Switching to Lithium (LiFePO4) Batteries: A Complete Yacht Guide
A LiFePO4 bank nearly doubles the usable capacity on board, but it only works properly when the whole charging system is rethought, not just the batteries.
Switching a yacht to lithium (LiFePO4) batteries is one of the most effective upgrades you can make to the house bank. For the same weight and volume you get far more usable energy, faster charging and noticeably fewer generator hours. But a lithium battery is not a plug-and-play part: if the charge sources, protection and monitoring are not designed together, the system ends up both inefficient and risky.
Why LiFePO4 is a different chemistry
LiFePO4 (lithium iron phosphate) cells have a nominal voltage of 3.2 V; four cells in series make a 12.8 V nominal battery. The key difference from lead-acid is that most of the capacity can be used safely. With AGM or gel batteries you normally stay above 50% depth of discharge to protect their life; with LiFePO4, using 80–90% is normal.
The discharge curve is also very flat: from 90% down to 20% the battery sits between roughly 13.3 and 13.0 V. That is good news for your equipment, but it means you can no longer estimate state of charge from voltage. A shunt-based battery monitor (for example a Victron Energy SmartShunt or BMV) is therefore an essential part of any lithium system.
| Property | AGM (12 V) | LiFePO4 (12.8 V) |
|---|---|---|
| Practical usable capacity | ~50% | ~80–90% |
| Approx. weight (100 Ah) | 28–32 kg | 11–15 kg |
| Charge efficiency | ~80–85% | ~95–98% |
| Absorption voltage | 14.4–14.7 V | 14.2–14.4 V |
| Float voltage | 13.5–13.8 V | 13.4–13.5 V (or off) |
| Charging at low temperature | Possible | Must be blocked below 0–5 °C |
The BMS: the brain of a lithium system
Every LiFePO4 battery needs a BMS (Battery Management System). The BMS monitors cell voltages and temperature, balances the cells and cuts charging or loads when conditions become unsafe. There are two basic architectures:
- Drop-in batteries with an internal BMS: the BMS sits inside the battery case and disconnects the battery itself when there is a problem. Installation is simple, but the disconnect usually comes without warning.
- Systems with an external BMS: batteries such as the Victron Lithium Battery Smart send cell data to an external BMS (smallBMS, Lynx Smart BMS, VE.Bus BMS). The BMS tells the chargers to stop charging and the loads to stop consuming; the battery is never suddenly dropped from the circuit.
Whichever architecture you choose, you need to know in advance what the BMS will cut and when. At anchor at night, is it acceptable for the fridge, navigation lights and anchor alarm to switch off together when one cell reaches its low-voltage limit? In a good design, non-critical loads are shed first, the crew is warned by an alarm, and the battery is only isolated as a last resort.
On a yacht, an external-BMS architecture is generally the safer choice. In particular, if the battery disconnects suddenly while the alternator is charging, the resulting voltage spike (load dump) can damage both the regulator and sensitive electronics.
Adapting the charge sources to lithium
Shore charger and inverter/charger
Your existing charger must offer a LiFePO4 profile or adjustable absorption and float settings. Typical settings for a 12 V system are 14.2–14.4 V absorption, a short absorption time and 13.4–13.5 V float. Older fixed-profile chargers often need replacing. We cover this in detail in our guide to choosing an inverter/charger for a yacht.
Alternator
Because a lithium battery has very low internal resistance, it will draw as much current as the alternator can deliver. A standard alternator with an internal regulator may run at full output for hours and overheat. There are two common solutions:
- DC-DC charger: a DC-DC charger (for example an Orion-Tr Smart) is fitted between the alternator and the lithium bank; current is limited and the correct charge profile is applied.
- External regulator: the alternator is controlled by a programmable regulator with a temperature sensor; this is preferred on high-capacity systems.
Which approach fits depends on alternator output and how you use the engine. We explain it step by step in our article on alternators, external regulators and DC-DC charging.
Solar
MPPT solar charge controllers usually support a lithium profile. What matters is that the controller communicates with the BMS, or that the BMS can stop it charging.
Protection: fuses, cables and the main switch
A lithium bank can deliver thousands of amps into a short circuit. The main fuse must therefore have a high interrupting capacity; on larger banks a Class T fuse is the standard approach. Under the ABYC E-11 standard, the main fuse should be as close as possible to the battery positive terminal, generally within 18 cm (7 in).
Cable sizes must be recalculated for the new charge and discharge currents, because a lithium system carries higher currents for longer. A 3 kW inverter on a 12 V system, for instance, can draw more than 250 A at full load; even over a short run that may call for cable in the 70–95 mm² range. Crimping every lug with a hydraulic tool, tightening terminals to the manufacturer's torque and sealing them with heat-shrink are the foundations of a trouble-free installation. We compare the right fuse types in our guide to ANL, MRBF and Class T fuse selection.
A step-by-step conversion plan
- Measure your energy needs: log your current daily consumption (Ah) with a battery monitor for at least a few days.
- Size the bank: roughly two to three days of consumption is a practical starting point for lithium.
- Choose the BMS architecture: external BMS or drop-in with an internal BMS? Draw out the load and charge control paths.
- Review the charge sources: confirm a lithium profile for the shore charger, inverter/charger, alternator and MPPT.
- Upgrade the protection: Class T or another suitable main fuse, cable sizes and the battery isolator.
- Keep the start battery separate: the start battery can stay AGM, charged from the lithium bank via a DC-DC charger or a suitable charge splitter.
- Mounting and ventilation: install the batteries in a dry, secured location at a reasonable temperature. In a Bodrum summer, engine room temperatures climb; keep the batteries away from engine heat where possible.
- Commissioning: first full charge, cell balancing, battery monitor synchronisation and testing of every alarm and disconnect scenario.
Common mistakes
- Running lithium and lead-acid batteries in parallel in the same bank.
- Leaving the alternator unprotected and running it at full load for hours.
- Trying to judge state of charge from voltage.
- Charging below 0 °C in winter (on batteries without low-temperature protection).
- Not sizing the main fuse for the battery's short-circuit current.
When a lithium conversion is planned properly it takes the strain off the entire energy system: the generator runs less, the inverter can be used with confidence and the bank delivers season after season of reliable service.
Work with Aslan Yacht on your lithium conversion
From our workshop in Turgutreis we come to the marinas and boatyards around the Bodrum peninsula, inspect your existing system on board and plan the conversion from consumption logging through to commissioning. Learn more about our energy and battery systems service, or get in touch with us to discuss your boat.
Frequently asked questions
Can I connect a lithium battery in parallel with my existing AGM batteries?
No. Different chemistries have different charge voltages and internal resistance, so one bank will always be over- or undercharged. The engine start battery can stay AGM, but the house bank should be a single chemistry.
Should I store a LiFePO4 battery fully charged?
For long-term storage, a state of charge of roughly 50–70% is ideal. Over the winter lay-up, disconnect the loads, switch float charging off or keep it low, and leave the BMS protection active.
Does a lithium battery create a fire risk on board?
LiFePO4 is far more thermally stable than other lithium-ion chemistries. The risk usually comes not from the cells but from incorrect fusing, loose connections and installations without a BMS.
