Converting a Sailboat to Lithium: A Refit

The boat was a 1998 Catalina 34, tiller-steered, weekend-cruised, and cursed with a bank of three flooded golf-cart batteries that were tired before the owner ever met them. He wanted to run a fridge overnight at anchor without listening to the engine charge at 6 a.m. That is the whole reason most people start down the road of a sailboat lithium conversion, and it was his too.

He called me in April. We splashed the finished refit in late May. What follows is exactly what we decided, what it cost, the two things I would do differently, and the numbers that mattered.

The boat and the goal

The house bank (the batteries that run your lights, fridge, and electronics, as opposed to the one that starts the engine) was 300 amp-hours of flooded lead-acid on paper. In reality he had maybe 120Ah of usable capacity, because you should not pull a flooded battery below 50 percent without killing it early.

His fridge drew about 5 amps and ran roughly half the time in summer, call it 60Ah a day just for cold beer and food. Add cabin lights, the chartplotter, and phone charging, and he was looking at 90 to 100Ah of real daily draw at anchor.

You can see the math problem. His old bank was empty by midnight.

The goal was simple: two nights on the hook, no engine, fridge running the whole time. If you want the full decision framework before you spend a dollar, our Lithium Boat Battery Upgrade Guide walks through sizing a bank for exactly this kind of use.

Why lithium, and why not just more lead

We could have bolted in another 300Ah of golf-cart batteries. Cheaper up front, sure. But that is another 400 pounds low in a boat that already sails on her ear in a gust, and it still only gives you 150Ah usable.

Lithium iron phosphate, usually written LiFePO4 or just LFP, flips that trade. You can safely use about 90 percent of the rated capacity, it weighs a third of lead per usable amp-hour, and it charges far faster.

The short version of the comparison looked like this.

If you are still weighing the two chemistries in general terms, we broke it down head to head in Lithium vs AGM for Boats: Which Wins?. For this owner, the weight savings and the two-night endurance settled it.

Drop-in or a proper build

Here is the first real fork in the road. A drop-in lithium battery is a sealed box that fits the same footprint as a Group 31 lead-acid battery and has the battery management system (the BMS, the electronics that protect the cells) hidden inside.

They are easy. You disconnect the old battery, set the new one in the same tray, reconnect, and you are 80 percent done.

We went with three 100Ah drop-ins wired in parallel for this boat, because the owner is not an electrician and wanted something he could understand. If you are deciding on that route, Are Drop-In LiFePO4 Batteries Worth It? covers where they shine and where they bite you.

The alternator was the hard part

This is what nobody tells the first-time convert. Your batteries are the simple swap. Your charging system is the project.

A stock marine alternator is built to top off a lead battery that fights back as it fills. Lithium does not fight back. It will happily accept everything the alternator can produce, at full output, for as long as it takes.

On this boat that meant the 80-amp alternator, which was rated for short bursts, was suddenly being asked to push 80 amps continuously into a hungry lithium bank. That alternator was not built to run flat out for an hour. It gets hot enough to cook its own diodes.

We had two options. Install an external alternator regulator with a temperature sensor that backs off the output to protect the alternator, or install a DC-to-DC charger that limits the current pulled from the alternator to a safe number.

We chose a 50A DC-to-DC charger. It cost about $280, and it meant the alternator never sees more than 50 amps, which it can sustain all day. It also gave the lithium bank a proper lithium charge profile without touching the engine’s original wiring.

Shore charger, wiring, and the BMS

His old shore-power charger was a three-stage lead-acid unit that topped out at 14.4 volts and then dropped to a float voltage that lithium does not want. We replaced it with a 40A charger that has a proper LiFePO4 setting, about $220.

The existing 2/0 battery cables were oversized for the old bank and perfectly fine for the new one, so we reused them. We did add a 300A Class-T fuse within 7 inches of the positive terminal, which the American Boat and Yacht Council calls for on any battery bank. You can read their standards summary at abycinc.org.

The BMS is the brain that keeps lithium safe. It disconnects the bank if voltage runs too high, too low, or if the cells get too cold to charge. If you want to understand what it actually does before you trust one, How a Marine Battery BMS Actually Works lays it out plainly.

The cold-weather mistake I almost made

Lithium has one hard rule: you cannot charge it below freezing without permanently damaging the cells. Not should not. Cannot.

This boat lives in New England and gets hauled in October, but the owner sails into November. On a 38-degree night the cabin can drop below freezing by dawn, right when the solar panel wakes up and tries to charge.

The drop-ins we chose have a low-temperature cutoff built into their BMS, so charging simply stops below about 32 degrees. If yours do not, you need a bank with cell heaters or a charge controller that reads temperature. I have seen a $1,200 bank ruined in one cold week because nobody thought about it.

What it cost and how long it took

People always want the number, so here it is, honestly.

Call it $3,800 in parts. Labor was two long weekends, mostly spent chasing wiring and mounting the new gear, not the batteries themselves.

If your existing charging and wiring are already in good shape, you might spend less. If you need a new alternator regulator or a full rewire of a crowded bank, budget more. Planning that bank layout well saves hours later, and our Marine Battery Bank Setup Guide is the map I hand people first.

How she performed, and the two things I would change

The result did exactly what he asked. Two nights at anchor, fridge running the whole time, and the bank came back to the dock at 40 percent. The boat also floats a hair higher, and she is noticeably stiffer under sail with 100 pounds gone from down low.

First change: I would have installed a small alternator temperature alarm anyway, just for peace of mind. The DC-to-DC charger protects it, but I like a second set of eyes on heat.

Second change: we mounted the battery monitor’s display too low in the nav station to read at a glance. A ten-dollar decision that annoys him every trip. Mount your gauge where you actually look.

If you take one thing from this refit, let it be this: budget your time and money for the charging system, not the batteries. The cells are the easy, exciting part. The alternator, the charger, and the cold-weather protection are where a good sailboat lithium conversion is really won or lost. Do those three right and the boat will hand you quiet nights on the hook for a decade.