NTSB cites bypassed battery management system in $5m yacht fire

The probable cause of a fire that destroyed the 82-foot (25-metre) yacht Flagship in Miami was the thermal runaway and explosion of its 24-volt lithium-ion battery bank, the US National Transportation Safety Board (NTSB) has determined.

The agency found that the vessel's inoperable battery management systems (BMSs) led to a practice of manually charging the batteries, which compromised their safe monitoring.

The fire occurred on April 28, 2024, while the uncrewed yacht was docked at the RMK Merrill Stevens Shipyard on the Miami River. A security camera recorded an explosion originating from the hatch leading to the 24-volt battery bank at 10:31. Shoreside firefighters moved the vessel to a sea wall where they extinguished the fire, but the yacht eventually sank. The vessel was declared a total loss, valued at $5 million.

According to the NTSB report, the vessel had experienced problems with its battery systems since its construction in Turkey. A survey in December 2023 noted that the BMSs for the three 48-volt battery banks were not fully functional and the batteries were fully discharged.

To charge the batteries, the vessel manufacturer, shipyard personnel, an electrician, and the owner's representative developed a procedure to bypass the BMS of each battery bank with an electrical wire. This allowed for manual charging with portable chargers but meant the charge level was no longer automatically maintained.

The NTSB concluded that this practice could have resulted in overcharging or deep discharging, causing a battery cell to become unstable and eventually leading to a thermal runaway and explosion.