Sonar buoys are equipped with lithium batteries as energy storage. The battery cell used is from the manufacturer SAFT, utilizing Li-SO2 chemistry. Lithium batteries have previously shown a tendency to catch fire through thermal runaway. During thermal runaway of lithium batteries, large amounts of fire gases are generated which can be dangerous to humans. The gases may however vary depending on the battery cell and chemistry. The battery cells underwent experiments with Accelerated Rate Calorimetry (ARC) where the thermal pattern of the cell was investigated. It is clear from the results that thermal runaway occurs at a temperature of 150 ° C. Further, thermal runaway was initiated through needle penetration in a setup where the fire gases could be measured. Qualitative and quantitative analysis of the fire gases was carried out as a basis for a toxicological assessment of the fire gases. The toxicological assessment showed that the amount of sulphur dioxide, acetonitrile and formaldehyde that is formed can give rise to severe irreversible damage to humans. This is based on the assumption on an enclosed space without ventilation of 100 m3 and a 10 min exposure. These results should be considered when planning storage and handling of the buoys, especially on board vessels using these. It is of further interest to investigate whether the thermal runaway can spread to nearby battery cells within the same battery and how the battery chemistry can affect the toxicological effect.