2004 Prius, "READY" light, no start

Hi all,
I have the opportunity to get a free 2004 Prius from a friend of a friend that's a non-starter. When I went to look at it the 12v battery was flatter than roadkill so I hooked it up to jumper cables (positive in the fuse box and negative on strut nut behind it, hidden menu in MFD showed solid 13.8v with jumper cables connected) and the car boots up no problem, it comes up with "READY" on the dash and tries to start the engine to charge the HV battery, but it just makes occasional clunks for a while like it's trying to start and then eventually says "PROBLEM" on the MFD and shows a square red icon that has a battery on it and "MAIN" underneath (as well as the big fat red triangle on the combination meter, and the READY light shuts off). If I move the drive select lever before the error occurs, the car beeps at me and instructs me to put the car back into Park else it won't charge. I plugged in an OBD-II dongle (OBDLink LX) and scanned for codes with Torque Pro and it found no codes, either before or after the "PROBLEM" message appears. If I go into IG-OFF and then back into IG-ON, the READY light returns and the car tries the same cycle of trying to start before throwing an error. I didn't have custom PIDs setup beforehand so I wasn't able to pull the module voltages or hybrid battery SoC in the limited time I had. Would I be somewhat correct in guessing that the HV battery pack has been drained excessively and needs to be charged outside the car?

 

Resolution to this issue - engine was seized up solid. Replaced the engine with a low-mileage one from a local wrecking yard and all is well, hybrid system and battery are totally healthy.

 

It was actually easier than expected, I left the transaxle alone and just pulled the engine out the top, once you have the inverter removed and the wiring harness out of the way the engine slides out pretty neatly between the frame rail and transaxle (and saves the headache of removing the axles, dropping the engine out the bottom and trying to seperate and reattach the transaxle and engine outside of the car. I did have to remove the crank and water pump pulleys as they were clashing with the frame rail but other than that the engine came out as-is.