Calculate Your Marine Battery Bank

This Marine Lithium Battery Calculator is purpose-built for boat electrical systems. It uses a pre-populated marine load table covering the most common onboard equipment — navigation electronics, bilge pumps, VHF radios, marine refrigerators, cabin lights, chartplotters, autopilots, and electric trolling motors. Select your vessel type for a quick pre-fill, adjust the loads to match your actual equipment, then set your system voltage, days at anchor, and depth of discharge to get a recommended battery bank size in both amp-hours and kilowatt-hours.

Marine battery sizing requires more care than land-based applications because of the vibration, humidity, temperature swings, and corrosive salt air environment. LiFePO4 (lithium iron phosphate) is the preferred chemistry for marine use due to its inherent thermal stability, resistance to thermal runaway, long cycle life of 3000 to 6000 cycles, and significantly lower weight compared to AGM or wet-cell lead-acid alternatives. A typical 100Ah LiFePO4 marine battery weighs 25–30 lbs versus 60–70 lbs for an equivalent AGM, a critical advantage for boat performance and stability.

Marine Lithium Battery Calculator

Quick Start — Select Your Vessel Type
Step 1 — Marine Load Table (edit to match your boat)
Marine Equipment Watts (W) Hours/Day Daily Wh
Total Daily Marine Load 0 Wh/day
— OR enter daily energy use directly —
Step 2 — System Parameters
Leave blank to use the load table total above
Most small to mid-size boats use 12V
Number of days without engine run or shore power
80% DoD is recommended for marine LiFePO4
For boats with AC appliances via an inverter
Rated capacity of each individual battery unit

Battery Sizing by Vessel Type

Marine battery requirements vary significantly by vessel type and how the boat is used. The application cards below give typical LiFePO4 battery bank specifications for the most common vessel categories. Use these as a starting point before running the calculator with your specific loads.

⛵ Day Boat

Used for day trips with return to shore power each evening. Basic loads: navigation lights, bilge pump, VHF radio, chartplotter, and small cooler.

50–100 Ah at 12V LiFePO4

🚢 Weekend Cruiser

Overnight and weekend use at anchor or marina. Adds cabin lighting, marine refrigerator, and occasional inverter use for 120V loads.

100–200 Ah at 12V LiFePO4

⛵ Sailboat / Bluewater

Extended offshore passagemaking or coastal cruising. Autopilot, radar, AIS, satphone, and refrigerator running continuously. Solar supplementation common.

200–400 Ah at 12V or 24V LiFePO4

🚤 Powerboat

Higher hotel loads including entertainment systems, air conditioning, and galley appliances. Often 24V or 48V house bank with separate starting battery.

200–400 Ah at 12V or 24V LiFePO4

🏠 Liveaboard

Full-time residential loads on a boat: air conditioning, washer/dryer, entertainment, full galley. Similar to small off-grid home in energy demand.

300–600 Ah at 24V or 48V LiFePO4

⚡ Electric Trolling

Small fishing boat with electric trolling motor as primary propulsion. Motor load dominates; size based on motor amp draw and hours of trolling per trip.

100–300 Ah at 12V or 24V LiFePO4

Marine Load Reference Table

The table below shows typical power consumption for common marine equipment. Use these figures as a reference when filling in the load calculator above. Actual power draw varies by manufacturer, model, and usage pattern — check your equipment's documentation for exact figures.

Marine EquipmentTypical WattsTypical Hours/DayTypical Daily WhNotes
Chartplotter / MFD15–40W8–16h120–640 WhHigher with radar overlay
VHF Radio (receive)3–8W24h72–192 WhTransmit draws 25–50W briefly
AIS Transponder5–15W24h120–360 WhClass B: continuous operation
Navigation Lights5–25W LED10–12h50–300 WhLED vs incandescent varies greatly
Autopilot30–120W8–20h240–2400 WhVaries with sea state and vessel size
Radar20–60W4–12h80–720 WhStandby / transmit modes differ
Marine Refrigerator40–80W avg24h cycle400–800 WhCompressor fridge; duty cycle ~50%
Bilge Pump (auto)50–150W0.1–0.5h5–75 WhShort duty cycle normally
Cabin LED Lights10–40W4–8h40–320 WhDepends on number of fixtures
Anchor Light3–10W LED10–12h30–120 WhRequired when anchored overnight
Depth Sounder5–15W8–12h40–180 WhCombined with chartplotter often
Windlass (anchor)500–2000W0.05–0.2h25–400 WhHigh peak current, short duration
Trolling Motor30–600W2–6h60–3600 WhVaries by speed setting
Inverter (microwave)900–1200W0.15–0.5h135–600 WhBrief high-current AC load
Satellite Phone / Comms5–15W2–6h10–90 WhStandby + active use combined

Why LiFePO4 for Marine Use

Lightweight — 50–70% Less Than AGM

Weight is critical on boats. A 100Ah LiFePO4 marine battery weighs approximately 25–30 lbs, compared to 60–70 lbs for an equivalent AGM battery. Replacing a 200Ah AGM bank with LiFePO4 can save 70–90 lbs, which improves performance, reduces fuel consumption, and raises the waterline on weight-sensitive sailboats and powerboats.

Full Usable Capacity at Stable Voltage

LiFePO4 maintains a nearly flat discharge voltage curve throughout most of its capacity range. Unlike AGM batteries, which show significant voltage sag as they discharge, a LiFePO4 bank delivers full power to electronics and navigation equipment even at 20–30% remaining charge. This eliminates the need to oversize the bank just to maintain adequate voltage at low states of charge.

3000–6000 Cycle Lifespan

LiFePO4 batteries in marine use are rated for 3000 to 6000 cycles to 80% depth of discharge, compared to 200–500 cycles for AGM. A liveaboard cycling their battery bank daily would replace an AGM bank every 1–2 years but could use the same LiFePO4 bank for 8–15 years. The higher upfront cost of lithium is typically recovered through fewer replacements over the life of the boat.

Fast Charge Acceptance

LiFePO4 accepts charge at up to 1C (100A for a 100Ah battery), much faster than AGM's typical 0.2C limit. This allows shorter engine running time to recharge the bank — a major advantage for offshore sailing where engine hours are expensive and fuel is limited. A properly sized alternator with an external regulator can fully recharge a LiFePO4 bank during a single engine run lasting 2–3 hours.

Thermal Safety in a Marine Environment

LiFePO4 chemistry is the most thermally stable lithium chemistry available. It does not produce thermal runaway under normal or even moderate abuse conditions, and is rated safe for the elevated temperatures found in engine compartments, tropical climates, and poorly ventilated battery lockers. This safety advantage is particularly important in enclosed marine spaces where fire risk is more dangerous than on land.

Compatible with Solar and Wind Charging

Marine solar and wind charging systems work optimally with LiFePO4 because lithium accepts virtually all the current a solar panel or wind generator can produce, right up to full charge. AGM batteries enter absorption phase at around 80% charge, significantly slowing the charge rate. LiFePO4 banks absorb solar energy more efficiently, which means smaller solar arrays are needed to maintain equivalent daily recharge levels.

Marine Battery Charging Sources

Boats have multiple battery charging sources, and proper sizing requires understanding how much charge each source provides. The main alternator on a diesel or petrol engine is typically the primary charge source for cruising boats. A 12V alternator with an external regulator and temperature compensation can deliver 80–150A to a LiFePO4 bank, providing 1–2 kWh of energy per hour of engine runtime. Without an external regulator, factory alternators typically limit output to protect against overheating, reducing charging efficiency significantly.

Solar panels are an ideal supplementary charge source for anchored boats and bluewater cruisers. A 200W solar panel in a tropical location with 6 peak sun hours produces approximately 1000–1200 Wh per day, enough to cover the hotel loads of a modest cruising boat. Larger bluewater sailboats often carry 400–600W of solar to maintain energy independence for weeks at anchor without engine runs. Marine solar charge controllers must be waterproof and rated for the combined voltage of the solar array.

Shore power connected to a smart battery charger is the most reliable charging method for boats in marinas. A 30A 120V shore power connection delivers approximately 3.6 kW, and a good quality charger can convert this to 20–40A of DC charging current to the battery bank. Wind generators are effective in anchorages with consistent breeze and provide overnight charging when solar is not generating. For offshore passages, many bluewater sailors combine solar, wind, and alternator charging to maintain balanced energy flow throughout a passage.

House Bank vs Starting Battery on Boats

Most boats require two separate battery systems: a house bank for powering all onboard electronics and hotel loads, and a dedicated starting battery for engine cranking. Lithium batteries can serve both functions, but it is common practice to use a dedicated lead-acid start battery alongside a LiFePO4 house bank for redundancy. A flat or damaged house battery bank will not affect engine starting if the start battery is kept charged and isolated from the house bank.

The house bank carries the most demanding workload — it is cycled deeply and frequently by electronics, refrigeration, and lighting. This is where LiFePO4 provides the greatest long-term value. The starting battery, by contrast, is rarely deeply discharged and has a light duty cycle. Using an affordable AGM starting battery alongside a premium LiFePO4 house bank is a common and cost-effective approach for boats transitioning to lithium.

House bank and starting battery should be isolated using a battery management relay, echo charger, or DC-to-DC charger so that the alternator charges both banks without allowing the house bank to pull down the start battery. Smart battery isolators and lithium-specific DC-to-DC chargers handle this automatically, protecting the start battery from deep discharge while ensuring both banks receive proper charging voltage for their respective chemistries.

Waterproofing and Marine Installation Requirements

Marine battery installations face environmental challenges that land-based systems do not. Salt spray, humidity, condensation, and potential water ingress require all electrical components to be rated for marine environments. Marine-rated LiFePO4 batteries typically feature an IP65 or IP67 waterproof enclosure, marine-grade tinned copper terminals, and a BMS with moisture protection. Using a non-marine-rated battery in a bilge-adjacent or exposed deck location is a significant safety and reliability risk.

Battery mounting must account for the motion of the vessel at sea. Batteries must be securely strapped or boxed to prevent movement under the forces of rolling, pitching, and heeling at sea. Loose batteries can damage terminals, break connections, and create fire hazards. ABYC (American Boat and Yacht Council) standards require that batteries be secured against movement in any direction and that the installation meets specific ventilation, fusing, and labeling requirements. Following ABYC standards is required for marine insurance in many jurisdictions.

All wiring in a marine battery installation should use tinned copper marine-grade cable. Standard automotive or household wiring corrodes rapidly in the marine environment due to electrolytic action in the presence of salt moisture. Use heat-shrink adhesive-lined connectors for all terminal connections, protect cable runs with conduit or cable clamps, and use marine-grade circuit breakers or fuses rated for DC marine applications at the appropriate voltage and current rating.

Related Tools

Use these related calculators to continue your marine battery system design:

Frequently Asked Questions

What is the best lithium battery for a boat?
LiFePO4 (lithium iron phosphate) is the recommended chemistry for marine use. It is the safest lithium chemistry, resistant to thermal runaway, tolerates the vibration and temperature extremes of a marine environment, and offers 3000–6000 cycles of useful life. Choose a marine-rated LiFePO4 battery with an IP65 or higher waterproof BMS from a manufacturer that provides marine installation support and documentation.
How many Ah do I need for a sailboat?
A typical coastal cruising sailboat needs 150–250 Ah at 12V LiFePO4 for comfortable overnight and weekend use. A bluewater offshore sailboat with autopilot, radar, AIS, and refrigerator running continuously often needs 200–400 Ah. Full-time liveaboard sailboats can require 300–500 Ah. Use the calculator above with your actual load list for the most accurate sizing for your specific boat and usage pattern.
Can I use a car lithium battery on a boat?
Not recommended. Automotive lithium batteries are not rated for the marine environment — they lack waterproof enclosures, marine-grade corrosion-resistant terminals, and vibration-resistant construction. Marine-rated LiFePO4 batteries meet ABYC electrical standards, carry IP65 or higher moisture ratings, and are designed for the shock and vibration loads of a vessel at sea. Using a non-marine battery on a boat also creates insurance and liability issues.
Should I use 12V or 24V for my boat?
Most small to mid-size boats under 35 feet use 12V, which is compatible with the widest range of marine electronics, autopilots, and windlasses. Larger vessels above 35–40 feet, twin-engine boats, and vessels with high-power bow thrusters or electric winches often benefit from 24V to reduce cable size and voltage drop. If you are building a new system, choose based on the voltage of your most power-hungry equipment — particularly the autopilot and windlass.
How do I charge a lithium battery bank on a boat?
Use a lithium-compatible smart charger for shore power charging. For alternator charging, install an external voltage regulator (such as a Wakespeed WS500 or Balmar MC-618) programmed for lithium charging profiles to avoid overloading a factory alternator. For solar, use a lithium-compatible MPPT solar charge controller. A DC-to-DC charger (B2B charger) between the start battery circuit and house bank provides safe alternator charging without risking the factory alternator.
How much solar do I need to maintain a marine battery bank?
A rule of thumb is to install enough solar to cover your daily average consumption. If your boat uses 1200 Wh/day and you have 5 peak sun hours at your cruising latitude, you need 1200 ÷ 5 = 240W of solar panels. In northern latitudes or cloudy climates, use 6–8 peak sun hours for conservative sizing. For bluewater cruising, 300–600W of solar plus a wind generator provides reliable energy independence in most conditions.
Does a marine LiFePO4 battery need a BMS?
Yes. Every lithium battery installation requires a battery management system (BMS) to prevent overcharge, over-discharge, overcurrent, and overtemperature conditions that can permanently damage lithium cells or cause safety hazards. Most marine-grade LiFePO4 batteries include an integrated internal BMS. When building a DIY LiFePO4 bank from prismatic cells, a high-quality external BMS rated for the pack's maximum charge and discharge current is required.
Can I mix LiFePO4 with my existing AGM batteries?
No — LiFePO4 and AGM batteries must not be connected directly in parallel. They have different voltage profiles, charging algorithms, and internal resistance characteristics. LiFePO4 charges to 14.4–14.6V while AGM charges to 14.4–14.8V but requires different absorption and float stages. Mixing chemistries causes improper charging of one or both battery types. Always replace the entire house bank with lithium at the same time, and use a DC-to-DC charger to manage charging between different battery banks of different chemistries.