An unusual self-discharge of a field lithium-thionyl chloride battery submodule

A lithium thionyl chloride submodule in a Peace-keeper site in May 1992 was found to be bulging to a considerable extent. When an attempt was made to discharge the submodule in place, the battery voltage was about 6.1 V in two hours, vs. the expected 10.2 V. Thus, it appeared that at least one cell had experienced self-discharge. A subsequent discharge and dissection of the submodule conducted at NAVSURFWARCENDIV Crane showed that one cell in the submodule had indeed self-discharged, but no overt reason could be determined. A number of unusual observations did indicate however that the self-discharge had occurred very slowly, and was therefore a probable consequence of a soft short through a high resistance path. This paper discusses the discharge and dissection processes and the failure analysis findings, along with the unusual attributes of this particular failure