Upgrading a boat to lithium: everything around the battery

The short answer. For most boats, going lithium means checking or changing five things: the charging settings on every charger aboard, alternator charging (usually through a DC-DC charger), a shunt-based battery monitor, a fuse at the battery rated for what a lithium bank can dump into a short, and tinned marine wire sized for the new loads. This guide takes you through all five, one at a time.

A white trawler yacht cruising past a wooded island
Photo: Bengt Nyman from Vaxholm, Sweden / Wikimedia Commons / CC BY 2.0 (licence)

You’ll need

  • Your lithium battery’s manual. Its numbers beat anything general we can tell you.
  • The manuals or apps for your shore charger, solar controller and any DC-DC charger
  • A DC-DC charger for alternator charging
  • A shunt battery monitor and a negative busbar
  • A main fuse rated for your bank, plus branch fuses
  • Tinned marine cable, tinned lugs and adhesive-lined heat shrink

The full parts checklist, with product links, is near the bottom.

You can drop a lithium battery into the spot where your old lead-acid one sat, bolt up the same cables and go fishing. Plenty of people do. The trouble shows up later: an alternator that runs hot on a long run home, a gauge that says “full” right until the lights go out, a charger still set to equalize a battery that never needed it.

The battery is the easy part. The real job is everything around it, and it’s a job you can do well, because it’s your boat and you’ll take the time to get it right. This guide doesn’t pick a battery brand. Pick yours first and keep its manual open while you read.

Why lithium changes the system

Lithium iron phosphate (LiFePO4) is the type most boat batteries use. It behaves differently from lead-acid in three ways that matter here.

It takes charge hard. A lead-acid battery slows its own charging as it fills. Lithium will accept close to whatever current the source can give for most of the charge.

Its voltage barely moves. Between nearly full and nearly empty, a lithium battery’s resting voltage changes very little. The voltmeter that worked as a fuel gauge on lead-acid is close to useless now.

It has a brain. Every lithium battery has a battery management system (BMS) that can cut the battery off to protect it, say when it’s too cold to charge or a cell gets too high or too low. When the BMS opens, everything on that battery loses power at once. Whatever was charging it suddenly has nowhere to send its current.

Each step below deals with one of those.

Step 1: Set every charger for lithium

Lead-acid chargers often run stages a lithium battery doesn’t want: a long float, an equalize cycle, a “desulfation” mode. Most lithium batteries want a bulk and absorption charge at the voltage their maker lists, then little or no float, and never an equalize.

Go through every source that charges the house bank:

  1. Shore charger. Many newer chargers have a lithium setting. Older ones may not. Some lithium makers say a lead-acid profile is fine for their battery and others say it isn’t, so read your battery’s manual. If the charger can’t be set to match, replace it. The Victron Blue Smart IP22 12/30 charger has a lithium mode you pick in the app.
  2. Solar controller. Same check. An MPPT controller with a lithium preset or user-set voltages, like the Victron SmartSolar MPPT 100/30, can be matched to the battery maker’s numbers.
  3. Alternator. This one gets its own step, next.

One more setting worth knowing: most lithium makers say not to charge below freezing unless the battery has built-in heating or a low-temperature cutoff. If the boat stays in the water through a cold winter, check what yours does.

Step 2: Charge from the alternator through a DC-DC charger

This is the part people skip. Your engine’s alternator was built to charge lead-acid, which tapers off as it fills. Hook it straight to a lithium bank and it may run at or near full output for a long time. Many alternators aren’t built for that and can overheat.

There’s a second risk. If the BMS disconnects the battery while the alternator is charging, the alternator suddenly loses its load. That voltage spike can damage the alternator’s diodes or regulator.

The common fix is a DC-DC charger between the start battery side and the lithium house bank. It does three jobs:

  • It limits the current to a set number, so the alternator isn’t pushed flat out.
  • It charges the lithium bank with its own correct profile.
  • It keeps the start battery separate, so the house side can’t flatten it.

Here’s the order:

  1. Find out how much current your alternator can safely give up. That’s a question for its maker or a marine electrician, not a guess.
  2. Fit the DC-DC charger between the start side and the house bank. The Victron Orion-Tr Smart 12/12-30 isolated DC-DC is a common pick at 30 amps.
  3. Keep the lead-acid start battery for the engine if you like. Many boats go lithium on the house bank only, and that pairs well with a DC-DC charger. Just don’t put lithium and lead-acid in parallel in the same bank.

What about a plain combiner or voltage-sensing relay, the kind that links batteries when the engine runs? With lithium it doesn’t limit current, so the overheating problem comes back, and many lithium makers advise against it. Some combiners are built for lithium. Check the battery maker’s guidance before you use one. [Guide 3: Alternator charging lithium: Orion-Tr vs Cyrix vs a straight combiner] goes through the choices.

Step 3: Fit a shunt monitor (count amps, not volts)

With voltage this flat, the only good fuel gauge is one that counts the current going in and out. That’s a shunt-based monitor. A low-resistance shunt on the battery negative measures every amp, and the monitor keeps a running total of state of charge.

Two common choices:

Wiring either one comes down to three moves:

  1. Put only the battery negative on the battery side of the shunt.
  2. Move every load, charger and ground to the other side.
  3. Gather those negatives in one clean spot after the shunt, like a Victron Busbar 250A 4-point.

Anything wired straight to the battery negative bypasses the count, and the reading drifts. Get this right and your gauge tells you the truth. [Guide 4: SmartShunt vs BMV-712] compares the two in more detail.

Step 4: Fuse the battery

A lithium bank can push a huge current into a dead short, far more than a similar lead-acid bank. So the main positive cable needs a fuse as close to the battery as you can practically fit it, and that fuse has to be able to break that much current safely. That rating is called interrupt capacity. It’s the number that matters most here.

  1. Main fuse. Many lithium makers call for a class T fuse here because of its high interrupt rating. A Egis class T fuse block holds one right at the battery.
  2. Branch circuits. ANL fuses are common for branch runs, and an Egis ANL fuse block covers them. Their interrupt rating is lower, so check your battery maker’s spec before you use one as the main fuse.
  3. Battery switch. Add one on the house bank so you can kill power for work or storage: Egis XD 500A battery switch.
  4. Low-voltage load disconnect. A Victron Smart BatteryProtect 100A cuts loads before the battery runs flat and can be set for lithium. It’s made for loads only. Don’t run charging current back through it.

ABYC’s E-11 standard covers where fuses go on a boat, how close to the battery, and how to size them to the wire. Your insurer or surveyor may ask whether the install meets it. [Guide 9: Fusing a lithium bank: Lynx, MRBF and class T] covers the main fuse choices.

Step 5: Wire it with tinned cable

Lithium invites bigger loads. People add an inverter, a larger windlass or a fridge, and the old cable may not be up to it.

Use tinned marine cable. Each copper strand is coated in tin, which slows the corrosion that creeps along bare copper in salt air. Marine cable is also finely stranded, so it handles vibration and tight bends better than stiff automotive or welding cable.

  1. Size the cable for the current and the length of the run there and back. The Wire gauge calculator uses the 3% voltage-drop target common for boat circuits.
  2. Fuse each cable to protect it.
  3. Crimp the ends with tinned lugs that match both the cable gauge and the stud size: Pacer tinned lug 2/0, 3/8 in stud, 10-pack for main cables, Pacer heat-shrink ring terminal 8 AWG, 25-pack for smaller runs.
  4. Seal each one with adhesive-lined heat shrink.

[Guide 8: Marine wire: why tinned, and which gauge] has the sizing detail.

Your outboard, model by model

What your engine’s maker says about its charging output and battery, one page per model:

Parts checklist

What most boat lithium upgrades add. Not every boat needs every line.

Or start from the Boat lithium upgrade kit (placeholder: kit not built, margin check by TYLER pending), which groups these, and swap out what you already have.

When to call a marine electrician

Doing it yourself doesn’t mean doing it alone. Bring in an ABYC-certified marine electrician when:

  • You’re not sure what your alternator can handle, or it has an external regulator.
  • The boat has 24V systems, an inverter-charger, or shore power changes in the same job.
  • You’re adding a bank big enough that the main fuse and cable choices aren’t obvious.
  • Anything on board runs hot, smells, or trips without a clear reason after the change.

The AC side and the main battery fuse are the two places where a mistake can start a fire, so those are the places a second set of eyes is worth it.

Related guides: Shore power charging: Blue Smart IP22 vs IP65, Battery switch for a lithium boat, Cerbo GX: do you need one on a boat?

Bundles for this job: Boat lithium upgrade kit, small, Boat lithium upgrade kit, large

Quick answers

Do I need a DC-DC charger if I have solar? Solar doesn’t change the alternator problem. If the engine charges the lithium bank, a DC-DC charger is still the usual fix.

Why does my battery show 13.3V and still go flat fast? Lithium voltage stays flat until it’s nearly empty. Use a shunt monitor to see the real state of charge.

Amp Hour Supply

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