HV Battery relays not engaging

Hi everyone,
Been reading this forum for a couple of years and have been successful at remedying any issues with the 2009 Prius...up until now. This one has me stumped.
We have two Prius' in the family, both 2009, both red (I realize color is not associated) my wife drives one and daughter the other. A few days ago my daughter's car got the red triangle ️ of death after it hadn't been driven in 6 days. We read the codes and there was a U0293 ecm communication fault. The auxiliary battery voltage was a little low at 12.4v. My wife purchased a new battery thinking it was the issue since she'd had issues with her car that were resolved with a new auxiliary battery.
After installing the new battery, it still had the red triangle ️; I disconnected everything including the negative terminal off of the body. Upon reinstalling everything, the car powered up normally. My daughter drove it to work and when she left to go home, another red triangle ️ and the U0293 code had returned... I was unable to get it going that night, but we returned the following morning and after cleaning and reinstalling the battery terminals, the car once again started normally, we drove it around town for the next 2 days with no issues. Our daughter once again drove it to work and parked it until lunch. When she attempted to leave work, another red triangle ️... My wife went to her work, unplugged the positive terminal and let it sit for a few minutes, plugged it back in and the car once again started normally. The following day, another triangle ️ and no start.
Upon inspection, I realized I wasn't hearing the HV traction battery relays engaging, so I pulled the cover off the HV battery to check it out; I also uncovered the one in my wife's for reference. What I discovered was the voltages going to the relays on my daughter's car were around 9.6v, where the same points on my wife's car were 13.8. I also noted the contacts on the relays in my daughter's car had some minor corrosion, where my wife's were bright and shiny copper.

I currently have the auxiliary battery fully charged, and I read 12.6 volts at both the auxiliary battery and at the jump connection. I am currently at a loss as to where to go next.

 

Quick update:

While typing the above post, I had the auxiliary battery on charge, though we swapped batteries between the two Prius' and my wife's ran as always. Immediately after charging, the auxiliary battery voltage was 13.0v (pretty normal after a fresh charge). I hooked everything back up and powered the car on; it came on with no dash display (mph, gears, etc), but it drove fine. I disconnected the battery and reconnected it, and upon powering on again I had the display again. Checking voltage at the HV relays, I have 13.8v with the ready light on.
When the red triangle ️ of death is on, I get only 9.6v at the low-voltage side of the HV relays.

Swapping batteries between the two cars, my wife's Prius does fine with either battery, but the problem persists with my daughter's.

I am reading the codes through the OBD II port using a Bluetooth adapter and Torque pro. I am unsure which ECU is giving the U0293 code; is there a way to determine which ECU is giving the code?

I also ran battery diagnostics using Dr Prius app; according to the application, the HV battery is still doing its job.