Converting to Lithium: A Practical Guide to Upgrading Your House Bank

A good AGM house bank in the tropics is dead in three to five years. A properly installed LiFePO4 bank should run ten to fifteen. In my day job I spend a lot of time telling people to ignore sticker price and look at total cost of ownership, and batteries are the cleanest example I know: lithium costs roughly twice as much per amp-hour up front and something like a third as much per year of service. The math isn't close.

And yet the forums are full of lithium horror stories — fried alternators, BMS shutdowns mid-passage, banks ruined by one cold snap. Read those threads carefully and nearly every disaster traces back to the same handful of installation shortcuts. Lithium works. Botched lithium is more dangerous and more expensive than the lead-acid it replaced. Here's how I'd do the conversion with my own money.

The math, in numbers

Usable capacity. Lead-acid shouldn't be discharged below 50% — deeper cycles shorten its life dramatically — so a 400 Ah bank gives you about 200 Ah you can actually use. LiFePO4 discharges safely to 80–90%, so a 300 Ah lithium bank delivers 240–270 usable amp-hours. More usable energy from a smaller bank.

Weight. A 400 Ah AGM bank runs 120–130 kg. The 300 Ah lithium bank that replaces it: 40–50 kg. Taking 80 kg off a sailboat's waterline is not a rounding error, and on the performance cats we've been researching for our own trip, the builders treat house-bank weight as a design constraint, not an afterthought.

Charging speed. Lead-acid accepts charge well to about 80% state of charge, then absorption crawls — the last 20% can take 2–4 hours. Lithium accepts near-constant current right up to full, so the bank recharges in roughly half the time. Less engine, less generator, more of your solar actually doing something.

Cycle life. A quality AGM gives 500–800 cycles at 50% depth of discharge; a quality LiFePO4 gives 3,000–5,000 at 80%. Cycling daily, that's 3–5 years versus 10–15. And lithium doesn't sulfate, so the partial-state-of-charge existence a cruising bank actually lives — solar-topped, never quite full — doesn't slowly kill it the way it kills lead.

Buy drop-ins unless you like projects

Every LiFePO4 installation needs a battery management system. The BMS watches individual cell voltages, balances the cells, cuts charging on over-voltage or low temperature, and disconnects the bank before abuse becomes damage. It is the thing that makes lithium safe, and it is non-negotiable.

Your real choice is where the BMS lives. Drop-in batteries (Victron Smart, Battle Born, Lithionics, RELiON) have it built in — they go where the old lead-acid bank sat and the cell management is handled. Custom banks built from prismatic cells (EVE, CATL) use an external BMS: more flexibility, better value at large bank sizes, and considerably more ways to get it wrong.

My take: for a boat that's going offshore, pay the premium for quality drop-ins with a factory-integrated, warranty-backed BMS. The savings on a DIY prismatic build are modest, and I don't want a hand-configured protection system as the single point of failure in a salt environment a thousand miles from the nearest electrician. Tinkerers with dock power and time can disagree.

The alternator is where conversions die

Most lithium horror stories start at the alternator. A standard alternator with an internal regulator charges lithium at maximum output, continuously, because lithium never shows the rising voltage that tells the regulator a lead-acid bank is full. The alternator cooks itself.

Two fixes. An external smart regulator (Balmar, Wakespeed, Sterling) monitors battery voltage and alternator temperature and throttles output accordingly. Or a DC-DC charger (Victron Orion, Sterling) sits between the alternator and the lithium bank and feeds it a controlled charge regardless of what the alternator wants to do. The DC-DC route caps your charge current, but it's simpler and harder to get wrong — and on a boat, harder-to-get-wrong usually wins my vote.

The rest of the charging chain

Solar. Most modern MPPT controllers (Victron SmartSolar, EPEver, Renogy) have a LiFePO4 profile built in — switch it in settings. Absorption for lithium runs about 14.2–14.6V on a 12V system versus 14.4–14.8V for AGM. No lithium profile? The controller gets replaced.

Shore power. Modern inverter-chargers (Victron MultiPlus and Quattro, Mastervolt, Magnum) have lithium profiles; update firmware and select it. An older charger with no lithium profile gets replaced too — the overcharge risk isn't worth the savings.

Wind and hydro. Most charge through a controller that can be set for lithium. Verify before you assume.

Make the BMS talk to the chargers

A well-designed system doesn't just protect the bank — it tells the charging sources what to do. When the BMS sees a full bank or an over-temperature cell, the chargers should ramp down before the BMS ever has to act.

Victron handles this elegantly inside its own ecosystem: BMS, solar controllers, and inverter-charger share a data network and manage charge rates automatically. Mix brands — say a Victron BMS with a Balmar regulator — and you're configuring the handshake yourself through relay signals or CAN bus.

Skip this step and you've built the classic failure: the BMS disconnects to protect the bank while the alternator keeps pushing current into an open circuit. Voltage spike, fried electronics, damaged alternator diodes. This one failure mode is responsible for most of lithium's bad reputation, and it is entirely preventable at design time.

Sizing: smaller than you think

Size for 2–3 days of autonomy with no charging — same principle as lead-acid, different arithmetic. A modern cruising boat running autopilot, refrigeration, instruments, and (in our case) four kids' worth of fans and freezer stock burns around 250 Ah a day. A 400 Ah lithium bank at 80% usable gives 320 Ah — about 1.3 days. A 600 Ah bank gives roughly two. For most cruising boats, 400–600 Ah of LiFePO4 is the practical range, and that's what I've penciled into our own refit budget.

Resist the urge to oversize. Lithium is expensive per amp-hour, and because it recharges so much faster, you don't need the fat buffer lead-acid demanded. An oversized bank is money and weight bolted into the bilge for no benefit.

The cold-weather footnote

LiFePO4 cannot be charged below 0°C without permanent damage — the cells take the charge, but lithium plates onto the anode and capacity is gone for good. Tropical cruisers can ignore this. But our planned route includes New Zealand in winter, and anyone cruising Northern Europe or the US Northeast is in the same boat: you need a BMS with a low-temperature charge cutoff, and possibly a heating pad in the battery compartment. Quality BMS units include the cutoff as standard. Verify before buying — especially on DIY builds with aftermarket BMS units.

Installation notes

Figure a 2–4 day project for a competent electrician or experienced DIYer. The sequence: pull and recycle the old bank; mount the lithium batteries hard — strapped or boxed for a 90-degree knockdown, because lithium cells tolerate physical shock worse than lead; wire with properly sized cable (typically 2/0 or 4/0 AWG for a house bank), a master disconnect, a Class T fuse on the positive bus, and a shunt-based monitor (Victron BMV or SmartShunt), all crimped tinned-copper ring terminals, no solder.

Then configure and test each charging source individually, set the BMS limits to the battery manufacturer's spec, and — this is the step people skip — test the BMS disconnect and confirm every charging source actually stops. Cycle the bank fully once, and watch cell voltages closely for the first week.

The start battery stays lead-acid

Do not convert the engine start battery. A dedicated AGM start battery on its own charging circuit starts the engine no matter what the house bank, the BMS, or any lithium failure mode is doing. If the BMS ever takes the house bank offline mid-passage, you can still make power and get home. On an offshore boat, that redundancy is non-negotiable.

What it costs

Plan on $3,000–8,000 for a typical cruising boat, depending on bank size, brand, and whether you DIY the install. Over the lithium bank's lifespan you'd replace the lead-acid equivalent two or three times, which usually adds up to more than the lithium cost — the same buy-quality-once arithmetic I walk clients through on very different assets.

But honestly, the spreadsheet isn't the point. The point is a bank that charges fast, holds voltage steady, weighs half as much, and doesn't need babysitting in the tropics. Of every electrical upgrade on a cruising boat, this is the one with the highest return. Do it right the first time.

If you're budgeting a refit — or a whole boat — the ownership cost worksheet is where I'd start: it forces the lifetime-cost math on every line item, batteries included.

References: Victron Energy, Nigel Calder (Boatowner's Mechanical and Electrical Manual), Practical Sailor, Marine How To (Rod Collins), Cruisers Forum lithium conversion threads.