Selecting chargers and inverters for an electric boat

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.

This guide covers the four charger archetypes you'll meet, the sizing and selection criteria that matter, and which suppliers credibly cover each role.

→ The spec calculator outputs your battery kWh and recommended charge current →

The four charger archetypes

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

| Device | Input | Output | Typical role | |---|---|---|---| | AC battery charger | 230 V AC shore / generator | 48 V DC | Dedicated overnight charging at the dock | | Inverter / charger | 230 V AC + 48 V DC (bidirectional) | 230 V AC out + 48 V DC in | Combined unit: shore charging + AC appliances at anchor | | DC-DC charger | 12 V or 24 V engine alternator | 48 V DC | Bridges a low-voltage start system into the propulsion bank | | MPPT solar controller | Solar panel (Voc 50–250 V) | 48 V DC | Daily solar harvest into the propulsion bank |

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 — match charge current to pack capacity, not to outlet capacity

The right size for an AC charger or inverter/charger output is 0.2C to 0.3C of your pack capacity in Ah, capped by your shore-power outlet.

For a 200 Ah pack at 48 V (≈10 kWh), 0.2C is 40 A and 0.3C is 60 A of DC charge current. The 70 A class chargers (Victron MultiPlus-II 48/5000, Mastervolt ChargeMaster 48/75) sit right in that band and bring the pack from 20% to 80% in roughly 2 hours.

Two hard limits cap this:

• Cell C-rate. LiFePO₄ marine cells are typically rated at 0.5C maximum charge to preserve cycle life. A 200 Ah pack therefore caps at 100 A regardless of charger output.
• Shore-power circuit. A 16 A 230 V single-phase outlet delivers ≈3.7 kW. Allowing 90% efficiency, that's 3.3 kW into the battery — which at 51 V (LFP absorption) is 65 A DC. Going larger than this just causes the charger to hit its AC current limit and de-rate.

Rule of thumb: size charger DC output to min(0.3 × pack_Ah, AC_outlet_kW × 0.9 / pack_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.

For propulsion installs above ≈15 kW continuous, 96 V is increasingly common — and there are far fewer charger options at 96 V than 48 V. If you're picking 96 V, plan to source chargers from Sterling Power, ELCON, or industrial Schneider/Eaton ranges rather than the recreational marine catalogue.

3. Charge profile must match the cell chemistry

Most marine chargers were designed in the lead-acid era. Their default profiles end with a long float stage at 13.5 V (or 54 V for 48 V banks) that holds the battery on a trickle indefinitely. This is wrong for LiFePO₄. Float-charging lithium causes plating at the anode and shortens cycle life.

A LiFePO₄-compatible charger should:

1. Bulk-charge to absorption voltage (typically 14.4 V / 28.8 V / 57.6 V depending on bus voltage).
2. Hold absorption only long enough for cell balancing (1–2 hours typical).
3. Drop output to zero or "storage" mode rather than enter a continuous float stage.

Confirm the charger has a "Lithium" or "LiFePO₄" preset, or that absorption time is configurable and float can be disabled. Victron units do this through the VictronConnect app; Mastervolt and Sterling expose it via DIP switches or their proprietary apps.

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:

• Direct CAN bus. Charger and BMS share state-of-charge, cell voltage, and over-voltage warnings on the same CAN. Charger reduces output before BMS needs to disconnect. Available on Victron + Lynx Smart BMS, Mastervolt MasterBus, and most Bellmarine-supplied integrators.
• 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. BMS pulls a relay open to inhibit the charger when any cell hits over-voltage. Supported by REC Active BMS, Batrium, and most prosumer BMS units.

For new installs, prefer CAN integration. For retrofit on existing packs, the relay-inhibit signal is fine and works with almost any charger.

5. AC input flexibility

Marina shore-power varies from 6 A 110 V (small US marina) to 32 A 400 V three-phase (large European marina). A charger or inverter/charger that automatically detects the input and adjusts charge current to whatever's available saves a lot of fault calls.

The key feature is "PowerAssist" or "PowerControl" — Victron's name for shared input current management. With PowerAssist, you set a maximum AC input limit (say 16 A); the inverter/charger then dynamically reduces charging current if the boat's AC loads (kettle, watermaker, AC) start drawing from the same input. Without this feature, plugging in a 2 kW kettle while charging at 16 A 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).
• Headroom: 30–50% over nominal to avoid running near thermal limit in a warm engine room.

A typical sailboat needs 3000–5000 VA. The Victron MultiPlus-II 48/5000 hits this sweet spot. Smaller boats (no AC, no induction) can drop to 2000 VA; larger boats with full air-conditioning and an electric galley climb to 8000–10 000 VA.

7. Marine rating and installation environment

Standard industrial inverter/chargers fail quickly in a boat. Look for:

• IP21 minimum for engine-room mounting (drip-resistant from above).
• Conformal-coated PCBs.
• Class A or B EMC compliance to avoid wrecking your VHF and AIS reception.
• DC and AC terminals with proper marine-grade lugs (not screw clamps designed for solid-core house wiring).

The Victron and Mastervolt marine ranges meet all of this out of the box. Industrial Schneider/Eaton units often need a custom enclosure to be acceptable in a marine install.

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 (Voc) ≥ 1.25× the cold-day open-circuit voltage of your array. A 24 V panel (Voc 36 V) wired in 4S has a Voc of 144 V on a cold morning — pick a controller with ≥150 V input. Victron SmartSolar 250/100 covers 250 V input × 100 A output.
2. Output current ≤ 0.3C of pack. Same rule as AC chargers. A 200 Ah pack handles up to 60 A of MPPT output.
3. Bus voltage compatibility. 48 V MPPTs are common; 96 V MPPTs are limited to a handful of industrial brands.
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) and a 48 V/200 Ah pack, the Victron SmartSolar MPPT 150/70 or 250/100 sits in the right operating point with margin for cold-weather Voc spikes.

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.

Sizing is straightforward: pick the largest DC-DC charger your alternator can support. A typical 90 A 12 V alternator delivers about 1 kW continuously; a 50 A DC-DC 12→48 V converter (Sterling Power BB1248, Mastervolt MagicBoost) consumes ≈1 kW from the alternator and delivers ≈25 A into the 48 V pack. Larger 80 A 12→48 V units exist but require a 200 A+ alternator to feed them without overheating the engine.

A worked example: 35 ft sailboat repower

System: 48 V / 200 Ah LiFePO₄ pack (≈10 kWh), 15 kW saildrive, 2 kWp solar array, 16 A 230 V shore power.

| Role | Recommended unit | Output | Reason | |---|---|---|---| | Inverter/charger | Victron MultiPlus-II 48/5000 | 70 A DC charge, 5000 VA inverter | 0.35C charge fits 0.5C cell limit; 5 kVA inverter covers all AC loads + induction hob | | Solar MPPT | Victron SmartSolar 250/100 | Up to 100 A DC output | Handles 2 kWp at any panel-string configuration to 250 V | | BMS comms | Lynx Smart BMS over Cerbo GX | n/a | All charging devices share state on VRM | | DC-DC | Not needed | n/a | No separate 12 V start battery — propulsion pack covers everything via 48→12 V converter |

Total charge sources into the 48 V bank: ≈170 A peak (70 A AC + 100 A solar). The 200 Ah pack at 0.5C limit accepts 100 A, so the BMS or charger orchestration must coordinate to not exceed that. Victron's Distributed Voltage and Current Control (DVCC) on the Cerbo GX handles this automatically.

Supplier examples for chargers and inverter/chargers

The chargers and inverter/chargers below are the units most commonly specified in European and US marine installs in 2026. The criteria above should drive your choice — supplier preference is secondary.

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.
• Trusting the "marine" badge. Plenty of "marine" chargers are industrial inverters in a blue case. Check for IP rating, conformal coating, and EMC compliance before buying.
• 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.
• Ignoring float behaviour. A charger that never drops float current will eventually damage a LiFePO₄ pack. Verify the lithium profile is real, not a cosmetic relabel of an absorption-then-float lead-acid curve.

Putting it together

The shortest path to a working install:

1. Decide bus voltage (48 V for ≤15 kW; 96 V above that).
2. Use the calculator to size pack Ah from your runtime × kW.
3. Multiply pack Ah by 0.3 — that's your target AC charger DC output.
4. Pick an inverter/charger that hits that DC current at your shore-power input voltage and supports a real lithium profile + BMS comms.
5. Add an MPPT sized to ≥0.3C output and PV input voltage ≥1.25× cold-day Voc.
6. Add a DC-DC charger only if you're running a separate 12 V start battery on an engine alternator.

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