Lithium Batteries

LiFePO4 lithium batteries for boats: less weight, more cycles

The LiFePO4 lithium battery (lithium iron phosphate) has completely changed electrical systems on board. Compared with an equivalent lead-acid battery it weighs around half as much, delivers practically its full rated capacity and offers between three and ten times the cycle life.

At Repuestos Marinos we supply 12V lithium batteries for boats with an integrated BMS, designed for house loads as well as for fishing and electric trolling motors, alongside conventional marine starting and deep cycle batteries.

LiFePO4 versus lead-acid, AGM and gel

Usable capacity. A lead-acid battery should not be discharged below 50% if you want it to last; an AGM will take perhaps 60-70%. A LiFePO4 can be discharged to 80-90% with no penalty. In practice, a 100 Ah LiFePO4 performs like a 200 Ah AGM.

Weight. Around half for the same usable capacity, and less than a third if you compare genuinely usable capacity. On a fishing boat or a sailing yacht, where every kilo counts and sits low down, the difference is noticeable.

Cycles. From 2,000 to 5,000 cycles against the 300-500 of a lead-acid deep cycle battery. With normal leisure use, that means many years.

Charging. It accepts far higher charge currents and holds a stable voltage almost to the end of the discharge, so your equipment never notices the drop. It does need a charger with a lithium profile: using a lead-acid charger may leave it short of a full charge or, worse, damage it.

Price. More expensive up front, clearly cheaper per cycle over its lifetime.

Lithium batteries for fishing and electric motors

For fishing from a small boat, LiFePO4 has become the standard for powering electric trolling and positioning motors, the sounder and the livewell. The reasons are obvious: it holds a stable voltage, so the motor delivers the same thrust at the end of the day as at the start; and its low weight lets you carry more range without weighing the boat down.

To size it, work out the motor's average draw in amps and multiply by the hours of use you expect. Add the consumption of the sounder, livewell and electronics, and leave a 20-30% margin. Take a look at our Motores Fueraborda Eléctricos category too.

Installation and safety

BMS. Every quality LiFePO4 battery has a battery management system (BMS) that protects it against overcharging, deep discharge, overcurrent and extreme temperatures. Check that your battery has one and what maximum current it will take.

Charger and alternator. Use a charger with a LiFePO4 profile. If you are going to charge from the engine alternator, consider fitting a regulator or a DC-DC converter: standard alternators are not designed for the low internal resistance of lithium and can overheat.

Temperature. Lithium must not be charged below 0 °C. If the boat winters somewhere cold, choose a model with an integrated heater or low-temperature protection in the BMS.

Mounting and protection. Secure the battery firmly and protect the terminals. Fit a main fuse as close as possible to the positive terminal, sized to the maximum current of the installation.

Do not mix chemistries. Never connect a lithium battery in parallel with a lead-acid one: their charge curves are incompatible and the lithium will end up feeding the lead-acid battery.

Winter storage, warranty and shipping

To lay the boat up for months, leave the LiFePO4 at around 50-60% state of charge and disconnect it. Unlike lead-acid, lithium does not sulphate or deteriorate from sitting at half charge; what harms it is spending long periods at 100% or completely flat.

Shipping in 24-48 h to mainland Spain and the Balearics, 3-5 days to the Canary Islands and the rest of the EU. Orders placed before 14:00 leave the same day. Returns within 14 calendar days, plus technical support by phone and WhatsApp.

Frequently asked questions about marine lithium batteries

What advantages does a LiFePO4 battery have over an AGM?

It weighs around half as much, takes discharges of 80-90% against 60-70% for an AGM, offers 2,000 to 5,000 cycles against 300-500, and holds a stable voltage almost to the end of the discharge. In practice, a 100 Ah LiFePO4 performs like a 200 Ah AGM. It is more expensive up front but clearly cheaper per cycle.

Can I charge a lithium battery with my current charger?

Only if it has a LiFePO4 charge profile. A lead-acid charger may leave it short of a full charge or, in the worst case, damage it. If you are going to charge from the engine alternator, consider fitting a regulator or a DC-DC converter, because standard alternators are not designed for the low internal resistance of lithium.

What is the BMS on a lithium battery?

It is the electronic management system that protects the battery against overcharging, deep discharge, overcurrent and extreme temperatures. Every quality LiFePO4 battery has one built in. Check that your battery includes it and what maximum discharge current it will take.

Can a lithium battery be charged in cold weather?

Not below 0 °C: charging lithium at sub-zero temperatures damages the cells permanently. If the boat spends the winter somewhere cold, choose a model with an integrated heater or with low-temperature protection in the BMS.

Can I connect a lithium battery in parallel with a lead-acid one?

No. Their charge and discharge curves are incompatible: the lithium, with its lower internal resistance, would end up feeding the lead-acid battery and neither would work properly. If both live on the same boat, they must be on separate circuits with their own management.

How do I store a lithium battery over winter?

Leave it at around 50-60% state of charge and disconnect it. Unlike lead-acid, lithium does not sulphate from sitting at half charge; what harms it is spending long periods at 100% or completely flat.