Open Letter to the eFoil Community

Dear eFoil Riders and Partners,

Last week, a SiFly eFoil battery caught fire. We believe it is important to address this openly and transparently.

We are deeply concerned that this happened. A fire is a frightening and dangerous event. We are relieved that no one was injured and that there appears to be no property damage beyond the battery itself. The situation was handled quickly and responsibly, and for that we are grateful. We are in direct contact with the affected customer to understand the full circumstances of the incident.

We want to be honest about what we know, and what we don't.

Based on what we know, the battery involved was missing its vent plug, a small but critical component that seals the battery enclosure and keeps water out. When the battery entered the water without this plug installed, water was able to enter the housing. This caused an internal short circuit, heat build-up, and ultimately a fire.

Because this component is central to what happened, we want to explain its role clearly. The vent plug seals the battery enclosure to keep it watertight during use. It also allows air pressure to equalize inside the housing during temperature changes, which protects the seals from mechanical stress over time. In the unlikely event of internal pressure build-up, the vent provides a controlled release path that reduces the risk of more dangerous heat build up. Pressure vents of this kind are standard components in marine and automotive battery systems. When the plug is properly installed, the enclosure is sealed and safe for water use. When it is missing, the housing is open to water and the battery cannot be used safely.

We do not yet know at which point the vent plug went missing, whether during packaging, shipping and customs handling or later. Our review is ongoing. What we can say is that an unsealed battery must never be placed in water.

We are introducing two immediate changes to our production and shipping process. A tamper-evident seal will be applied around every vent plug before shipment, so that any removal or disturbance of the component is immediately visible to the customer upon receipt. We are also adding detailed photo documentation of each battery prior to packaging, and we intend to make this accessible to customers so they can verify the condition in which their unit left our facility.

All SiFly batteries are assembled and tested in Bulgaria. Our production process includes automated safety checks, full pack diagnostics, and vacuum and pressure testing to confirm that each enclosure is correctly assembled and watertight. This testing is performed through the same opening where the vent plug is subsequently installed. After a final visual inspection, batteries are packaged and prepared for shipping.

SiFly batteries include a Battery Management System (BMS) that continuously monitors voltage, current, temperature, and short-circuit conditions. Riders can monitor battery status in real time through the SiFly mobile app, and BMS data can be uploaded automatically to our cloud server when a WiFi connection is configured. The battery enclosure is rated to IP68 for marine use and includes an internal humidity sensor. When the enclosure was flooded due to the missing vent plug, the internal monitoring systems were immediately affected. The battery had no way to issue a warning.

We encourage all SiFly riders to inspect their battery before and after every ride. Check that the vent plug is present and fully installed, and look for any signs of physical damage to the casing, connectors, or plug. If you suspect your battery may have been exposed to water internally, remove it from use immediately, place it in a well-ventilated area away from people and flammable materials, and contact us immediately. Do not attempt to charge or use it. If a battery begins to show signs of thermal reaction while out on the water, placing it in the water can help reduce temperature and slow cell discharge. This should be treated as a last-resort emergency measure, not a substitute for prevention. If you have any concern about your battery's condition, please reach out to us before your next ride.

Safety remains our highest priority, and we continuously work to improve the design, safety, and reliability of our products.

We are grateful for the trust you place in us, and we remain committed to transparency, safety, and continuous improvement.

The SiFly Team
06.03.2026