ECU Replacement vs Battery Issues

Hello All Prius guru's, fangirls/boys and shade tree mechanics. 1st time poster here, 2nd time Prius owner. I recently picked up a 2005 with 170,000 miles. The hybrid battery has been tinkered with by the previous owner, who replaced 3 cells.

The car has been acting strangely lately however, and I opted to replace the 12v battery with a yellow top optima. This seemed to solve the starting issues. Unfortunately, after sitting overnight or for long periods during the day, the hybrid traction battery seems to be draining. When parked with a 1/2 or full charge it will start and show nearly drained. Recently, the RED TRIANGLE of DEATH along with the check engine light have come on. Checking the codes, I get P0A11F (HV ECU failure) and the dreaded P0A80 (Hybrid battery problems).

My questions to the forums are these: are these codes possibly related? Given that the car's hybrid battery has been recently serviced, would it be wise to replace the ECU and see if that alleviates my troubles? Any recommendations on the first step to investigate?

Many thanks in advance to anybody out there who has prior experiences with anything like this!