Thanks to an ongoing tangible reduction in prices, the life cycle costs of lithium boat batteries are now close to those of conventional options. Upgrading an existing boat to the newer technology is therefore increasingly feasible, writes Rupert Holmes
Larger new cruising yachts, especially those at the quality end of the market, have been routinely fitted with lithium boat batteries for the past few years. Arcona, for instance, says up to 90% of their larger yachts now leave the factory equipped with them. Equally, the technology is increasingly embraced in the racing world, whether at the top end on IMOCA 60s and Fast 40s, or smaller IRC yachts competing in RORC’s offshore races.
Benefits include a huge reduction in physical size and weight, along with a radically increased number of charge-discharge cycles. Typically the best lithium boat batteries will withstand four or five times the number of cycles compared to most deep cycling lead acid batteries.
This is a significant factor in reducing their long-term costs, although it has to be remembered savings will often not be realised until five years after installation, when conventional boat batteries may be nearing the end of their lifespan. As prices of lithium ion relative to capacity continue to fall, total life cycle costs are likely to drop below those of lead acid batteries.
However, a lithium boat battery is not a straightforward drop-in replacement for lead-acid batteries. Instead, a comprehensive and unified upgrade of boat battery management systems and regulation for all charging sources is needed to eliminate the possibility of thermal runaway creating a self-sustaining fire.
Despite the improving economic case for lithium boat batteries, this is rarely the prime driving force behind owners’ motivation. Quite simply they are better suited to today’s increasingly complex and power-hungry yachts. Lithium boat batteries can even make it possible to run air conditioning through the night without resorting to the disruption of a generator.
Equally, a lithium-based system may be able to store enough power to make it feasible to change from gas to electric induction cooking, and from a petrol tender to an electrically powered one.
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Each of these changes have safety benefits in that they get rid of other sources of potential fire or explosion. However, there is so much power concentrated in a lithium boat battery that its chemistry is more lively than that of conventional batteries, with a potential thermal runaway situation able to create a self-sustaining fire that’s impossible to control.
Granted, this is an extraordinarily unlikely event, but boats have been lost as a result and multiple safeguards are essential.
A properly designed installation will have three comprehensive lines of defence against thermal runaway. At the heart of the system the individual cells of each battery have electronics to keep the state of charge balanced across adjacent cells and to trigger isolation of the cell in the event of an over-temperature situation.
At the next level up each battery module also has a more sophisticated embedded battery management system that can also shut the unit down. Both the cell level and battery level monitoring are, in theory, backstop solutions that should never need to be called upon.
Each battery module also communicates with the boat’s overall battery management system, which is the first line of defence. This overarching system should warn the user if they are approaching operational limits, and will automatically decrease charge or discharge rates to reduce load on the battery. If necessary it will also isolate specific battery modules.
Both rapid discharge and rapid charge can cause problems, so all elements of the charging system must be part of the battery management system. Standard alternator regulators, for instance, are designed to charge at 14.4V (or 28.8V for 24V marine systems), but a nominal 12V lithium ion phosphate battery will only reach 12.8V when fully charged – they are formed of four 3.2V cells, rather than six 2.2V cells for a standard lead-acid battery. A standard charging regime would therefore overcharge the lithium units, risking overheating and potentially shortening their lifespan.
That’s why a system-wide approach, encompassing power management, charging and monitoring must be adopted. All elements have to be designed from the outset to be compatible with lithium power storage and sized to match each installation.
Hardware bought from reputable suppliers will satisfy all these conditions. However, it’s as well to be aware that there are also very low cost, unbranded cells available for online purchase that have no embedded safety or management systems.
At the moment the use of reputable and experienced suppliers is all the more important, given the marine industry is still on a learning curve with this technology and specific standards for lithium ion battery installations have yet to be developed for marine use.
Specifying lithium boat batteries
To prevent damage from storing batteries at a full state of charge, many manufacturers only allow a maximum charge of 90%. Equally, many don’t allow discharge below 10-20% of total capacity, partly reflecting the self-discharge rates of a battery in storage, which can reach 3% per month. In practice, it’s therefore worth banking on being able to use only 70% of the rated capacity of a lithium ion battery bank.
This of course is far better than conventional lead acid alternatives, where in real-world use only 30% of rated capacity is available, but not by as large a margin as some vendors of lithium boat batteries might suggest.
In terms of battery chemistry we’re most likely to see lithium-ion phosphate (LiFePO4) batteries, a chemistry that was discovered in the late 1990s, advertised for service or start batteries. These are excellent for this purpose and are generally well priced.
However, the automotive industry uses lithium nickel manganese cobalt chemistry, which has a better response to fast rates of charge and discharge, which also makes it a better for choice for electric propulsion on the water.
With Torqeedo alone having supplied more than 100,000 electric drives for marine use, including huge numbers of electric outboards, LiMNC batteries represent the bulk of lithium boat batteries used afloat. Storing LiMNC batteries in a fully charged state reduces their lifespan, so charging regimes will need to reflect this. With the motor industry continuing to drive down costs of LiMNC batteries, we’re likely to see wider use of this chemistry in the future.
Temperature ratings for batteries are also important. While automotive batteries have both heating and cooling systems (usually run off the vehicle’s AC system) to keep cells in the 0-45˚C range, this is not the case for other battery types. Even in temperate climates engine room temperatures can easily top 60˚C in summer; while sub zero winter temperatures could leave a battery unable to start an engine.
Charge and discharge rates
Whatever the chemistry, it’s important to match batteries to both charge and discharge regimes. If you need batteries to charge quickly via a large alternator they must have a high charge acceptance rate.
Equally, if running large loads such as overnight air conditioning, check the specified discharge rates of the battery match or exceed the maximum power drawn by your aircon system at start up.
When updating an existing electrical installation don’t fall into the trap of replacing like with like in terms of available capacity. Part of the advantage of changing to lithium boat batteries is being able to run a wider range of power hungry appliances, so consider additional equipment you may want to add over the next few years, such as coffee machines, induction cookers, freezers, watermakers and dive compressors.
A bigger battery bank also enables better use to be made of opportunistic ways to charge batteries, such as when motoring in a calm, which reduces the number of occasions on which the main engine or genset has to be run for battery charging.
At METS last year Ken Whittamore, managing director of Triskel Marine, which developed the ultra-efficient Integrel charging system, told an audience that 20 years ago a typical cruising yacht used around 1kWh of electricity per day. Today, his Hallberg-Rassy 42, which is equipped with electric cooking, water heating and tender outboard, uses 5-6kWh daily.
A similar size boat with air conditioning could double these numbers, while values rise rapidly for larger yachts – 50-60ft multihulls running air-conditioning can use 20-30kWh per day. To put these figures in context, an average size north European home uses around 10-12kWh daily, averaged over a year.
It’s a testament to the efficiency and variety of charging systems now available that such large power usage can often be achieved without recourse to a generator. This in turn makes cruising yachts more resilient in terms of reliability, and their ability to be self-sufficient for longer without needing to refuel.
Over the past few years the overall trend has been to increase the energy density, and therefore capacity, of batteries without a corresponding increase in their physical size or cost. Historically the general trend has been for the available technology to advance at about 5-7% per year – a trend that Torqeedo says it expects to continue.
“Beyond that, it’s hard to say when step changes will come,” says vice-president of program management, Thomas Wiedemann. “However solid-state and other new lithium technologies are still very new and very, very expensive, with significant challenges for widespread adoption.”
He predicts short-term gains over the next two to four years may come in optimising how we charge lithium boat batteries, including more efficient and cost effective forms of charging while under way. Solar power, for instance, continues to fall in price, while a greater range of companies now offers panels in custom sizes and shapes that can be tailored to neatly fit the available deck or coachroof space.
Looking further ahead, the automotive industry is putting a huge effort into extending the total lifespan of batteries. To date, car manufacturers have generally guaranteed the batteries of their electric vehicles for 200,000km (125,000 miles), or eight years. However a recent report in the Economist revealed three companies – one in China and two in North America – that are expected to launch a production battery that will be good for one million miles. That’s a figure that’s necessary for electrically powered buses and trucks, but such technology would also render a yacht’s batteries as fit-and-forget items that have the potential to last as long as the vessel itself.
The case for lead acid batteries
It’s still too soon to automatically discount lead acid batteries. While they’re less suitable for high-demand applications, there are still many boats for which they are appropriate – and this technology has also been advancing over recent years.
They also avoid the big humanitarian and environmental issues currently associated with mining cobalt and, unlike lithium ion batteries, recycling is easy and cost effective.
First published in the October 2020 issue of Yachting World.