Lithium-ion batteries have a long list of advantages over traditional batteries: they are lighter, hold their charge better, can handle more charge/discharge cycles and do not have charge-memory problems. But most of us are aware that in certain circumstances they can present a fire risk, from exploding hoverboards to self-igniting cell phones. Even our humble power banks cannot be stowed in the holds of planes. So how safe are Li-ion batteries aboard yachts?
Equipped with built-in protection and fail-safe management systems, the short answer is that the latest generation of Li-ion batteries for marine use are extremely safe. But battery fires have always been a potential hazard on boats and as the science of batteries has evolved, so has the nature of the threat they present. Thankfully, no casualties resulted from the fire that destroyed 40.88-metre motor yacht Kanga while she was at anchor off Dubrovnik on the morning of 7 September 2018, but the episode underlined the need for better understanding of Li-ion batteries to keep the hazards in check. 
Although the surfboards had in-built protections to guard against short circuits, over-charging, extreme temperatures and water ingress, the crew had complained about sea water ingress into three of the batteries and brownish-coloured water leaking from them. The safety investigation could not determine whether this was only sea water, sea water mixed with electrolyte, or the product of a reaction.
If moisture or water seeps into a Li-ion cell it can react with the Lithium salt in the electrolyte to release Hydrogen Fluoride. As the concentration of HF increases, the colour of the electrolyte could change from colourless to a yellowish-brown to a reddish-brown during storage. Furthermore, sea water can cause a short circuit because of its high conductance and corrosive properties. 
After the first fire alarm aboard Kanga was heard at approximately 08:20, it took less than 25 minutes for the fire to spread from the lower deck to the sun deck, accompanied by explosions. Attempts to fight the fire were abandoned and, after shutting off all the fuel valves, the captain gave the order to abandon ship.
Conclusions:
• The crew did not seem fully aware of the hazards associated with the Lithium-ion batteries, which resulted in an inadequate assessment of the risks involved with these batteries, even after three of the four batteries were found leaking.
• The garage space of Kanga was not considered a service space, within the meaning defined by the Commercial Yacht Code 2015, and therefore additional measures to prevent the spread of, as well as to extinguish the fire, were not deemed necessary to be provided in the garage.
• Unlike SOLAS II-2/5.2.1.1, which requires the main inlets and outlets of all ventilation systems to be capable of being closed from the outside of the spaces being ventilated, as well as the means of closing to be easily accessible, the Commercial Yacht Code 2015 placed this requirement only for ventilation ducts/fans of the machinery spaces and galleys.
• Only Section 4.2.8.1 of the Commercial Yacht Code 2015 required ventilators to be provided with permanently attached means of weathertight closure, which were to be easily accessible. The Code did not address the means of closure from the point of view of control of air supply, which could pose a potential for fire growth.
• The only means to detect a fire in the garage were through a photoelectric smoke detector and a fire patrol.
• There was no gas detector fitted in the garage, which could have provided an early warning of the situation in the garage; probably even before the activation of the fire alarm triggered by the smoke detector.
Recommendations
Following the investigation, Transport Malta has recommended:
Floating Life International S.A. to:
• Review the procedures on the frequency of and intervals between fire patrols onboard;
• Where applicable, consider additional means of surveillance of various spaces onboard, to ensure early warning of a fire;
The flag State Administration to:
• Either revise Information Notice 20 or issue a new Information Notice to address the potential hazards of Li-ion batteries;
• Review the Commercial Yacht Code 2015 to address the storage of Li-ion batteries and equipment powered by Li-ion batteries;
• Review the Commercial Yacht Code 2015 to address the means of closing ventilation systems onboard yachts for fire protection;
• Review the ‘Commercial Yacht Initial Inspection Report’ and the ‘Commercial Yacht Survey Report’ to take into account the equipment stored/intended to be stored in a garage space, and which may necessitate additional safety and fire-protection measures.