LiFePO4 vs AGM vs gel batteries on a boat: an honest comparison

More and more yacht owners are replacing lead-acid house batteries with lithium iron phosphate (LiFePO4). The benefits are real, but so are the pitfalls. Here is a straightforward comparison of the three types you are most likely to consider.

Note: this guide is written by the team behind Sail Mallorca, which supplies and installs FORSTER lithium batteries in Palma. Other good marine lithium brands exist, and the principles below apply to all of them.

The short answer

• LiFePO4 gives you far more usable energy for its size and weight, lasts many times longer and charges faster. It costs more up front and needs a charging system set up for it.
• AGM is a robust, affordable lead-acid battery that tolerates a wide range of chargers. It is heavy and only about half of its rated capacity is usable for a long life.
• Gel handles deep discharge a little better than AGM but is more sensitive to charging voltage and slower to charge. It is now less common on yachts.

Side by side

The figures below are typical ranges. Always check the data sheet of the specific battery.

	LiFePO4	AGM	Gel
Usable capacity	Around 80 to 100% of rated Ah	Around 50% for good lifespan	Around 50 to 60%
Typical cycle life	Several thousand cycles	A few hundred cycles at 50% depth	Several hundred cycles
Weight for the same usable energy	Roughly a third to half	Heaviest	Heavy
Charging speed	Fast, accepts high current until nearly full	Slows down well before full	Slowest
Voltage under load	Stays flat until nearly empty	Drops steadily	Drops steadily
Price per battery	Highest	Lowest	Medium
Needs a lithium-ready charging system	Yes	No	Gel profile recommended

Why usable capacity matters most

A 100 Ah AGM battery should normally not be taken below about half charge if you want it to last, so in practice you get around 50 Ah. A 100 Ah LiFePO4 battery can routinely deliver most of its rating. That is why a lithium bank can often replace a lead-acid bank of roughly twice the nominal capacity, at a fraction of the weight.

What changes when you switch to lithium

This is where many upgrades go wrong. Lithium batteries are not a drop-in replacement on most boats.

1. Chargers and solar controllers need a lithium profile with the right absorption and float voltages.
2. The alternator can be overloaded, because a lithium bank will draw as much current as the alternator can deliver for long periods. A DC-DC charger or a smart external regulator is the usual solution.
3. Battery management system (BMS). Every LiFePO4 battery relies on a BMS that disconnects it on over-voltage, under-voltage, overcurrent or extreme temperature. Your boat must cope safely with that disconnect, especially the alternator.
4. Cold charging. LiFePO4 must not be charged below freezing. In Mallorca that is rarely an issue on the water, but it matters for boats wintering further north. Some batteries have built-in heating.
5. Battery monitor. A shunt-based monitor set up for lithium gives you an accurate state of charge, which voltage alone cannot.
6. Starter battery. Many owners keep a lead-acid battery for the engine and use lithium only for the house bank.

When AGM is still the right choice

• The boat is mainly on shore power and rarely cycles its batteries.
• The budget is tight and the charging system is old and not worth upgrading.
• You need a simple replacement before the season and have no time for a full system check.

Our advice

Start with an energy audit: what you use each day and how you charge. Size the bank from that, then plan the charging side before you buy batteries. If you are in Palma and want a second opinion, the Sail Mallorca team offers consultations on onboard energy systems, and any good marine electrician can do the same.