Selecting chargers and inverters for an electric boat — sizing, comms, and supplier examples

A marine electric propulsion system rarely runs from one energy source. A typical install pulls from shore power at the dock, solar at anchor, an alternator under engine, and sometimes a generator on long passages. Each of those needs the right charging device — and on top of that you usually want an inverter to run AC appliances from the same battery bank.

Get the charging hardware right and the system "just works": you arrive, plug in, and leave with a full pack the next morning. Get it wrong and you fight under-sized chargers, broken charge profiles, and BMS shutdowns for the life of the boat.

Below: the four kinds of charging device, how to size each from your battery's own limits and your shore supply, and a worked example from the configurator's 35-footer.

→ The spec calculator works out your battery kWh and the charge time for a given charger →

The four charger archetypes

In a 48 V LiFePO₄ propulsion install, you'll typically use some combination of these:

Most cruising boats end up with an inverter/charger + an MPPT, and add a DC-DC charger only if they keep a separate 12 V start battery driven by an alternator.

Selection criteria for chargers and inverter/chargers

1. Sizing — between the battery's limit and the shore supply

Two things cap useful charge current, and the charger should sit between them:

• The battery's maximum charge current. It's on the datasheet. Victron's Lithium NG 51.2 V/100 Ah, for example, accepts 100 A continuous (1C) per battery, so five in parallel accept 500 A. Other batteries are rated lower, so check yours.
• The shore supply. A 16 A, 230 V outlet delivers about 3.7 kW. At 90% charger efficiency that's about 3.3 kW into the battery, or 65 A at 51.2 V. A bigger charger on that outlet just runs at its input current limit.

Then pick a charger that refills the pack in the time you have. A MultiPlus-II 48/5000 charges at up to 70 A, so it takes 20–80% of a 500 Ah bank (300 Ah) in about 4.3 hours, if the supply can feed it. The shore power guide covers supply-side sizing.

Rule of thumb: charger DC output ≤ the smaller of the battery's maximum charge current and (supply kW × 0.9 ÷ battery voltage).

2. Voltage class — pick the bus voltage first

This sounds obvious, but voltage mismatches are one of the most common purchase mistakes. A 48 V charger does not drive a 96 V pack, and most inverter/chargers are sold in fixed 12 / 24 / 48 V variants.

The configurator moves boats to 96 V from 15 kW, and there are far fewer charger options at 96 V than at 48 V. Victron's MultiPlus-II range, for example, tops out at 48 V, and so do its SmartSolar MPPTs. At 96 V, ask the motor or battery maker which chargers they support.

3. Use the battery maker's charge settings

Lead-acid profiles don't suit LiFePO₄: they absorb at the wrong voltage, and some add equalisation or temperature compensation that lithium doesn't want. Set the charger from your battery maker's figures. For Victron's 51.2 V Lithium NG, those are:

1. Absorption: 56.8 V, for 2 hours, which gives the cells time to balance
2. Float: 54.0 V
3. Temperature compensation: off (Victron says it isn't required for lithium)

Other makers specify different values. Confirm the charger lets you set absorption voltage, absorption time and float, or has a preset that matches your battery. Victron's chargers are set up through VictronConnect, and with a Lynx Smart BMS and DVCC they follow the BMS's limits automatically.

4. Communication with the BMS

A charger that ignores BMS state can overcharge a pack with one weak cell. The BMS sees the over-voltage and disconnects the contactor — but every disconnect under load stresses the contactors, the FETs, and (in the worst case) welds them closed.

Look for one of these integration paths:

• Over the network. The BMS publishes charge limits and the chargers follow them, reducing output before the BMS has to disconnect. Victron's Lynx Smart BMS NG does this through a GX device (DVCC).
• Voltage-derived charge control. Charger reads pack voltage and respects a programmed maximum. Acceptable for stable LFP packs with quality cells; less safe with second-life or unmatched cells.
• External charge-enable signal. The BMS opens a contact to switch the charger off when any cell reaches its limit. The Lynx Smart BMS NG has an allowed-to-charge contact for chargers that don't speak DVCC, and REC's 2Q has an opto-relay output.

For new installs, prefer CAN integration. For retrofit on existing packs, the relay-inhibit signal is fine with any charger that has a remote on/off input.

5. AC input flexibility

Shore supplies vary widely from marina to marina. The feature to look for is an adjustable AC input current limit that accounts for the boat's other loads. Victron calls it PowerControl: you set a maximum shore or generator current, and the MultiPlus-II "will then take account of other AC loads and use whatever is extra for battery charging". PowerAssist goes further, adding battery power to the supply for short peaks. Without something like this, switching on a kettle while charging at the outlet's limit trips the marina breaker.

6. Inverter sizing — separate from charger sizing

If the same unit is a combined inverter/charger, the inverter VA rating drives a different sizing logic:

• Continuous: cover the largest single load that runs steadily (watermaker pump, induction hob).
• Surge: cover the peak inrush of motors (compressor start, AC unit).
• Heat: inverters derate when warm. The MultiPlus-II 48/5000, for example, gives 4,000 W continuous at 25 °C but 3,700 W at 40 °C, so size against the temperature where it will be mounted.

Add up the loads you'll actually run together. Victron's 48 V MultiPlus-II range runs from 3,000 VA to 15,000 VA.

7. Installation environment

Check the enclosure rating against where the unit will go. The MultiPlus-II, for example, is rated IP22, which means it needs a dry, protected space, not a wet locker. Mount it with the clearances the manual gives for cooling, because the output ratings assume them.

MPPT solar controllers — selection criteria

Solar feeds the propulsion bank through a Maximum Power Point Tracker (MPPT). The selection criteria differ from AC chargers:

1. Maximum PV input voltage must exceed the array's Voc at the coldest temperature you will ever see it. Note which way this goes: panel open-circuit voltage rises as temperature falls. Victron's mono panels gain 0.35% per °C below 25 °C. So the datasheet Voc is the warm-day figure, and the number you size against is higher. A panel with a 36 V Voc wired 4S gives 144 V at standard test conditions, but at −10 °C that string reaches about 162 V — which a 150 V controller will not survive. Size for the cold figure plus margin and you are into 250 V-class hardware; the Victron SmartSolar 250/100 covers 250 V input × 100 A output. Getting this backwards is the most common way people destroy an MPPT, and it happens on the first cold clear morning rather than at installation.
2. Total charge current within the battery's limit. Add the MPPTs to the shore charger: together they mustn't exceed the battery's maximum charge current, unless something (such as DVCC) coordinates them.
3. Battery voltage support. Not every MPPT supports 48 V: in Victron's SmartSolar range the 75/xx and 100/30–100/50 are 12/24 V only. None of Victron's SmartSolar models supports 96 V.
4. Networked monitoring. Pick a controller that joins the same bus (VE.Direct, MasterBus, NMEA 2000) as your other charging devices, so the helm display shows total state of charge from all sources.

For a 35 ft cruiser with 2 kWp of solar (4 × 500 W panels) on a 48 V bank, the array's series/parallel arrangement decides the controller: wire the four panels 2S2P and a 150 V-class controller has room, wire them 4S and you need the 250 V class. Work out the cold-weather Voc for the arrangement you actually intend to build before ordering either.

DC-DC chargers — when you need one

A DC-DC charger lets you use a 12 V engine alternator to charge a 48 V propulsion pack. They're useful when:

• You're keeping a separate 12 V start battery and house bank, with the propulsion pack as a third bank.
• You have a generator that natively outputs 12 V via its starter alternator.
• You're using a small auxiliary diesel as a range-extender without a dedicated 48 V genset.

Check the converter's power against what the job needs. Victron's isolated Orion-Tr Smart 12/48-8, for example, delivers 8 A at 48 V, roughly 0.4 kW. That's useful for keeping a bank topped up from an engine alternator, but far too small to recharge a propulsion bank meaningfully. Charging a propulsion bank from an engine needs a generator or alternator built for the bank's voltage.

The opposite direction is far more common on electric boats: a 48 V to 12 V converter feeding the house bus from the propulsion bank, such as Victron's Orion-Tr Smart 48/12-30.

A worked example: 35 ft sailboat repower

The configurator's 35 ft sailboat: 8 kW motor, 48 V, 25.6 kWh, which is five Victron Lithium NG 51.2 V/100 Ah batteries (500 Ah). Shore supply: 16 A at 230 V. Solar: 2 kWp.

Total charge into the bank can reach about 65 A from shore plus 39 A from solar, about 104 A. Five Lithium NG batteries accept 500 A, and the Lynx BMS publishes the limit that DVCC enforces, so there's plenty of margin.

Supplier examples for chargers and inverter/chargers

The chargers and inverter/chargers below are widely used in European and US marine installs. Treat them as a shortlist to check against your pack voltage, charge current and shore supply, not as a ranking. Our 48 V charger comparison sets the makers' own figures side by side.

For batteries that pair cleanly with these chargers (matching CAN bus, charge profile, and BMS comms), see the battery suppliers section in our LiFePO₄ guide.

Common selection mistakes

A few errors come up repeatedly when sailors put their first electric boat install together:

• Buying the biggest inverter/charger you can afford. If your shore-power feed is 16 A, an 8000 VA charger spends most of its life de-rated. Match charger output to outlet capacity, not to charger capability.
• Ignoring the enclosure rating. Check the IP rating against where the unit will be mounted, and keep it out of wet lockers.
• Skipping BMS-charger comms on day one. It's far cheaper to spec a CAN-integrated charger up front than to retrofit one when your first BMS shutdown happens at 2 a.m. in a marina.
• Using a generic lithium preset. Set absorption voltage, absorption time and float from your battery maker's figures.

Putting it together

The shortest path to a working install:

1. Decide the bus voltage. The configurator stays at 48 V up to 10 kW and moves to 96 V from 15 kW.
2. Use the calculator to size the pack from your runtime and power.
3. Note the battery's maximum charge current from its datasheet.
4. Pick an inverter/charger that uses your shore supply fully, without exceeding the battery's limit, and that can be set to the battery maker's charge values.
5. Add an MPPT that supports your battery voltage, with a PV input rating above the cold-morning Voc.
6. Add a DC-DC converter for the 12 V house bus.

Get those six right and the rest of the system follows.

Sources

Checked on 7 November 2026:

• Victron Energy — MultiPlus-II datasheet: 48 V models, charge currents, output at 25 °C and 40 °C, PowerControl and PowerAssist, IP22
• Victron Energy — Lithium NG batteries datasheet and 51.2 V manual: maximum charge current, absorption and float settings, temperature compensation
• Victron Energy — SmartSolar charge controller overview and BlueSolar mono panels datasheet: battery voltages per MPPT model, Voc temperature coefficient
• Victron Energy — Orion-Tr Smart product page: 12/48-8 and 48/12-30 models
• REC — REC 2Q BMS user manual: opto-relay output
• Lynx Smart BMS NG: see the sources in our Lynx Smart BMS NG guide

We are not affiliated with Victron Energy or REC.