Acoustic ultrasound interrogation and deformation measurements have been used simultaneously as supplementary battery monitoring methods during external overheating and external short-circuit safety tests of LG INR-18650 MJ1 (NMC 811- G-SiOx) Li-ion cells. The short-circuit experiments showed that the MJ1 technology is protected against this type of thermal abuse by the current interruption device (CID) integrated in the positive terminal of the cell. The results indicate that the strain gage signal is able to provide very rapid alert for this type of battery safety breach due to an abrupt change of the cell pressure. It precedes the time of the increase of the skin temperature by an order of magnitude. The thermal stability experiments carried out in adiabatic rate calorimeter on completely charged and overcharged batteries at open circuit conditions, showed that the MJ1 technology is susceptible to self-heating by slow internal exothermic reactions starting above 60 °C. The subsequent process of thermal runaway starts when the temperature exceeds 140 C. The results from the extended monitoring of the cells during the thermal stability tests showed that the acoustic ultrasound interrogation data combined with data mapping and clustering of the signal provides advantageous indication for early detection of slowly approaching battery safety breach events.