Auxilia Electric Propulsion and the Safety in Lithium Battery Systems

Systems and tools are available which are fully capabl of handling these risks, but it is necessary to bette understand both these risks as well as the tools available so that they may be appropriately selected and implemented.

lt is important that the protection systems match th failure modes and consequences of a particular battery sys tem. Auxilia Electric Propulsion, since the first application i 2008, has analized the different possibility of energy storag technologies in the market and, through the years, supplied electric systems that required batteries 
In particular, the last 2 projects, they used lithium batterie with a special design in order to: 

• Decrease the dimensions

• Decrease the weight

• Maximize the life time

• Guarantee the best safety standard

In particular, this last point requires a particular attention be­cause, without it, ali other topics lose meaning. 
This article summarizes the safety aspects of Li-ion batteries containing liquid electrolyte with Nickel Manganese Cobalt Oxide (NMC) and Lithium Iran Phosphate (LFP) cathode chemistries. These batteries are the most common for mariti me applications at the publication time of this article. Battery technology is in rapid development, and new advancements might influence the presented statements. 
There are severa! failures that generate an increase of temper­ature; this can become a thermal runaway and this will gener­ate fire and gases. Gas and heat management. The off-gases in a lithium-ion battery is known to be flammable as well as toxic. This presents an explosion risk in enclosed spaces. Gas release is considered especially important for early stage detection of thermal runaway. Off-gas in the early stages of thermal runaway events wili be colder than off-gas release in the later stages. The early otf-gas can therefore become heavier !han the air, collect­ing al floor level. Il should lherefore be considered if gas-detec­tion relateci to room explosion risks should be applied at both levels, close to the floor and close to the ceiling. Solely relying on Lower Explosion Limit sensor(s) and celi voltage levels to detect early stages of a thermal runway event is insutficient. 
Both the Li-ion Tamer sensor and smoke detector, when placed close lo or inside the affected module, proved the most reliable means of pre-thermal runaway warning. The early detection of thermal runaway has also proven that a celi can be disconnect­ed, effectively stopping the overheating process. 
In arder lo realize the most potenlial of a forced extraction duci, a high extraction point in the room has proven lo be the key fac­tor. This ensures thai the required air changes per hour stays low while stili providing the necessary dilution of explosive gases in the space. Fire suppression. The core of a lithium-ion battery fire - the celi itself - is typically noi accessi bi e and extremely difficult lo extinguish, having elements of multiple types of fire (metallic, chemical, etc.) as weli as being exothermic and potentially pro­ducing its own oxygen. However, a single celi lire is typically not of significant concern with regard lo safety or survival of the ship. The prime concern is thai a battery is made up of tens of thou­sands of cells, and this fire will tend lo propagate to additional cells - thus increasing lhe heat load and increasing the likelihood thai it will propagate further, lo a worst case of having involved lhe enlire battery system. Thus, extinguishing the fire at the sin­gle cell level is not the focus of fire suppression systems. The key role of fire suppression systems is lo absorb heat and reduce the degree of propagation, or the number of batteries which will be involved in the fire. Direct injection of foam shows the besi heat mitigating performance compared with ali tested methods. This method had the highest potential for module-to-module fire mit­igation, especially when designed for sufficient capacity to flood the modules/racks over longer time periods. In cases where al­ternative ship integration concepts are lo be evaluated - such as a battery installed without a dedicateci battery room - this may be a particularly attractive approach to evaluate the equal level of safety. High pressure water misi protection provides good heat mitigation at module leve! in addition to providing full battery space protection from external fires. Il also has good gas absorp­tion and gas temperature reduction capabilities. 

In conclusion 

The required ventilation rate and the amount of lire suppression materiai depends on the number and the size of the battery cells involved in the fire. lf the complete battery system calches fire, the suppression and ventilation will not be able to mitigate the lire and explosion risks. 
There are many different battery system designs or engineering approaches thai may focus on miligating certain chalienges. In addition, there are many ditferent tools thai may be used for mit­igating certain risks. Understanding the risks of a given battery design is the key and ensuring sufficient systems are in piace to produce an acceptable level of risk.