Battery chemistry plays a vital role in electric maritime vessels. Charging infrastructure still lags behind the automotive industry, but the two sectors share the same desire for technologies that enable greater range and, more importantly in a marine environment, critical safety properties. Bram ter Meulen, founder of battery technology company Ionbase, believes lithium iron phosphate (LFP) is most suitable for maritime applications, primarily because of its high safety levels. “LFP is an inherently safer chemistry than other lithium chemistries such as nickel manganese cobalt (NMC) or nickel cobalt aluminum oxide (NCA),” he explains. In rare cases of cell short circuits, following events are less extensive than in the more popular NMC cells.
Lithium titanium oxide (LTO) is also considered a very secure chemistry. “LTO technology can be completely submerged in ocean water, so events such as a complete short circuit condition do not create unsafe conditions on board,” says Magnus Eriksson, founder and chief strategy officer of Echandia, which uses heavy-duty Toshiba cells in its battery systems. Resilient to external heat and general abuse and therefore helping to prevent thermal runaways, LTO does not form dendrites, unlike some other lithium-ion chemistries. Short circuits are therefore avoided and battery safety increased. It can also function safely at temperatures as low as -30°C.
LTO offers other benefits, too. Nicky Mayenburg, technical sales manager at Vuyk Engineering Rotterdam, says LTO was chosen for an electric ferry design project due to its fast-charging properties. “During our design process, we compare different battery chemistries, such as NMC, LFP and LTO. There is a focus on weight, price, charging times, lifecycle, safety and the allowable depth of discharge. LTO batteries were a good choice for our project, due to their fast-charging rates.”
However, the types of chemistry used for a vessel project are determined by many factors. “There is no best solution for all projects,” Mayenburg adds. High-energy-density, low-weight batteries are more suitable for marine solutions, but improvements with a larger depth of discharge range are needed to reduce the required installed battery capacity. “Ship/vessel design is based on the Archimedes principle, so less weight on board results in a more competitive and energy-efficient design,” he explains.
“Other aspects such as lifetime expectancy, how much energy storage capacity will be needed on board over a certain time period, reliability, total cost of ownership, even battery second life – everything goes into the equation,” Eriksson says, confirming that LTO is useful in more heavy-duty applications with tough operational cycles, characterized by many daily charging cycles and high-power throughput. Electric passenger ferries are
a good use case, their frequent departures necessitating fast recharging and long battery lifetime expectancies – LTO can recover up to 90% of its capacity after 20,000 charge/discharge cycles.
“Our battery system is designed to typically last for at least 12 years,” reports Eriksson. Disadvantages of LTO include low specific energy, high weight and cost, but its fast-charge capability, safety and long life outweigh the negatives aspects.
“We use cylindrical LFP cells because they have a very long cycle life (compared with NMC and NCA),” says ter Meulen. “The trade-off is a slightly lower energy density, but for marine this is almost never an issue as weight is often added for stability in the first place.”
Use cases and operational loads are equally important when it comes to more personalized vessel specifications and battery chemistry combinations, explains James Weller, head electrician at Spirit Yachts. “At Spirit, every yacht is customized to her owner, so we don’t have a standard specification. Each project starts with a conversation with the owner about how they will use and keep the yacht, and who will be on board. Once we have a wish list, we calculate the operational house loads of the yacht and the battery capacity required.”
Spirit then uses this information to form the basis of supplier discussions to determine a tailored package and specification, whether it be a fully electric – with no backup generator – or a hybrid-drive vessel. “We look at the whole yacht holistically, ensuring we have the correct balance of power consumption, management and regeneration,” explains Weller.
“The most important design input for an electric vessel is the operational profile,” Mayenburg says. “It is important to define accurately how long the vessel sails and at what speed, to define power consumption and required battery capacity for the operation.”
Charging intervals and times, loading and unloading cycles, and required lifetime influence total installed battery capacity. According to Eriksson, the need to squeeze more energy and active material into cells to increase energy density characterizes chemistry development and safety.
“Because of safety incidents on electric ferries, there’s an increased awareness about the fundamental safety aspects with the choice of cell chemistry,” Eriksson says. Battery systems with integrated firefighting systems are now more commonplace, with safety legislation more regularly revised. “The sector was first in setting up an adequate safety regulation in which the safety and functions of onboard lithium-ion battery systems have to be tested and verified.”
Marine battery testing standards are similar to those of other industries, particularly in thermal runaway propagation throughout the battery system. Specific regulations such as DNV-GL and Bureau Veritas certification guarantee component and system safety. Cooling methods and BMS software requirements also need to be proved.
“One part of the certification process is to validate safety functions, all the software and hardware involved in those functions and those embedded around the installations, to prevent any dangerous situations occurring,” explains Eriksson.
“Mandatory tests are the same as in other sectors except for automotive,” ter Meulen comments. “EMC testing and other product certification are also important. Additional DNV certification and different IEC standard tests can also be carried out.”
Firefighting solutions in particular are being constantly improved. “Previously, using seawater might have been recommended,” says Eriksson. “This was not necessarily a good solution because it is conductive and could make things worse. If ocean water is injected into a battery compartment, it can cause short circuits or speed up thermal runaway events that can lead to dangerous and explosive gases.”
Echandia’s batteries are completely air-cooled, creating additional security. Eriksson explains, “With LTO safety, if there’s a fire next to the lithium battery, our battery solutions can withstand over 300°C ambient temperature without going into thermal runaway; a lithium NMC battery, which is the most common, has a safe limit of around 85°C – a huge difference.”
Ter Meulen says that the use of smaller cylindrical cells means that if thermal runaway occurs in one cell, total energy released is also smaller. “The chance for a thermal runaway chain reaction to other cells is reduced.”
Dedicated battery room
Ensuring battery safety is a vital part of the ruggedization process for marine applications. “A dedicated battery room is needed in the ship, and inside that compartment gas monitoring ventilation and firefighting solutions need to be installed, with no other equipment allowed,” says Eriksson. “Specifically for LTO batteries, under normal operation no fresh air needs to be introduced, identical to an air-conditioned room,” comments Mayenburg.
Echandia’s specifications for an LTO battery string installation include a ventilation system that must support a flow at least 150m3/hr/string with a pressure drop no greater than 50Pa; recirculated air average temperature must not exceed +/-20°C; and relative humidity in a typical battery room must never exceed 85% RH. Improved requirements for water tightness in battery systems have also come into play, as there have been incidents where water has eaten into battery compartments in non-obvious manners.
Marine installations involve harsher environments than those on land, particularly with regard to salinity and ocean moisture. “Salt is one aspect that is considered now, before it enters into a battery system, to ensure there is no corrosion,” says Eriksson. Other considerations include substantially higher shock and vibration loads on battery systems, which need to be designed to withstand high levels of mechanical abuse.
Material choices are also of great importance, says ter Meulen: “It is very important to use corrosion-resistant materials such as anodized aluminum, stainless steel and silicon seals.” Choices of components inside the battery are vital, too, he continues: “Each of our 48-6500 batteries are built up of 352 cylindrical LFP cells (22p16s – 22 cells in parallel and 16 strings in series); other batteries generally consist of 16 (1p16s) or 32 (2p16s) prismatic cells. We have a large number of cylindrical cells in parallel to increase battery life and reliability.” Cylindrical cells are physically and chemically more stable due to their shape, says ter Meulen, and better for withstanding shocks and vibrations.
Other benefits of cylindrical cells include degradation. “Not all battery cells degrade at the same pace,” explains ter Meulen. “Battery energy capacity is limited to its weakest link (string with the lowest energy capacity). When a string consists of one or two cells and those cells degrade faster than the other strings, the entire battery is constrained by those one or two cells. It follows that, statistically, when only using 16 or 32 cells the probability of that happening is much higher compared with a battery with 22 cells in each string.
“Subsequently, if a string ‘dies’ (a battery cell/s stops functioning altogether), the entire battery is useless as a ‘chain link’ is broken. If a string consists of multiple cells in parallel, however, only a small portion of that string stops working. This cell can be isolated from the battery pack while the other cells continue working.”
Lithium chemistry batteries are said to be big investments, so battery enclosures are designed to protect them. “We have developed a custom 3.5mm-thick anodized aluminum enclosure, built to endure marine environments for years,” says ter Meulen. “It also acts as a heat sink, transferring battery heat to its surroundings, while protecting it from shocks and puncture.”
In thermal runaway cases, LFP cells ignite to temperatures of 400° and ter Meulen says that Ionbase’s 6063 T6-grade enclosure material has a solidus point of 616°C. “In addition, a metal ventilation device is placed in the enclosure for pressure equalization in the event of a cell vent,” he adds.
A modular battery system layout also has ruggedization advantages, and Eriksson says that all common battery types are modular in one way or another. “More importantly, it’s about how effects can be scaled in production, and how to minimize overhead cost centers,” the expert explains. “Modular systems can be the same product, multiplied to increase the energy capacity. While this type of standardization means that there is less variation in terms of quality, it often results in a more streamlined quality control function in production. This, in itself, improves ruggedization.”
This article was originally published in the April 2023 issue of Electric and Hybrid Marine Technology. To view the magazine in full, click here.