The amount of battery electrical vehicles (BEVs) carried as cargo on ro-ro ships is increasing. The possibility of thermal runaway in a lithium-ion battery makes BEVs a different fire risk compared to internal combustion engine vehicles (ICEV). One of the challenges that arise is how to detect a thermal runaway early. Current detection systems in ro-ro spaces generally consist of smoke and/or heat detection. To identify potential techniques and challenges for detection of a thermal runaway, as early as possible, tests with batteries and detectors are needed. Tests with one battery cell were performed inside an ISO container (with almost negligible ventilation) as well as in an open room with moderate ventilation (14 air changes per hour). Point-type detectors (two smoke and heat detectors, one CO detector, and one LEL detector), thermal imaging, video analytics, and light detection and ranging (LIDAR) were evaluated in the tests. A total of 14 tests were conducted. The detectors were evaluated in different positions relative to the battery cell and comparative tests with wood-sticks were performed to investigate the detectors’ ability to detect a more conventional source of fire. Based on the results, it can be concluded that early detection of thermal runaway in batteries is possible in principle. However, detection is a matter of circumstances e.g., ventilation, gas/smoke production and the location of the detector(s). The result indicates that detection in a small and confined space is relatively manageable, but detection in a large and open space could be more of a challenge. If the gas/smoke is cooled down it may sink and spread along the floor/deck, instead of rising and spreading along the ceiling. This would be a challenge with current smoke detectors installed in the ceiling. Shielding may be a problem, especially with LIDAR and thermal imaging. Future research should address full-scale tests, and it is recommended to include Optical Gas Imaging (OGI) as a mean of detection.