With ever-increasing power demands on today’s cruising yachts, Duncan Kent looks at lithium boat batteries and explains what’s needed to guarantee a safe and trouble-free system

How to install lithium boat batteries

For blue water cruising yachts, the modern solution to increasing electrical demand is to install a lithium-ion battery bank, particularly if one plans to eliminate the use of LPG for cooking.

However, lithium-ion installations can be complex and problematic, and if not done correctly can be a serious fire risk.

First and foremost, the only type of lithium-ion cell chemistry currently recommended as safe for use on board a boat is Lithium-Iron-Phosphate (LiFePO4), usually abbreviated to LFP.

These cells are virtually fireproof in themselves, having been tested extensively by fire authorities in several countries, although they can still cause a fire (as can any battery) if installed or used incorrectly.

Lithium boat batteries make induction cooking on board a possibility

Lithium boat batteries make induction cooking on board a possibility

Use of automotive lithium-ion blends with such elements as nickel, cobalt and manganese are strongly discouraged for use on a boat as they are far more prone to ‘thermal runaway’ should they fail.

This effectively rules out the use of old electric car batteries as there is no real way to transfer the same complex protection system they were originally designed to operate under.

Electrically propelled yachts will usually have a higher voltage system (usually 48V, 72V or 96V), which needs a very carefully devised control system.

For these purposes it’s tempting to incorporate lithium-ion cells of higher energy density, such as Lithium Cobalt (LiCoO2), but this would require professional design and installation, and be very expensive.

Besides, if you can’t insure your boat with this type of battery installed, then why would you do it?

Why choose LFP batteries?

The main benefits of LFP batteries are that they will accept a very rapid, high-current recharge, and they can be discharged to almost empty without the need to be regularly recharged to 100% State of Charge (SoC) as you have to with Lead-Acid (LA) batteries.

In fact, they are happier to sit between 20%-80% SoC most of the time. You can even discharge LFP completely without doing them any harm, although most built-in Battery Management System (BMS) will shut them down at around 12V, which is equivalent to around 10% SoC.

A burnt out lithium battery

Thermal runaway and fire is the risk of badly installed lithium boat batteries. Credit: Alamy Stock Photo

The same when they are fully charged – the BMS should shut off the charging source automatically at around 14.2V to prevent them from being overcharged.

LFP batteries will also provide a much greater number of charge cycles than the equivalent capacity LA battery, and finally they are also considerably lighter than any type of LA battery, which can make a big difference to a sailing yacht’s balance and performance.

Converting to LFP?

With the ability to accept and discharge very high currents, any associated wiring and circuit protection for LFP batteries must be up to the task and tailored to suit.

All LFP banks require a comprehensive Battery Management System that offers reverse polarity protection, individual cell balancing, charge voltage and current limitation, management and emergency disconnection, battery and alternator temperature sensing, discharge current limitation and management – plus visual/audible alarms.

It’s also worth noting that, with many brands of so-called ‘drop-in’ LFP batteries (ones with an integral BMS), it might not always be possible to join more than two together in series or parallel to form a larger bank.

Lead acid remains the preferred option for start batteries, as LFP battery management systems can prevent sufficient rapid output. Credit: Graham Snook/Yachting Monthly

Lead acid remains the preferred option for start batteries, as LFP battery management systems can prevent sufficient rapid output. Credit: Graham Snook/Yachting Monthly

If you need greater capacity, you’re often better off building a custom bank from individual 3.2V cells and attaching a single external BMS to control the entire bank simultaneously.

If you do plan to construct your own battery bank from individual cells, then you need to buy new, grade A cells.

Many of the budget buys online from China are pre-used cells, often changed out from a data storage bank’s Uninterruptible Power Supply (UPS) or similar.

Although you might be lucky, it really isn’t worth the gamble as returning them will be virtually impossible.

Adding lithium boat batteries is more complicated than a straight swap

Adding lithium boat batteries is more complicated than a straight swap

Ideally, the cells will have been capacity tested and arrive at a very similar voltage level, but you will still need to initially balance them to be sure.

Although possible, it isn’t advisable to use LFPs for engine starting or anchor windlass and bow thruster operation. Most won’t work anyway as such an instant heavy current draw these devices demand can often exceed their BMS’s output threshold.

The element of an LFP installation that causes most concern is how to set the system up safely for charging.

As with most things on a boat there are personal choices and practical decisions to consider, and many of these will vary depending on the size and type of craft.

Maximising cell life

LFP batteries have a very low resistance compared to Lead-Acid (LA) types, thereby enabling them to be charged and discharged at a much higher rate – even up to 1C (1 x capacity, or 100A for a 100Ah battery).

However, they are generally charged at between 0.5C-1.0C until the charge current drops to between 0.015C-0.03C, when charging must cease so as not to overcharge the cells.

A blue battery bank

Building your own battery from individual 3.2V cells with an external Battery Management System will let you create a larger bank

Some manufacturers even recommend you stop charging at 0.1C to further reduce cell stress and extend their lifespan.

Unlike LA batteries, the SoC cannot be determined by battery voltage alone, as this can reach its peak when the LFP battery is only half-charged.

Furthermore, a fully charged 12.8V LFP battery has a rested voltage of between 13.4V-13.6V, way above the 12.7V of a regular lead-acid battery. At 20% SoC
it will still read 13V, when a LA battery will be at 11.8V.

Alternator charging

One of the most confusing areas of an LFP upgrade is how to set up alternator charging to suit your installation and any existing equipment you wish to retain.

As a LFP battery has a very low internal resistance it will draw as much current from the alternator as it can, and if allowed to do so uncontrolled it will overheat the alternator windings, destroying them in a few minutes.

If you leave a LA engine start battery connected directly to the alternator output and incorporate a Voltage Sensing Relay (VSR) to charge the LFP leisure bank (as you would normally with a LA domestic battery bank), you will very likely overload the alternator, VSR or both.

External battery management systems are needed for larger banks

External battery management systems are needed for larger banks

In fact, even if the VSR survived this scenario it would not work as it should due to the higher resting voltage of the LFPs forcing the relay to remain permanently activated.

One recommended solution is to install an external smart alternator regulator, which will have alternator and battery temperature sensors fitted and will limit the charge accordingly to ensure the alternator stays within its operating temperature range, while allowing as much charge as safely possible to reach the LFP bank.

An alternative, popular for its simplicity, is to send the alternator charge direct to the start battery but then to install a DC-DC (B2B) charger between that and the LFP house bank.

Continues below…

The main advantage of this method is that a DC-DC charger will limit the current being drawn from the start battery, and thereby the alternator, whilst also ensuring the output charge parameters are fully compatible with the LFP house bank.

It also means the alternator diodes are protected if the LFP BMS shuts down for any reason as it can continue to charge the start battery as normal, and a DC-DC charger is also considerably cheaper than a smart regulator.

The only real downside is the charge going to the bank will be limited to the DC-DC charger’s maximum output, although this can be overcome by installing more than one DC-DC charger in parallel if a greater total charge current is required.

Temperature controlled

There is a ‘shoestring’ method that I’ve come across recently, often called ‘long wire regulation’.

It involves connecting the alternator (with standard in-built regulator) directly to the LFP bank, and then regulating the amount of current drawn by adjusting the size and length of the cable, using Ohm’s law to calculate the required resistance to reduce the charge current so that it is within the alternator’s safe working load limits.

If you sail in cold climates beware, lithium boat batteries won't accept charge when below 0°C

If you sail in cold climates beware, lithium boat batteries won’t accept charge when below 0°C

While this theory does work, using a wire as a resistor will heat the wire and is not recommended by professional marine electrical institutions and would certainly be frowned upon by insurance companies in the event of an incident.

A problem with ‘drop-in’ type LFP batteries is once the BMS decides the battery is fully charged it will simply disconnect the charging source.

If this is a solar or DC-DC charger it’ll be fine. But if it’s your alternator, then instantly cutting off the load will undoubtedly blow its output diodes.

Finally, it’s worth noting that LFP batteries won’t accept any charge at 0°C or less.

So, if your battery box regularly drops to such temperatures, you’ll need to place them onto heat pads to keep them at 5°C or more.

A DC-DC charger can link lithium and lead acid batteries

A DC-DC charger can link lithium and lead acid batteries

This will result in some consumption of power but at least they will then accept a charge.

There is an important physical aspect to bear in mind too when converting to LFP batteries.

Because the higher charging rates will place a heavier load on your alternator, you’ll need to ensure your drive belt is up to the job.

Make sure it’s the correct type of belt, in good condition and properly tensioned.

A standard vee-belt is only sufficient for alternators rated up to 75A.

Above this and you’ll need to convert to a heavy-duty belt and pulleys such as the serpentine multi-channel belt drive.

Solar charging lithium boat batteries

Because of the LFP’s ability to accept a high current bulk charge, some owners choose to charge them exclusively from solar energy.

This works well, in reasonably sunny climates anyway, in that you can pour as much power into the bank as the panels will produce during their most productive part of the day.

A boat sailing with solar panels

Solar panels work well with lithium boat batteries, as they can store high rates of peak charge and don’t waste solar energy as slower-charging lead acid batteries do. Credit: Richard Langdon

It also means that the BMS can simply switch out the PV panels without damage when the batteries have achieved their prerequisite SoC.

The obvious downside if you cruise all year round is the shortage of sun in the winter.

In this case I would advise you either add alternator charging via a DC-DC charger or keep a small portable generator and an LFP-compatible mains charger on board for those short, cloudy winter days.

220V mains charging

Most modern marine battery chargers now come with a LiFePO4 charging regime built-in to ensure the charging voltages are kept within the correct parameters.

As LFPs hate to be continuously charged, even in float mode, using an LFP-compatible charger allows them to be safely left unattended.

A decent MPPT solar charge controller will help make the most of your solar power

A decent MPPT solar charge controller will help make the most of your solar power

It is possible, however, to use a standard, single stage LA battery charger in AGM mode, provided you monitor the state of the batteries constantly and disconnect the charger immediately the desired SoC or peak voltage is reached (definitely no higher than 14.6V – 14.2V is safer).

One vital function to check if using a non-LFP shore power charger is whether it is configured to perform an automatic de-sulphation (equalisation) cycle, wherein the charging voltage is increased to 15.5V or more for several hours.

This must be disabled as it will destroy the LFP cells if activated.

Hybrid systems

After years of warning boaters never to mix different battery chemistries together in a single bank, ‘experts’ proclaimed that, in the case of LA/LFP, the reverse is true and that a hybrid LA/LFP bank is perfectly safe and can even offer the best of both worlds.

We’re not talking about a 50/50 LA/LFP split here, more usually just a single LFP to boost the capacity of an existing LA house bank.

The theory behind it is that the LA battery modulates the excesses of the LFP, whilst being maintained in tip-top condition itself.

RELION is a good example of a LFP battery with an internal BMS

RELION is a good example of a LFP battery with an internal BMS

The LFP battery, which is simply connected in parallel with the LA house bank, will accept the bulk of any charge until it reaches the pre-programmed upper voltage threshold on the BMS (usually set for around 90% SoC).

At this point the BMS will shut the charge down to the LFP and the LA bank will continue charging alone.

When discharging, the LFP will naturally be drained first until its voltage reaches 12.8V, at which point the LAs will also start to supply power.

Once the bank drops below the LFP’s BMS low voltage threshold it simply switches off and lets the LA batteries continue to work as normal.

Which lithium boat batteries should you choose?

The point of such a hybrid system is that the LA batteries have far less work to do and are kept permanently topped up by the LFP battery.

It also means that the alternator diodes are safe when the BMS shuts down as the charge is transferred to the LA bank.

Despite this idea being tested and proven to work safely by a number of qualified marine electricians, both the International Standards Organisation (ISO) and the American Boat and Yacht Council (ABYC) have stated that mixing battery chemistries such as Li-ion and LA is not recommended and will not be approved for certification.

A power charge point at a marina on a pontoon

Consider whether shore power is part of your regular charging regime

It is the latter that marine insurance providers will listen to when formulating their policies.

We would only trust LFP batteries on a boat. Other existing li-ion batteries currently available are just not suitable for the marine environment.

However, we would recommend that you consider installing a LFP house battery bank when you next upgrade as the benefits of rapid charging, greater useable capacity, longer life and ability to leave them half-charged for long periods, far outweigh the slight increase in initial cost and the need to change a few aspects of the charging system.

We also recommend fitting solar charging facilities, however small, as these work hand-in-hand with LFP cells, even in the less sunny climes of the UK.

Every boat is different, depending on the equipment, electrical loads anticipated and type of usage.

It is well worth seeking the advice of a professional before just going ahead and adding a lithium battery into your boat.

There’s much more to it than just a straightforward swap.

Mixed battery set up options

The three most common installation methods using LA engine start and LFP house batteries are:

  1. Alternator to start battery/solar to house battery. Pros: Simple & safe with least risk of failure. Cons: Any excess charge will be wasted once each independent battery full.
  2. Alternator to start battery/solar to house/start to house via DC-DC charger. Pros: Alternator protected from sudden disconnection and house LFPs charged at correct LFP parameters. Cons: DC-DC charger limits maximum charge rate.
  3. Alternator and solar to LFP house battery/DC-DC from house to start battery. Pros: Maximum charge to LFP house bank so quicker to charge. Cons: Requires external alternator regulation to prevent overheating the alternator.

Glossary of terms

  •  LA – Lead Acid (battery)
  • Li-ion – Lithium Ion
  • LFP / LiFePO4 – Lithium Iron Phosphate
  • UPS – Uninterruptible Power Supply
  • DC-DC charger – battery to battery charger (aka: B2B charger)
  • BMS – Battery Management System
  • SoC – State of Charge
  • DoD – Depth of Discharge
  • PV panels – Photo-Voltaic (solar)

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