How to tell if I have a bad HV battery or HV battery computer?

My 2004 Prius with 148k miles (CA car, always driven in CA) threw a red triangle ! light and I noticed the fan at the back seat always blowing even when it's not hot inside the car. Sometimes the car drives normal, and sometimes it will run very underpowered. Disconnecting and reconnecting the aux battery cable will extinguish the red triangle light and restore engine/electric motor power for awhile. I managed to drive 40 miles home that way, and most of the way was freeway and it ran ok. The red triangle light was on but car still drove ok. I believe the rear seat fan was blowing constantly the entire drive home.

Is my HV battery defective for sure, or can it be the computer which is attached to the HV battery? I haven't scanned the codes yet.

For about a year, the red triangle light has been coming on intermittently, and all I do is disconnect and reconnect the aux battery to make it go out. Driving has been normal.

About two years ago, the dash would sometimes go dark. All lights out. After a few days it will work again.

 

The HV battery was replaced by dealer under warranty in 2011 at 20,000 odometer. So today that new HV battery has 128k miles and 8 years on it. But if it hadn’t been replaced are you suggesting that every single 2004 Prius on the road has had a HV battery replaced by now? When do most of them go bad - do 90^ of them die within 12 years?

 

I meant after how many years or miles do 90 percent of Gen 2 Prius HV batteries require replacement?

 

Ok I had the car scanned using AUTEL MaxiSys MS906 scanner.
Codes:
P0A80 - replace hybrid battery pack.
P3015 - battery velocity 5 weak.

Note the dash RTD, CEL and circle with brake pads symbols are lit.

I should inform you my dash sometimes goes dark. I’ve had the CEL light 6 months ago and the code said bad catalytic converters, I get the RTD light on sometimes after I wash my car or drive it in heavy rains but it goes away after the car dries up, but today the scan tool didn’t find the catalytic converter code. My MFD has stopped working properly after that solder joint came loose and the climate controls no longer show on the MFD. I’ve had to run my AC using the steering wheel buttons.

Assuming my main battery is defective and I wish to drive my car 5 more years, is the dealer $1650 new battery a better choice than the $1600 new aftermarket battery from newpriusbatteries.com?

I wish by now I can get a new battery for closer to $1200. Anyone?