The US National Transportation Safety Board (NTSB) has said in a report that inoperable battery management systems was the probable cause of a fire on yacht Flagship last year.
What Happened
​On April 28, 2024, at 1031 local time, a fire started on board the uncrewed yacht Flagship while it was docked at an enclosed bay of a shipyard on the Miami River in Miami, Florida. Shoreside firefighters moved the vessel to a nearby sea wall, where they extinguished the fire. The vessel eventually sank at the sea wall. There were no injuries, and no pollution was reported. The Flagship was declared a total loss, valued at $5 million.
Investigation
The NTSB report said the yacht Flagship was docked with no personnel on board at a shipyard on the Miami River in Miami when an explosion and fire occurred. Shoreside firefighters moved the vessel to a nearby sea wall, where they extinguished the fire. The vessel eventually sank at the sea wall.
The video from the security camera at the shipyard showed an explosion at 1031 that originated from the hatch for the space below the floor where the 24-volt lithium-ion battery bank was located. The explosion expanded from the hatch and into the interior of the vessel. Following the explosion, smoke and fire were visible on the Flagship, and the fire continued to grow, eventually engulfing the entire vessel. Investigators later identified extensive damage to the aluminum deck and hull structure in the area around and above the location of the 24-volt lithium-ion battery bank. Based on the security video and the location of the most extensive fire damage, a lithium-ion battery cell within the 24-volt battery bank was most likely the initiating source of the explosion and fire.
Since the vessel’s arrival in the US, the BMSs for the battery banks of the 24-volt system and the three 48-volt battery systems were inoperable; they did not safely monitor and maintain the charge level of the batteries. This problem with the BMSs was not immediately resolved. In the interim, the vessel manufacturer, shipyard personnel, an electrician, and the vessel owner’s representative developed a new procedure to increase the charging speed of the lithium-ion batteries by bypassing the BMS of each battery bank. A wire was attached to bypass each BMS so the batteries could be manually charged at a faster pace by an external charger. As a result, the charge level within the vessel battery banks was not automatically maintained by the BMSs, and, due to an unidentified electrical drain within the vessel, the batteries became fully discharged (0% charge) on several occasions for extended periods. In addition, routinely charging the batteries using a portable battery charger could have resulted in overcharging, since the external battery charger was not connected to and monitored by a BMS and the battery charge level was instead monitored by personnel at the shipyard. Therefore, the practice of externally charging the lithium-ion batteries without resolving the BMS issues resulted in fluctuations in the level of charge on the battery bank.
Extensive fluctuation in the level of charge within the batteries may have caused one of the cells to become unstable, eventually resulting in thermal runaway. A thermal runaway occurs when a battery cell overheats and combusts; it is a chemical reaction that can occur to any type of battery cell if it is damaged, shorted, overheated, defective, deep discharged, or overcharged.
The heat produced from a thermal runaway of a lithium-ion battery cell can exceed 1,100°F, which can easily cause the adjoining cells of the same battery bank to ignite, as well as cause significant damage to aluminum structures. Lithium-ion battery cell explosions are typically caused by a thermal runaway. Therefore, the explosion on the Flagship was likely the result of one of the cells of the 24-volt battery bank undergoing a thermal runaway.
Probable Cause
The National Transportation Safety Board determines that the probable cause of the fire on the yacht Flagship was the thermal runaway and explosion of the 24-volt lithium-ion battery bank due to the inoperable battery management systems, resulting in the practice of manually charging the lithium-ion batteries with a portable battery charger, which compromised the safe monitoring of the vessel’s lithium-ion battery systems.
