Duncan Kent investigates the technology used in lithium batteries and explains what to consider in matching them with a management system

How suitable are lithium batteries for use in the liveaboard marine environment?

Everyone has their own theories and experiences of what type of battery is best.

Traditionally, the choice was always big and heavy, open flooded lead-acid (FLA) batteries and many still swear by this simple technology.

Lithium batteries help make the most of solar power on a yacht

Modern solar panels are much more efficient and lithium batteries are able to make the most of their power. Credit: Alamy

The main benefits being that you can top them up with distilled water easily and test the capacity of each cell using a hydrometer.

They’re also easy to source and less expensive than newer tech batteries.

Their main drawbacks are their size, weight and tendency to lose their capacity over time.

The old saying ‘heavy is best’ referred to the fact that the heavier it was, the thicker the plates and the longer they would last.

A flooded lead acid battery

Flooded Lead Acid: Need topping up, but inexpensive and rugged

However, being so heavy persuaded many to build their service bank from easier to manhandle 6V batteries (often golf-cart batteries), connecting them in series pairs for 12V and then parallel for capacity.

It also meant there was less to lose/replace if one 6V battery failed.

The next level up are sealed lead-acid batteries (SLA), also known as Valve Regulated SLAs (VRSLA). Many prefer these for their maintenance-free, non- spill qualities, although they can’t be charged as vigorously as an open-cell battery due to their ability to only release excess gas pressure in an emergency.

Sealed lead acid battery on board a boat

Sealed Lead Acid: Maintenance free but slow charging and shorter life. Credit: Graham Snook/Yachting Monthly

Several decades ago, gel batteries were introduced, wherein the electrolyte was a solid gel rather than a liquid.

Although sealed, maintenance-free and able to provide a greater number of charge/discharge cycles, they were slower to charge and could easily be damaged, some even exploding if overcharged.

More recently, Absorbed Glass Mat (AGM) batteries became the rage for boats.

Lighter than regular LAs and with their electrolyte absorbed into a glass matting rather than free liquid or gel, they require no maintenance and can be mounted at any angle.

They can also accept a higher charge current, thereby taking less time to recharge, and they will survive a good many more charge/discharge cycles than most flooded cells.

Absorbed glass matt battery

Absorbed glass matt: Lighter, higher charge rate and longer service life

In addition, their lower self-discharge rate enables them to be left without charging for some considerable time.

Whatever technology you choose, the important thing to consider when planning to create an interconnected service bank is that all the batteries must be of the same chemical makeup, type, capacity and voltage.

You can’t mix SLA, Gel or AGM, and you certainly can’t link any of these with a Lithium-based battery in a single bank.

Lithium batteries

In the past few years most new developments in marine battery technology have involved Lithium-ion (Li-ion) cells.

Cost excepted, most swear by them in their various guises but they have to be installed, managed and maintained extremely carefully. If you are in any way considering moving towards a fully electric boat, then Li-ion batteries will be a necessity.

Their primary benefits are:

  • they are much lighter than any lead-acid derived marine battery;
  • they can be discharged to between 10-20 per cent of their total capacity (which almost doubles their ‘useable’ capacity when compared to LAs);
  • they will provide many more (up to 10x) charge/discharge cycles in their lifetime than any older tech battery
  • thanks to their low internal resistance, they can withstand a much greater charge rate, thereby drastically reducing the time taken to recharge them.

Li-ion batteries do have a few unfortunate drawbacks, however.

To start with they’re an expensive investment on their own.

Then they require an intelligent battery management system (BMS) to ensure the voltage level of each cell in each battery remains the same.

Furthermore, there are complex procedures required to prevent them becoming under-, or over-charged, either of which can kill them stone dead.

Lithium batteries are very fast charging

Lithium Ion: Very fast charge acceptance, can be discharged more deeply, and very light

The discharge rate of Li-ion batteries is also strictly limited so it’s also important to know the maximum discharge capabilities of a Li-ion battery before choosing which to buy, especially if you have heavy current draw items on board such as a windlass, bow thruster, electric cooker, water heater and kettle.

One notable disadvantage of Lithium cells, which can be a problem in the northern hemisphere, is their behaviour in extreme temperatures.

Although they will still supply power at -10°C, they won’t actually accept any charge if the ambient temperature is below freezing.

If you’re planning an extended cruise in high latitudes then, forget Li-ion.

And neither do they like being too hot – 40°C being the point at which some BMS choose to disconnect for safety purposes, so that clearly rules out their installation in engine compartments.

A wind generator on a yacht

A means of topping your batteries up is crucial for keeping them in good health

If you are thinking of installing a Li-ion system on your own boat then please read everything you can find on the subject first and even then I would recommend you seek professional advice from a power expert – at the very least during the planning stage.

It’s also worth checking with your yacht insurer before you shell out on an expensive Lithium-based battery bank.

Some insurers are very wary of them and some won’t even allow them to be covered unless the battery has been put in by professional installers.

Lithium Iron Phosphate

Early Li-ion batteries were Lithium-Cobalt (LiCoO2) and were energy dense but quite unstable and prone to ‘flame venting’ (catching on fire to you and me) when overcharged.

In 1996 phosphate was tried as cathode material rather than Cobalt, resulting in the now commonly available Lithium Iron Phosphate (LiFePO4) or LFP battery.

LFP batteries offer excellent performance and have low resistance, allowing high-current transfer rates and a cycle life way above any simple lead-acid battery.

They’re also one of the most thermally stable in the Li-ion group of batteries, although on very rare occasions the electrolyte can become unstable at extreme temperatures or if punctured, leading to thermal meltdown.

A Torqueedo 40kWh lithium battery

Electric cars are at the forefront of marine battery development – the Torqueedo 40kWh uses the same technology as a BMW I3

LFPs have two positive and two negative electrodes, separated by a liquid chemical electrolyte such as ethylene carbonate or diethyl carbonate.

Their lower nominal cell voltage (3.2V) reduces the energy density below that of LCOs and they also suffer from slightly higher self-discharge, requiring a sophisticated BMS to monitor and adjust the voltage of the individual cells.

When considering upgrading to Lithium it’s important to understand their limitations in order to create a safe energy store that will last a long time.

First and foremost, LFP cells don’t like being overcharged, preferring to remain around 90% charged most of the time.

When cycled regularly, charging closer to 100 percent does no harm, provided you stop charging at that point.

Continues below…

A Li-ion battery doesn’t like being float charged – it makes them overheat.

Most LFPs will accept at least half of their own capacity (0.5C) in charge current, ie. a 100Ah battery can easily take a constant 50A charge, and often up to 1C (depending on the make).

This results in very rapid recharging, but without proper regulation you can melt your alternator’s internal and external wiring and roast the batteries. It also makes good sense to increase the size of your alternator and cabling, and to improve its drive mechanism by incorporating heavy-duty belts and pulleys.

Lithium-ion Polymer (LiPo)

In these batteries, the electrolyte is a gel rather than liquid chemical and are the next step on.

Even more robust and stable, they provide better temperature tolerance than LFPs, although they offer lower energy density and a shorter lifespan.

They are also very costly and only being made for smaller items such as hearing aids and smart phones.

Lithium Titanate (LTO)

These are being tested in some electric vehicles (EV) as they exhibit a high-safety factor and are able to accept charge very rapidly.

Duncan Kent

Duncan Kent is a technical writer for marine publications and websites

Instead of the more common carbon, LTOs use lithium-titanate nanocrystals on the surface of its anode, that massively increase the useable surface area (rather like Lead Crystal LA batteries).

This unusual structure allows very high charge and discharge currents in the region of 5C-10C.

However, though some LTO cells can provide an extremely long cycle life of between 5,000-20,000 cycles (depending on DoD and temperature), they have a lower energy density than LFPs and other Lithium-ion batteries, and a lower discharge efficiency so their theoretical capacities don’t always ring true and are not really ideal for the typical marine domestic bank usage.

Li-manganese (LMO)

Current used in state-of-the-art Electric Vehicle (EV) technology, it might soon be available in marine energy storage systems.

LMO batteries are blended with lithium nickel manganese cobalt oxide to improve their specific energy and to prolong their life span.

This combination brings out the best in each system and is already used in the majority of EVs, including the Nissan Leaf, Chevy Volt and BMW i3.