Triangle/Check Engine/"Problem" - 2005 ECU replacement p0ac1

Hello everyone, I searched and searched but was unable to generate a hit with the code or find info on replacing the ECU...

Yesterday afternoon I had both the triangle and the check engine lights come on in my 2005 Prius (109k miles), in addition to the hybrid icon and "Problem" on the MFD. I took it in today and they gave me a code of P0AC1 with a diagnosis of needing a new battery ECU. They tested the current sensor and harnesses and it tested okay. The part needs to be shipped up from Chicago and that'll take a few days. They're quoting me $838, including the hour and a half labor.

Three weeks ago, after leaving the car sit for a day, I tried to start it and it was dead. So I jumped it and took it in for a 12v battery replacement since I was still on the original ($170 with labor.)

I also noticed that I posted a year ago that I had almost the same thing happen...with code P0AC0.