Looking at the pictures of the (fresh) 787 battery unit, the wiring layout and execution doesn't appear to be to the standard you might expect of a safety-critical device. Also, there seems to be just one solitary thermocouple in the middle under a gob of resin.
Contrast that with
this, the battery pack for a self-launching glider. Each cell is individually (and redundantly) monitored for voltage and temperature, with the facility to automatically heat the batteries to keep them in the ideal operating range if the OAT is low.
This isn't ultra-new technology, I think the above aeroplane has been around for at least a decade...