tewharaunz, interesting problem you've posed, with kiev providing a great start for your troubleshooting.
tewharaunz wrote:...Does anyone have any experience with their aux battery not charging in ready mode, but charging fine when plugged in?...
I never checked, but had always understood that the path from the dc-dc to the 12v battery was the same, irrespective of whether the car was in READY or charging.
tewharaunz wrote:..and when I turned it back on (ready), it was 12.9v.
Yes, a key clue that the dc-dc is not feeding that battery.
My own suspicion would be that LiFePO4 battery, as my first (crude) inclination would be to slap any old 12v lead-acid battery in there and see if the same problem occurs.
At that relatively low price, what is the Ah capacity of the 12v LiFePO4 battery? Could you provide a link to its specs, especially upper and lower voltage protection? What happens at the terminals when the input upper voltage limit is exceeded? Conversely, same question at the lower end - does a relay inside the 12v BMS open up the circuit? Also, is there any way you can check to see if the cells are balanced?
Four days of inactivity being enough to bring the voltage level down so the i-MiEV won't go into READY ... the next time that happens, could you measure that 12v, exactly? Be sure to check the calibration of your voltmeter.
One more thing - if I understand correctly, the car did not die on you on the road but just wouldn't go into READY when you tried to restart it. Is that right? Your commute must now be a nailbiter.
Wish you all the best and please do stay in touch to let us know of your progress.