The reported handset and charging cradle were returned for evaluation.Upon visual inspection it appears that a thermal event occurred on the handset.It is not possible to determine the exact sequence of events that led to the thermal event.However, there is evidence that a short occurred in the upper battery pack between the cells and printed circuit board.A visual inspection of the handset indicated blackening on the top of the upper battery pack and its printed circuit board.Cells 6 appears to have been involved in the thermal event, likely by supplying energy into a fault in the board.Additionally there was a slight melting of the handset plastic enclosure.Visual inspection of the upper pack printed circuit board indicated thermal damage involving charring on the printed circuit board materials, damage to the printed circuit board is apparent with blistering and exposed copper traces and a trace to trace short cannot be eliminated as root cause for this thermal event.The root cause for the fault in the board is unclear.Visual inspection of the upper pack cells with the blue shrink wrap removed, the cells were detached from the battery pack circuit board assembly and the voltages measured: cell 4: 177mv, cell 5: 176mv and cell 6: 0.039mv.Cells 6 appears to have been involved in the thermal event, likely by supplying energy into a fault in the board.Visual inspection of the upper pack cells on the opposite side indicated electrolyte leakage was apparent on the cells, the cells are swollen which can lead to failure.It is not possible to determine if the electrolyte leakage contributed to the thermal event, or if the thermal event caused the leakage.This concludes the investigation.
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