The essentials in 30 seconds
- Electricity on a sailboat is decided in this order: consumption, storage, charging, conversion, then protection and monitoring. The bank is sized based on what you consume and on what your charging sources restore each day.
- Start with the 24-hour balance: current of each device × operating hours. The calculator on this page does it for you.
- Switching to lithium is not just swapping batteries: you need a BMS suited to the brand, chargers set to the manufacturer’s specifications, and a solution for the alternator.
- Victron, Mastervolt and MG Energy do not organize the BMS the same way: external, integrated into the battery, or master controlling modules. This determines what you need to add to the battery.
- Solar: the MPPT regulator is chosen based on the open-circuit voltage of the panels in cold weather and on the charging current; actual production is far below the peak power.
Table of contents
- Five blocks, in this order
- Perform your energy balance (calculator)
- From consumption to capacity
- Switching to lithium: what really changes
- Charging: dock, engine and sun
- Convert: 12 / 24 V and 230 V
- Protect and monitor
- Energy refit and wintering
- Victron, Mastervolt or MG Energy?
- Limits and pitfalls
- Our verdict by program
- Sources
This guide compiles what we published in six separate articles: sizing a lithium bank, switching to lithium, charging from the engine and solar, converting 12 V to 24 V or 230 V, and preparing an energy refit. It follows the sequence in which decisions are made aboard, and for each piece of equipment it links to the manufacturer’s documentation.
Five blocks, in this order
A sailboat’s electrical system is decided in a precise sequence, and each block depends on the previous one: you do not choose a battery before knowing what you consume, nor a charger before knowing the battery.
| Block | The question to resolve | What depends on it |
|---|---|---|
| 1. Load | How many ampere-hours does the boat consume in 24 hours, at anchor and while sailing? | Everything else |
| 2. Storage | What capacity, voltage (12, 24 or 48 V), chemistry, and BMS? | Chargers, protections, cable cross-sections |
| 3. Charging | How much does the shore power inlet, alternator, and solar restore each day? | Actual cruising range |
| 4. Conversion | Is 230 V AC needed aboard, or a second DC voltage? | The inverter, combi unit, DC-DC converters |
| 5. Protection and monitoring | Fuses, battery isolators, state-of-charge measurement | The system’s safety |
Switching to lithium affects blocks 2, 3, and 5 simultaneously. That is why it is never just a matter of swapping batteries.
Performing a 24-hour energy balance
The balance is calculated item by item: the current drawn by each device (in amperes, at the boat’s voltage), multiplied by the number of hours it runs over 24 hours. The sum gives the daily consumption in ampere-hours (Ah). If you work in power terms, divide watt-hours by the bank voltage: 600 Wh per day equals 50 Ah on a 12 V bank and 25 Ah on a 24 V bank.
On a cruising sailboat, the refrigerator and autopilot often weigh the most because they run for long periods. Their actual power draw depends on temperature, icebox insulation, sea state, and autopilot settings: start with the values in the manual, then adjust them with a battery monitor once you have one installed.
The calculator below performs this calculation, deduces the required battery capacity, and checks whether your charging system can keep up. All values are editable; the profiles provided are examples, not measured data.
Calculate your lithium bank — 24 h energy balance
The 3-step method: what you consume in 24 h, the required LiFePO4 capacity, and whether your charging can keep up. All values are editable.
Daily consumption: —
A battery bank is only as good as its ability to recharge. Without a suitable charging setup, the Ah sizing remains theoretical.
Educational estimate to help plan your project — it does not replace an on-board power balance (measured loads, alternator condition, cable sizes, BMS settings). Solar assumption: Wc × 4 h equivalent × 0.7 real efficiency.
From power draw to capacity
Calculating the required battery capacity involves three steps:
- Daily consumption × days of autonomy without recharging. Two days at anchor without engine or sun means twice the daily consumption.
- Divided by the usable depth of discharge (DoD). This is not a personal choice: it is the value at which the battery manufacturer specifies its service life. Examples: Victron quotes 2,500 cycles at 80 % DoD for its Smart and NG lithium batteries (5,000 at 50 %); Mastervolt about 3,500 cycles at 80 % for its MLI Ultra batteries at 25 °C and limited currents; MG Energy Systems over 4,000 cycles at 80 % for its LFP 25.6 V 304 Ah battery at 25 °C, end-of-life set at 70 % of initial capacity. These figures are not directly comparable: each manufacturer sets its own test conditions.
- Do not work in “Ah lead-acid” terms. A 200 Ah lead-acid bank from which you only use half provides 100 Ah of usable capacity; with lithium used at 80 % DoD, 125 Ah deliver the same energy (Skysat calculation: 100 ÷ 0.8).
- Round up to available sizes. Batteries come in discrete capacity steps, and not all can be wired in parallel or series the same way.
12, 24 or 48 V? At equal power, doubling the voltage halves the current: an inverter delivering 2 000 W draws about 170 A from a 12 V bank and about 85 A from a 24 V bank (Skysat calculation: power divided by voltage, inverter losses not included, which further increases the actual current). Less current means thinner cables, lower voltage drop, and smaller protection devices. On the other hand, some navigation electronics remain 12 V and require a DC-DC converter. A boat already wired for 12 V often stays on 12 V; a major refit is the right time to reconsider.
Switching to lithium: what really changes
A lithium iron phosphate battery (LiFePO4, or LFP) accepts and delivers high currents, maintains a stable voltage, and can be discharged more deeply than a lead-acid battery. These advantages come with a downside: it must be monitored cell by cell by a BMS, and every charging source must comply with the manufacturer’s specifications. Switching to lithium is therefore not just a battery swap; it’s a full review of the entire charging chain.
The BMS: three architectures by brand
The three brands we carry implement the BMS differently. This is the first point to check, because it determines what else you need to buy beyond the battery.
| Brand | Where is the BMS | What needs to be added |
|---|---|---|
| Victron Energy (Lithium Smart and NG batteries) | External: the battery monitors its cells but does not protect itself | A Victron BMS compatible with the battery family: smallBMS, VE.Bus BMS V2 or Lynx Smart BMS for Smart; smallBMS NG, VE.Bus BMS NG or Lynx Smart BMS NG for NG. The two families are not interchangeable. |
| Mastervolt (MLI Ultra 12/3000, 12/6000, 24/6000) | Integrated in the battery | An external safety relay, mandatory, and a type T fuse (max 500 A, breaking capacity of at least 15 kA). The MLI Ultra connection kit includes the mount, relay and fuse holder, without the fuse. The MLI Ultra 1250 models have an integrated relay. |
| MG Energy Systems (LFP batteries) | Distributed: each module has a slave BMS, controlled by a mandatory master BMS | An MG Master LV (or HV), which integrates contactor, pre-charge, fuse holder, distribution and shunt. |
The details of the three ecosystems are in our Victron, Mastervolt and MG Energy lithium BMS comparison.
Chargers: the battery manufacturer’s profile
Each manufacturer specifies its charging voltages. For its Smart and NG lithium batteries, Victron requires 14.2 V absorption for at least two hours, then 13.5 V float (28.4 V and 27 V for 24 V systems), and prohibits charging below 5 °C. Mastervolt requires 14.25 V then 13.5 V for its MLI Ultra batteries (28.5 V and 27 V for 24 V). A charger without an adjustable lithium mode is unsuitable; our guide docking or combi charger for lithium banks reviews compatible models and their control via the BMS.
Alternators: the most often overlooked point
A lead-acid battery inherently limits the current it accepts as it charges; a lithium battery continues to absorb high current. An alternator designed for lead-acid can therefore run at full power for extended periods and overheat. If the BMS cuts charging while the alternator is delivering power, the alternator must be stopped or decoupled cleanly. Three documented approaches, as per the manufacturers:
- A DC-DC charger between the alternator and the lithium bank. The Victron Orion XS limits its input current, charges only when it detects the engine running, and has an input that the BMS can control. The starter battery remains connected to the alternator, while the lithium bank is charged via the DC-DC charger.
- A BMS-controlled alternator regulator. With Mastervolt, the Alpha Pro III has an MLI profile and cuts the alternator’s excitation current on the battery’s “Stop charge” event. At MG Energy Systems, the AFC 300 regulator controls alternator excitation based on BMS commands.
- An alternator and regulator sized for lithium, such as Mastervolt’s Alpha alternators paired with the Alpha Pro III.
Which one suits your sailboat depends on the alternator installed, the power you want to recover from the engine, and the battery brand. This is typically the kind of decision to make during a power audit.
Charging: dock, engine and sun
At the dock
The shore charger is selected based on three criteria: an adjustable lithium mode matching the battery manufacturer’s specifications, a way for the BMS to stop it, and a current between what’s needed to restore the deficit during your time in harbor and what the battery can accept (Victron recommends 0.5 C for its Smart and NG batteries, i.e., half the capacity in amperes). All details are in our shore charger guide for lithium banks.
At the engine: alternator and DC-DC charger
The DC-DC charger is the simplest solution for charging a lithium bank from the alternator without modifying the alternator itself (see above). Two families exist:
- Non-isolated: input and output share the same negative. This is the case for Victron’s Orion XS (50 A, up to 1,400 W at 28 V for the XS 1400) and Mastervolt’s Mac Plus (50 A in 12/12 V and 24/12 V), which feature a lithium charging profile.
- Isolated: input and output are electrically separated (200 V galvanic isolation on Victron’s isolated Orion-Tr Smart). Choose this when the two circuits must remain separate; Mastervolt also offers isolated converters, the Magic.
The DC-DC current must stay within what the alternator can continuously supply, once other engine consumers are served.
In the sun: panels and MPPT regulator
An MPPT charge controller adjusts the panel voltage to match the battery’s, maximizing power output. With Victron, its two numbers say it all: a SmartSolar 100/30 accepts up to 100 V open-circuit on the panel side and charges up to 30 A. Panel open-circuit voltage rises when it’s cold: Victron recommends keeping an extra 10 % margin when nights approach or drop below 10 °C. With Mastervolt, the SCM25 (25 A, 75 V max open-circuit) and the SCM60 MPPT-MB (60 A, 145 V, connected to MasterBus) have a profile for lithium batteries.
The power rating on a panel is measured under laboratory conditions. On a sailboat, orientation, rigging shadow, season, and latitude reduce it sharply: estimate daily output, not peak power. The calculator above uses a conservative assumption (peak power × 4 hours × 0.7), to be adjusted with your own measurements. Rigid and flexible panels are chosen first by available surface and mounting method (arch, bimini, roof); see our solar panels and solar charge controllers.
Convert: 12 / 24 V and 230 V
DC-DC converters
On a 24 V system, some navigation electronics remain 12 V: a 24/12 V DC-DC converter powers them, such as the Victron Orion-Tr Smart 24/12-20A (isolated) or the Mastervolt DC Master 24/12. Size the converter not on average consumption, but on the sum of devices that may run together, and allow margin for a VHF in transmit or a radar at startup. The full range is in our DC/DC collection.
The inverter
An inverter produces 230 V from the battery. To power motor-driven appliances or sensitive electronics, choose a pure sine wave inverter, such as the Mastervolt AC Master or Mass Sine. Three figures matter in a spec sheet:
- Continuous power, not peak power or 30-minute power. Example from Mastervolt: the Mass Sine 24/1500 delivers 1 200 W continuously; the 1 500 W in its name applies only for 30 minutes.
- Peak power, which must cover the start-up of a motor (compressor, pump, tool).
- No-load consumption, which impacts the power budget if the inverter is left on. The Mass Sine 12/2000 draws 6 W in idle mode and 0.6 W in economy mode, according to Mastervolt; the small Victron 12/500 VA inverter draws about 1 W in ECO mode.
Rule of thumb: a boat’s refrigerator usually runs directly on 12 or 24 V. There is no need to power it through an inverter.
The inverter-charger combi
A combi combines shore charger, inverter, and transfer relay, and can limit power drawn from the shore supply or supplement it with battery power. The Victron MultiPlus-II and the Mastervolt CombiMaster and Mass Combi Ultra are compared in our shore charger vs. combi guide.
Protect and monitor
- The main fuse must be installed as close as possible to the battery. Its rating is determined by the battery manufacturer’s instructions and the cable cross-section it protects. A lithium bank can deliver extremely high short-circuit current: the fuse’s interrupting rating is as important as its current rating.
- The battery switch isolates the bank for maintenance or lay-up. It does not replace the controlled disconnection managed by the BMS.
- The battery monitor (a shunt that measures current in and out) calculates state of charge. It converts your theoretical balance into real numbers. Depending on the brand, it is integrated into the BMS or installed separately.
Energy refit and lay-up
An energy refit follows the same sequence as this guide: start with the load analysis (ideally measured over a season with a monitor), then the battery bank, charging sources, power conversion, and protections. Replacing batteries without reviewing chargers, the alternator, and protections is doing half the job.
For lay-up, follow the battery manufacturer’s storage instructions: state of charge, temperature, disconnection of loads, and periodic checks. These vary by brand and chemistry. Our lay-up checklist for electronics and batteries compiles Victron and Mastervolt storage guidelines and recommissioning checks.
Victron, Mastervolt or MG Energy?
The three brands cover batteries, BMS, and monitoring, but not in the same way:
- Victron Energy offers the broadest ecosystem (batteries, external BMS, chargers, combis, DC-DC converters, MPPTs, supervision via Cerbo GX); its lithium batteries require an external Victron BMS.
- Mastervolt integrates the BMS into the MLI Ultra battery and enables communication between the battery, chargers, combis, alternator regulator, and MPPT via its MasterBus.
- MG Energy Systems combines LFP modules with a master BMS that includes a contactor and shunt; MG does not manufacture chargers under its own name and documents compatibility with Victron and several charger manufacturers.
Choice mainly depends on what is already onboard. Criteria-by-criteria breakdown: Victron, Mastervolt or MG Energy: which lithium ecosystem?. To compare the BMS units themselves, see our marine lithium BMS comparison.
Limitations and pitfalls
- Size the bank before knowing consumption. An oversized bank that the charging system never fills is useless.
- Keep chargers and alternator regulators designed for lead-acid. They must apply the lithium battery manufacturer’s voltage values and be able to shut off on BMS command.
- Mixing families. With Victron, Smart battery BMS units are not interchangeable with NG battery BMS units; with MG, a 230 Ah battery cannot be mixed with 280 Ah or 304 Ah batteries in the same bank.
- Forget the relay or fuse. An MLI Ultra 3000 or 6000 without its external safety relay is not installed according to Mastervolt specifications.
Skysat has not performed any testing for this guide: it relies solely on manufacturer documentation, and the examples marked “Skysat calculation” are order-of-magnitude estimates.
Our verdict by program
- Coastal cruising, returning to port every night or almost: a modest bank, a shore-power charger in lithium mode, and often nothing more. Lithium is not essential if the current bank is sufficient and the chargers do not need replacing.
- Cruising with several nights at anchor: this is where lithium changes the game, provided you revise the charging setup: DC-DC charger or regulator on the alternator, and solar panels sized for realistic output.
- Long-range cruising or living aboard: switching to 24 V should be considered as soon as a powerful inverter or a watermaker is part of the equation; plan for a battery monitor and supervision (Cerbo GX, MasterBus or CAN depending on the brand).
Sources
Manufacturer documents consulted on 22 October 2026 (printed versions indicated where available):
- Victron Energy: Lithium Smart Battery manual (rev. 21, 01/2026) and Lithium Smart battery datasheet; Lithium NG 12.8 V manual (rev. 11, 03/2026) and NG battery datasheet; Lynx Smart BMS manuals (rev. 17, 01/2026) and Lynx Smart BMS NG (rev. 12, 02/2026); VE.Bus BMS V2 datasheet; Orion XS 12/12-50A datasheet and manual (rev. 10, 01/2026); Orion-Tr Smart isolated datasheet and manual; SmartSolar MPPT manuals (rev. 07, 08/2024 and rev. 10, 02/2026); VE.Direct inverter datasheets; MultiPlus-II 230 V manual (rev. 14, 07/2026).
- Mastervolt: MLI Ultra manual (rev. 10) and quick guide; ChargeMaster Plus manual (10000016594/06, 09/2025); Alpha Pro III manual; Mac Plus manuals (10000021042/02) and Magic (10000002796/03); SCM25 manuals (10000009291/02) and SCM60 MPPT-MB (10000009952/03); AC Master and Mass Sine datasheets.
