Hail Mary: How to Tell If Hybrid Battery Really Dead?

I have a 2005 Prius with 160K miles on it. A few days ago, I starting having difficulty, almost sluggish acceleration as if the car couldn't get into first car and only would drive naturally after slowly gaining enough speed to get into 2nd gear. Strange. Of course, the "Warning Exclamation" light went on a few hours later.

I immediately went to my repair guy to find out the diagnostic error codes. There were several, most notably, UO293 -- a communication error between the engine and HV battery, I'm told. Anyway, my repair guy was quick to say the hybrid battery is failing and to replace it. The Toyota dealer wanted $125 to do the same diagnostic but was also quick to say that it was probably the battery due to the mileage on the car.

The car is only worth $4,000-$4,500 with the hybrid battery working. And with new hybrid batteries running around $3K after installation, the Toyota dealer only offered $1,000 because he can't sell it (due to mileage) and would simply take it to auction. Basically, the car is not worth much if the hybrid battery is indeed dead.

Drove it several times today (even with warning light on) and eventually the battery meter went down to one bar and the car was basically revving with no power. 0-30 in about 20 seconds and then the engine could effectively move the car at near normal speeds. Ugly, sad.

Hail Mary Question: Does this sound like the hybrid battery is indeed dead or am I being misled by repair guys and the dealership?

Thanks for your help!

 

Appreciate the answer. 12V battery must be at least 5 years old, not certain. I'll read up on your links and give it a try. It could be the 12V?