter solar as maintenance charging.
Why does my battery read 12.6V but die within an hour?
Voltage measures pressure, not quantity. An old or sulphated battery holds a healthy resting voltage but has lost most of its capacity, so it collapses as soon as you load it. A load test — or a shunt monitor watching what actually comes out — tells you the truth.
Do I need an electrician, or can I do this myself?
Competent owners routinely install their own 12V and 24V DC systems, provided the fusing and cable sizing are right. Anything on the 230V AC side — shore inlet, RCD, distribution, inverter output wiring — should be installed and tested by a qualified marine electrician. On a coded commercial vessel the electrical installation forms part of survey.
What's the single best upgrade for the money?
A shunt battery monitor. It costs a fraction of a battery bank, and it stops you slowly destroying one through chronic undercharging — which is what actually kills most leisure batteries.
Not sure which bits you actually need?
Send us your calculator results, or just tell us the boat and how you use her. We'll spec the bank, charging and protection to suit — and we'll tell you honestly if you don't need half of it.
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