triangle of death and check engine light

long story. 3 weeks ago i had to replace the inverter pump on my 2006 prius. while i was waiting for the part i took everything out and checked the voltage on all the cells in my battery pack. they all checked out at 7.7volts.
i also checked the 12 volt battery an it was at 12.6 volts. today i was 60 miles from home and the triangle of death and the check engine light came on. rather then pull over and shut the car off i decided to try to make it home. for the first 30 miles the screen showed the battery completely empty. after that it started to show the battery pack being charged and helping to propel the car. occaisionally it would show the battery empty but soon go back to showing charging and discharging. i tried to turn on the cruise control but it would not come on. (i think that is a clue but i don't know what it means). i made it all the way home and turned off the car and tried turning it off and on a few times and it worked fine. i disconnected the 12 volt battery and checked the voltage and it was at 12.7 volts. disconnecting the battery shut off the triangle of death and the check engine light. any ideas as to what could be going on?