Aentron details the risks associated with lithium-ion battery fires on large vessels and briefly outlines an innovative lithium-ion battery fire suppression system for maritime vessels.
Of all the threats that universally scare ship captains the most, sinking is probably at the top of the list. But fire is no less of a threat. Indeed, a fire, even if you manage to put it out, can easily lead to a loss of life from either the flames or the fumes. When it comes to preventing a fire on-board it is critical for ship builders to ensure that the latest and best fire-fighting equipment is made available relative to the risk and size of the vessel. This is a sensible strategy that applies to all conventional vessels with combustion engines. Unfortunately, it does not apply to lithium-ion-based battery propulsion systems and has generally led to a justified reluctance within the maritime industry to adopt the technology for fear of a lithium-ion battery fire breaking out.
If one analyses the key factors driving the current boom in the adoption of electric boat technology on inland waterways and lakes, one can draw a clear correlation between legislative pressures, e.g. no new boat licences for combustion engines, whereas cost and ecological factors play only a secondary role in many boat owners’ considerations. Hence, it seems that through legislative means hearts and minds can be both changed as well as motivated to implement greener shipping technologies.
The lithium-ion battery will certainly play a significant role in the transition to greener inshore and offshore shipping technologies. However, this article will address the perceived risks associated with lithium-ion batteries and how new technologies and solutions are emerging to reduce and for the most part eliminate the potential risks associated with lithium-ion batteries.
In recent years lithium-ion battery technology has made rapid inroads as the technology of choice for land-based mobility applications, i.e. in the form of full-electric or hybrid drive-train. In the maritime sector and especially for large vessels, the potential offered by full-electric or hybrid application technologies has only recently been recognised. In Europe, national and regional authorities have legislated mainly for reasons of water quality, air pollution and noise pollution. This has led to the practical extinction of the combustion engine in many waterways.
This, of course, has spurred both the availability of and innovation in electric power-train technologies. Unfortunately, the same cannot be said for both offshore and inshore waters, where the ‘business as usual’ mentality reigns. Only in very limited cases such as in harbour areas are there legislative activities to reduce local emissions. For large offshore operating vessels liquefied petroleum gas (LPG) will more than likely play the major role in emissions reduction. However, for estuaries, harbours and passenger ferries, as well as water taxis, lithium-ion battery-powered vessels will enable cleaner and quieter vessels.
Why switch to lithium-ion battery technology?
For small watercraft (2.5m to 24m), lithium-ion technology is replacing combustion engines and pushing aside the hereto ubiquitous lead acid battery technology for electrically powered boats. The main attractions of the technology are the significant weight savings of up to 75% less than conventional lead acid, fast charge capability and near-zero maintenance potential. Consequently, lithium-ion batteries are rapidly making headway into recreational boats. It is not an exaggeration that many of the batteries, if properly designed, managed and protected, will outlast the boat they are installed in.
Nevertheless, new technologies or technologies expanding into new applications often bring just as many risks as advantages. Lithium-ion battery technology has had a bumpy history within the maritime sector. The early adapters were sailboat racers or private individuals experimenting with the technology. This often led to disastrous and, on occasion, fatal consequences. Often the technology was not ready, and battery suppliers did not understand the application’s needs and risks. However, risk managers as well as ship architects should be aware of their primary downside, i.e. fire/explosion risk.
Therefore, successful integration and, later, safe operation will depend on taking the proper precautions as much as setting the right design requirements to a lithium-ion supplier to minimise the fire/explosion risk. Lithium-ion batteries must be considered as a system and not as batteries in the traditional sense. Today, lithium-ion batteries, when supplied from a reputable supplier, are equipped with cells, sensors, and battery management electronics that would equal a laptop or mobile phone.
In the case of large vessels, the technology has only been utilised in very limited examples. Part of the slow uptake of the technology for larger vessels is the industry’s inertia in adopting new technologies, the perceived greater risk associated with integrating lithium-ion technologies, and the up until recently low priority to legislate greener technologies within the shipping industry. It should be added that this is changing due to recent initiatives from the European Commission as part of the LeaderSHIP programme.
Regardless of the propulsion type, hybrid or full electric, lithium-ion battery systems are getting larger and more powerful. Recently, aentron shipped a 600Vdc/960kWh/1,600Ah system with a total weight close to 4,500kg. Such large battery systems require special measures, and ship certification becomes challenging due to the scale and risks associated with such large systems. To reduce the associated risk of large lithium-ion battery systems and reduce certification costs, a specialised lithium-ion fire suppression system is urgently required by the industry. The currently available fire suppression systems are not up to the task and in many cases would worsen the situation if activated.
A burning issue that is hard to suppress
Large inshore or offshore vessels are measured to a different yardstick, where operational reliability and safety take precedence. As already elaborated at the beginning of this article, regardless of vessel size or class, one of the worst scenarios besides sinking that can befall a vessel is an on-board fire. A lithium-ion battery fire is one of the most dangerous and difficult fires to get under control and extinguish.
For the most part, a lithium-ion battery fire can at best be cooled, contained and suppressed. Extinguishing a lithium-ion battery fire with 100% certainty is not always possible due to the unpleasant issue of thermal runaway. Lithium-ion battery fires do not require oxygen to burn and can be considered by nature a chemical fire. As with any chemical fire, extinguishing by conventional means, e.g. water, can often exacerbate the situation to such a point that an explosion cannot be excluded.
The intensity and rapidity of a lithium-ion battery fire can vary depending on the lithium-ion cell chemistry. The two most common lithium-ion chemistries in usage both on and offshore are lithium nickel manganese cobalt (Li-NMC) and lithium iron phosphate (LiFePO4) chemistries. Both chemistries have their advantages and disadvantages. The LiFePO4 chemistry is ideally suited for high-temperature operations above 40°C, having a more robust chemistry to withstand high-temperature operation. Li-NMC is, meanwhile, far superior in both power and energy densities and has in recent years largely replaced LiFePO4 for many applications. Both chemistries’ thermal runaway temperatures are, relatively speaking, closely aligned at 180-220°C and 270°C for Li-NMC and LiFePO4, respectively.
Building better batteries is often not enough
While the causes of a lithium-ion battery fire are varied, the often-cited reasons can be summarised as follows:
- Flawed electrical and mechanical design;
- Production quality issues with the cells;
- Faulty or no battery monitoring; and
- Defective charging process.
Lithium-ion battery fires as a result of flawed electrical and mechanical design are often derived from not designing battery systems to a standard that meets the application requirements. Many reported lithium-ion fires can be attributed to substandard workmanship in integration. Admittedly, this has become less of an issue in recent years due to the availability of high-quality lithium-ion battery solutions at an accessible cost.
A further example of a potential lithium-ion battery fire source comes from vibration and shock damage from rough seas as vessels hulls can be slammed with waves of high G-force at frequencies of ten seconds over hours. Over extended periods the shock and vibration effects on plastic-encased batteries can lead to significant internal damage, which, as a consequence, leads to short-circuiting and battery fires. Such failure types are tackled by encasing batteries in aluminium or steel housing. They provide structural protection and, in the worst-case scenario, slow or contain a lithium-ion fire.
Lithium-ion battery fires through flawed production quality issues of the cells are extremely rare and often go unreported as the evidence is usually destroyed with the resulting fire. However, substandard cell manufacturing quality is estimated by industry experts to cause at least
2-5% of all lithium-ion battery fires. This risk can only be reduced by sourcing cells from proven cell manufacturers as well as picking the right cell technology such as cylindrical and prismatic cells, while avoiding the polymer cell technologies which are often not sufficiently robust enough to survive the extremes of the open water.
Often a source of lithium-ion battery fires comes from the battery management unit itself. This argument is based on the over-reliance on battery management electronics to prevent fire events. Installing a lithium-ion system without a battery management system (BMS) is still common and can lead to fatal consequences. The most common cause of a lithium fire besides charging-related fires is designing the battery safety concept exclusively around the BMS.
The BMS will confirm a lithium-ion fire, but it will not be able to stop or contain it once it breaks out. A BMS’s primary responsibility is to measure voltage, current and temperature. It is designed to prevent a fire but can do nothing about a lithium-ion fire when voltage, current and temperature are not the causes of such calamity. Hence, a battery developer is still obliged to design a battery with the assumption that the battery does not have a BMS. Subsequently, a battery fire must be stopped through so-called ‘external-measures’, e.g. metal housings, non-flammable materials, fuses, or fire suppression systems. Many battery developers as well as battery system suppliers overlook these points to focus on weight and cost design goals, while putting too much faith in the BMS to provide the backbone of the batteries’ safety concept.
Without question the main culprit of most battery fires is the charging procedure. What is often overlooked by system designers it that batteries spend more than 90% of their lifetime attached to a charger in float mode and the rest of the time either turned off or providing the power for the application. Hence, the risk exposure during charging is massively underestimated and, consequently, the main source of most battery fires. The causes are varied but typically include charging too fast and using the wrong or an unauthorised charger. Overcharging often results in a process called thermal runaway, which is a reaction within the battery causing internal temperature and pressure to rise at a quicker rate than can be dissipated.
Once a cell or group of cells enters a thermal runaway state, enough heat is generated to cause adjacent cells to also go into thermal runaway. This produces a rapid fire that flares up as each cell in turn ruptures and releases its contents. The result is the release of flammable electrolytes. As already stated, an enormous issue is that these fires can’t be treated like ‘normal’ fires and require specific training, planning, storage, and extinguishing interventions.
But what if the fire is in the lithium-ion battery housing?
Many lithium-ion fires very quickly enter thermal runaway due to difficulty in accessing the fire-point. All batteries are usually encased in either plastic or metal housings, making fire-fighting extremely difficult or near impossible. By the time you could access the fire-point by conventional means of extinguishing, it is already too late and too dangerous to approach due to the possible explosion and jet flaming risk. Worse still, attempting to extinguish with conventional means, e.g. water, foam or CO2, is often a bad idea as it can feed additional accelerant to the fire, i.e. water can cause a flare-up and explosion, hence dooming the vessel to a flaming end. It is exactly this situation that is making certification of large vessel with larger battery systems such a challenge.
Lithium-ion fire suppression: detection, containment, suppression, cooling, isolation
Prevention is better than cure: if a battery fire can be stopped in its tracks early, then fire extinguishing becomes superfluous. Within its battery products from 12-900Vdc, aentron has responded to all of the abovementioned issues by developing a highly robust modular solution. However, we decided to go a few steps further in response to the industry’s need to not only prevent and detect a lithium-ion battery fire but also to contain and suppress it. Therefore, we developed and tested with the support of the Technische Hochschule Ingolstadt (THI) an automated lithium-ion fire suppression system that can extinguish a lithium-ion fire within less than ten seconds from detection. The aentron fire suppression system follows the following key steps:
- Detection – electrolyte sniffer and temperature and voltage outliners;
- Containment – containing the thermal runaway or fire event within sufficiently robust housing;
- Suppression – injection of a non-toxic and non-conductive foam that chokes the flames;
- Cooling – cooling effect of foam (specially patented solution) eliminates flare-ups; and
- Isolation – controller removal of affected battery unit.
The detection is implemented by an innovative electrolyte leakage sniffer that triggers a pyro-switch independent of the BMS and removes the external energy source. aentron discovered from tests with the THI that a lithium-ion battery fire should be tackled without opening the battery housing which should be made of steel. This design strategy enables both the containment of the initial fire and possible explosion. The explosion comes from the rapid build-up of explosive gas mixtures that, through short-circuit spark, explode to often devastating effect. aentron is able to prevent this event by using the foam to rapidly dissipate and eject the explosive gases from the housing.
An automatic fire extinguisher system is then activated only if the detection and containment steps fail. In its simplest form, if the battery temperature continues to rise beyond +150°C, the fire suppression system deploys. Where possible, after the cooling step has succeeded, the affected battery section should be removed and isolated for later disposal.
Summary
The recorded evidence relating to lithium-ion battery fire behaviour is limited or deliberately kept out of the public eye. However, we know from intentionally provoked overcharging tests that, regardless of chemistry, lithium-ion batteries quickly enter thermal runaway. The fire may be a progressive burn-off or one that is explosive in nature. Both types of thermal events, as well as their negative by-products (jetted shrapnel, molten metal, burning electrolytes, and other matter), can be managed and contained in the appropriate battery housings. However, the explosion and subsequent flame-out is the source of the fire that jumps to other combustibles in the vessel.
In most instances, lithium-ion battery fires cannot be treated like common fires. The burn characteristics and toxic by-products release components. Together with the THI, recent tests of aentron batteries have shown that it is possible to have a lithium-ion fire even within a large battery system, but you can still contain the situation if certain design aspects and an intelligent fire suppression system are taken into consideration.
The discussed fire suppression system is intended to ease ship certification and accelerate the adoption of large lithium-ion battery systems. However, like recent developments of smaller watercraft within inland waterways, leadership must come through smart legislation and targeted innovation support. All new technologies have bottlenecks and drawbacks that need to be overcome, i.e. lithium-ion battery fire risk. The already discussed lithium-ion fire suppression system is one of those solutions that quickly enables stalled technologies to rapidly dissipate, enabling revolutionary change. But to move one must first be pushed, and recent European initiatives as well as national legislation give cause for hope on the matter.
