The new software is supposed to monitor the cells for voltage variations much more closely to prevent a Bolt Fire. The idea is to proactively look for the conditions that could lead to a fire, avoid them, and get the battery replaced before it happens.
It also monitors the battery after the charge completes to ensure nothing bad happens. As per GM’s Service Information Tool, which has some information on the new software and DTC codes, this is added:
[…] If conditions are right, thermal monitoring may remain active after vehicle OFF or at the completion of plug-in charging for 10 minutes up to 12 hours. With vehicle OFF thermal monitoring, the high voltage contactors will be closed and the K1 14V power module will support the 12-volt battery. […]
The following will occur only if thermal runaway is detected – regardless of Vehicle ON/OFF state:
– The vehicle parking lights will flash and the horn will sound continuously until the 12-volt battery is depletedGM’s Service Information Documentation for the new P0BBD DTC Code
Many Bolt owners report that their charging station shows power still flowing to the car after the charge has completed for 4 hours. This matches what’s mentioned above about the 14V power module and contactors closed. It means that the car will use the wall power to run the 14V system and monitor the battery for up to 12 hours after charging has completed. According to the above, it will also run when unplugged.