Switching to Lithium (LiFePO4) Boat Batteries: What Changes and When It's Worth It

Write down each battery on the boat and what it does: starting, house (lights, electronics, fridge, inverter) or trolling motor. Practical Sailor's conclusion for a typical boat is to keep lead-acid on the starting side and use LiFePO4 for the house (or trolling) side. Starting a big engine with lithium is possible with batteries designed and approved for it, but it is a separate decision; check your engine manual's battery requirements (type, marine cranking amps and reserve capacity) before replacing a cranking battery.

Each one must be compatible with lithium, or be changed:

• Onboard shore-power charger
• Engine alternator (inboard, sterndrive) or outboard charging output
• Solar or wind charge controller
• Battery isolator, automatic charging relay (ACR) or battery combiner
• Inverter/charger, generator-powered charger

Also note the loads: a windlass, bow thruster or inverter can draw hundreds of amps, and the battery and its BMS must be rated for that.

The BMS is the electronic brain that protects the cells. Points to check:

• Built-in or external: many "drop-in" batteries have the BMS inside. Others, such as Victron's Lithium Battery Smart line, need a separate external BMS, and Victron lists which of its BMS units suit marine systems with alternators and DC loads.
• Protection that meets ABYC E-13 as described above, including low-temperature charge protection.
• Ratings for continuous and peak discharge that cover your biggest load. Practical Sailor warns that some makers stretch "cold cranking amps" claims and suggests trusting conservative continuous ratings.
• An alarm you'll notice. Practical Sailor cautions against relying on a phone app alone for low-charge warnings; ABYC calls for a visible or audible alert.
• One larger battery is usually simpler than several small ones in parallel or series. Practical Sailor describes parallel batteries with separate BMSs falling out of balance and shutting down.

Lithium needs different charge voltages than lead-acid. Dakota Lithium, for example, publishes about 14.4 volts bulk/absorption and 13.6 volts float for its 12-volt batteries, with equalization, desulfation and lead-acid temperature compensation turned off; other brands differ, so use your battery maker's numbers. Dakota adds that a lead-acid charger may fully charge, undercharge or use the wrong stage on a lithium battery depending on its profile. Use a charger with a lithium setting the battery maker approves, or replace it. See the onboard battery charger guide for mounting and fusing.

This is the most important change on boats with inboard or sterndrive engines (and big outboards with high-output alternators).

• A discharged lithium bank will try to draw as much current as the alternator can make, for as long as it can. A standard alternator can overheat doing that. Practical Sailor puts it bluntly: don't connect an alternator directly to an LFP house battery.
• If the BMS suddenly disconnects the battery while the alternator is charging it, Battle Born warns the alternator and the boat's electrical system can be severely damaged.

The two common solutions:

• A DC-DC charger: the alternator charges the starting battery as usual, and the DC-DC charger feeds the lithium house bank at a controlled rate with a lithium profile. Battle Born calls this the simple, safe option (slower); Dakota Lithium calls it the appropriate choice for most installations with a factory alternator.
• An external alternator regulator designed for lithium, which controls alternator output using voltage, current and temperature. This charges faster but must be designed around your alternator and the BMS's disconnect behavior.

Dakota notes some lithium starting or dual-purpose batteries are approved for direct alternator charging in specific applications. Only rely on that if the battery maker approves it for your engine.

Never wire a lithium battery in parallel or series with lead-acid batteries; their voltages and charging needs are different, and one will be over- or undercharged. Keeping a lead-acid starting battery and a separate lithium house bank is fine when they are linked by the right device (usually a DC-DC charger). Check any existing ACR, isolator or combiner: many were designed around lead-acid voltages and may not behave correctly with a lithium bank. Ask your installer whether yours can stay or should be replaced by a DC-DC charger. Owners with older diode-type isolators often notice their batteries never seem to reach full charge from the engine: the diodes drop some voltage on the way to the battery, so charging is slower and may stop short of full. Lithium batteries want a specific charge voltage, which makes this worse, and it's one more reason installers usually swap a diode isolator for a DC-DC charger in a lithium conversion.

Check what your engine already provides, too. Yamaha notes that many of its larger outboards (150 hp and up) have separate isolator or charging leads meant for a house battery, which charge the starting battery first and then switch to the house bank. That system was designed around lead-acid, so ask the dealer and the battery maker whether it is approved for a lithium house bank before you connect one.

Dakota Lithium's rule: most LiFePO4 batteries should not be charged below 32°F (0°C) unless the battery has internal heating; warm the battery first or use a heated model. A good BMS will block charging when it is too cold, which is why a lithium bank may refuse a charge on a frosty morning. For winter layup, follow your battery maker's storage instructions and switch off or disconnect every load so small parasitic draws can't run the battery down to its BMS cutoff over the winter.

For 24- or 36-volt trolling motors, Dakota cautions to connect batteries in series only when that model permits it, and to use a charger that matches the full bank's voltage and chemistry (or one lithium-compatible output per battery, as the battery maker specifies). Don't keep the old lead-acid charger profile, and don't mix one new lithium battery with older lead-acid ones in the same trolling bank. The trolling motor wiring guide covers fusing and wire size.

A common question is whether the outboard will recharge the lithium trolling bank on the way home. On most small and mid-size outboards the charging output is modest and goes to the starting battery first, so plan to recharge the trolling bank with its lithium charger at the dock or at home. If you want the outboard to top it up underway, the usual safe route is a small DC-DC charger fed from the starting battery, sized to what the outboard's charging system can spare. Check your engine manual for its charging output.

• Secure the battery with hold-downs or a fixed box so it can't move in rough water, as ABYC E-13 requires.
• Keep it cool and dry. Practical Sailor advises avoiding direct sun and hot spots such as a hot engine compartment; the BMS should have an over-temperature cutoff.
• Fuse the main positive cable close to the battery, with a fuse type rated to interrupt the very high current a lithium battery can deliver. Your installer will size it per ABYC E-11 and the battery's specs.
• Use correctly sized tinned cable, crimped lugs and strain relief. Practical Sailor stresses that loose connections become hot connections, which can start a fire.
• Include a battery switch so you can fully isolate the bank.

A LiFePO4 battery's resting voltage barely changes across most of its charge range, then drops off sharply near empty. A voltmeter that reads "13.1 volts" can't tell you whether you're at 30 or 80 percent. A battery monitor with a shunt, or the battery's own app, counts amp-hours in and out and gives a real percentage.

For anything beyond a simple trolling-motor or electronics battery swap, hire an ABYC-certified marine electrician. A typical house conversion touches the charger, alternator, DC-DC charger, inverter, battery switches, fusing and monitoring, and each needs to be set for the specific battery. Ask for:

• A wiring diagram and a list of the charge settings used for every source
• Proof the battery is tested to a recognized standard and meets ABYC E-13
• Paperwork for your insurer and future surveyor

On the first outings, watch the battery monitor and app:

• Each charging source (shore charger, alternator via DC-DC, solar) shows charge current going in.
• The alarm sounds or lights in a test (your installer can show you how).
• No cable, fuse holder or charger runs hot after an hour of charging.
• Update the battery type and charge settings in your BoatMend boat profile.