Hypothesis development— I noted that the exhaust fan expels hot air just fine. The exhaust channel & vent is adjacent to the battery compartment. It seems that if shutdown was an OS or BIOS related feature/function, then if over-temp is sensed (by computer sensor), then a controlled shutdown would be initiated. However, all indications suggest that what is happening is an uncontrolled shutdown (failure mode) by power cutoff shutdown, as if the power button were pushed and held. It occurs to me that such a failure must be due to a signal not from the system hardware but from battery self monitoring sensor and circuitry which is designed to prevent its own combustion ignition by signa ling the operating system or BIOS to remove all loads (in addition to clipped AC input cutoff)…a signal, in effect, to perform an instant power down as if pushing and holding its own power switch. Can readers confirm or refute that hypothesis.
BTW, no data loss or harm is done when this happens since data inputs are continuously shadowed and saved to an auto-recover file on a virtual D:\ drive (for Windows files) and likewise by Google (for tabbed Web pages). Google automatically queries as to “Restore pages?” upon re-connection. Windows queries similarly with the next successive start-up of the previously cut off Office application.