LiFePO4 batteries on board: electric boating, three years on
Three years ago on this blog we published our first article on sustainable boating. Re-reading it today is quite something, because almost everything we then presented as a promise has come true —and faster than we expected. Lithium battery prices have roughly halved, electric outboards have gone from a curiosity to a normal option for tenders and light craft, and the question on the pontoons is no longer "does this lithium thing work?" but "how do I make the switch without messing it up?". This update is about exactly that: what has won the day, why, and the details that separate a well-done conversion from an expensive problem.
Why LiFePO4 has won on board
In a house, a battery's weight is an anecdote. On a boat, it's the variable that touches everything: speed, consumption, heel, trim. And here lithium iron phosphate doesn't compete with lead: it crushes it.
Let's run the numbers with the most common house battery, the 12 V 100 Ah. In AGM, that's about 28-30 kg, of which —respecting the 50% maximum discharge that lead tolerates without degrading quickly— you get about 0.6 kWh usable. The same nominal capacity in LiFePO4 weighs 11-13 kg and delivers more than 1.1 kWh usable, because it takes 90-95% discharges without flinching. Per unit of usable energy, we're talking about 50-60 kg of lead against 10-12 kg of LiFePO4: five times less weight for the same real energy. On a cruising sailboat replacing a bank of four AGMs, that's easily 80 kg freed from the bilge.
And weight is only the start. LiFePO4 lasts 4,000-6,000 cycles against lead's 400-600 —in practice, the life of the boat—, holds its voltage steady almost to the end of the discharge (goodbye to the fridge that quits with the battery "half full"), charges far faster because it accepts high currents without lead's endless absorption phase, releases no gases and holds no acid that could spill when heeling, and asks for no maintenance. On the chemistry and the safety in detail, everything we say about the thermal stability of LiFePO4 counts double in an enclosed space like a boat.
House and propulsion: two jobs, one chemistry
On board there are two very different electrical jobs. The house battery feeds the fridge, electronics, lights, windlass and autopilot: here LiFePO4 is today the obvious choice, and converting from lead is the most common project on the pontoons. Electric propulsion is the other frontier, and the one that has made the biggest leap since 2023: electric outboards for tenders and light craft, auxiliary motors for sailboats replacing the inboard diesel, and fully electric harbour launches. It's the ground we come from as a company —marine propulsion and storage systems are our origin— and where LiFePO4's energy density and discharge capacity make the difference between a motor that pushes and one that disappoints.
For a cruising sailboat, the combination we almost always recommend is the modest and effective one: LiFePO4 house bank, solar on the bimini or coachroof with a good MPPT regulator, and the realistic goal of spending whole days at anchor with the fridge and electronics running without starting the engine. It's the biggest quality-of-life upgrade per euro invested in recreational boating.
Switching from lead: where the hasty ones get hurt
Swapping lead for lithium isn't just pulling one battery and dropping in another, and the three classic mistakes have names.
The first is the alternator. LiFePO4 has a very low internal resistance and accepts all the current you give it; an alternator sized for lead's lazy curve suddenly finds itself running at 100% continuously, overheats and dies. The correct solution is a DC-DC charger between the alternator and the lithium battery (or a smart external regulator), which limits the current and protects the machine. Any conversion quote that leaves it out is an incomplete quote.
The second is the BMS and the sudden disconnect. When a BMS protects the battery —on voltage or temperature— it disconnects. If the alternator is charging at that moment, the resulting voltage spike can fry diodes and onboard electronics. A correct design plans for it: a quality BMS with early warnings, coordinated protections and, on serious installations, communication between battery, chargers and monitor so nothing cuts out by surprise.
The third is the environment. The sea means salt, humidity and vibration: suitably protected enclosures, corrosion-protected terminals, mechanical fixings designed for pounding seas and generously sized tinned cabling. And a chemistry note: LiFePO4 must not be charged below 0 °C —in the Mediterranean it's almost never an issue, but a BMS with a low-temperature charge cut-off is the insurance that turns it into no issue at all.
A piece of advice we give from experience and without varnish: the home-made pack with internet cells and a ten-euro BMS, which already looks like a bad idea on land, is downright unacceptable on board. At sea there's no emergency ladder.
The battery watched from the sofa
There's a quirk of boating that makes it the perfect use case for telemetry: most boats spend 95% of the time alone in the marina, and the owner visits every two or three weeks. A lead battery forgotten in port sulphates in silence; a monitored LiFePO4 sends voltages, temperatures and state of charge in real time, and any anomaly —a parasitic drain emptying the battery, a cell drifting out of balance— shows up on your phone before it becomes a lost Saturday at the dock. It's also the same verifiable-health infrastructure that the 2027 battery passport will require of every stationary battery. Real-time monitoring of every element of the system is part of our factory DNA, and boating is where it most clearly pays for itself.
In short
If in 2023 electric boating was a bet, in 2026 it's simply the sensible way to equip a boat: less weight, more usable energy, ten times more cycles and prices that are no longer a barrier. What still makes the difference is the installation: a DC-DC for the alternator, a serious BMS, real marine protection and, if possible, telemetry. You'll find our marine batteries and components in the shop (Marine section), and if you have a conversion in mind —from the house bank to propulsion— write to us: we'll tell you what fits and what doesn't, even if the answer is that your AGM can still do another season.
SolarBox Team