2005 Prius not accelerating well/ Hybrid battery code pulled

I’ve posted a few times because my 2005 Prius is having big issues for the first time ever.
In feb, my mechanic pulled the hybrid battery needs replacing code.
I’d been away during a very cold week and the red triangle came up but the car still ran fine, same mpg of 47-50.
Car has 174k miles.
Mechanic cleared the code and said see how fast it comes back on. Could be a couple days or months.
Came back on a few weeks ago, then went off, now back on.
Other symptoms - when braking, there now seems to be a slight slip when slowing down, like when you hit a slippery patch.
- Car sounds like its ‘revving’ more when driving
-fan in rear seat has been blowing on high last two days.
- today on the way home from a 30 mile outing, the car was accelerating really slowly after a red light, despite me pushing the gas pedal. It was also ‘revving’ and I saw no arrows indicating the engine was charging the hybrid battery and nothing from braking regeneration going to the hybrid battery. Also no arrows from the H battery.
- got home, parked it. Turned off then on
Engine sent a charge to the H battery, like usual upon start up.
I’m now not trusting it to drive.

I’ve had the regular battery 12v, replaced maybe 2-3 years ago.
Just had cabin and hybrid filters changed out and a new muffler put on.

Am I screwed here?
I’m going to get it to my mechanic to test the 12 v battery and pull the codes but I’m not hopeful given the other symptoms.

Other than this, the car is mechanically pretty sound.

 

I’m the only owner. I bought it in August of 2005, brand new. It’s not worth it for me to put a new battery in it because I’d want a newer car before I’d get enough use from a new battery. If I thought I’d keep it another 5-7 years, then new battery, no question.
I’ve been looking at Primes, and I guess this accelerates my buying time frame. I was going to wait another 6 months.

 

I don’t think I want to go down that road, because it’s so iffy. I have a few road trips coming up and can’t take a chance of being stranded 300 miles away from home.
Any idea what I could reasonably expect to get for the car if I sell it? Disclosing the issue.
The rest of the car is well maintained. Pretty clean. ( no kids, no dogs )

 

The warning code the mechanic got from the car was the one saying replace the hybrid battery.
It’s around $3500 new, installed from a dealer.

 

I think I’m going for a new car, the Prime. Spoke to a dealer today and $1500 was mentioned which is probably the best I’d get for it. Kbb is up to 3200.

 

I guess I’ll know soon what exact codes are coming up once I get the word on diagnostics. :/

 

One cell went bad and the hybrid battery needs replacing. I opted to get a new battery because this car will keep going and when I sell it, I’ll get my battery money back.