P0A80 and sometimes P3020

It could be the battery, have it load tested at an auto parts store.
If it’s bad, replace it. If it’s good, check the connections by testing the voltage at the jump point under the hood.
If that voltage is the same as at the battery,
It’s all good.
Then you’ll need to find a mechanic with tech stream diagnostics software or go to a dealer.
The codes will direct you to the proper repair procedure in service manual .
It may very well be the hybrid battery block 10.
Your options then would be new $2-3,000.00 or refurbished $500.-2,000. Or you could replace block 10 and Rebalance the battery.

 

Where are you located country or state you need to have the 12 volt known to be good I have a car where my 12 volt has been 11.9 volts for the last 3 years and the car runs just fine this is the same car that I just replaced the HV battery in realize your HV battery is over 20 years old so you are past it's useful life and are on borrowed time that is a fact you can play games and fool around and do all kinds of things to extend this pain or you can get yourself a battery and get back on the road if that's what you need to do I played with this for a few minutes and realized very quickly but I wanted to drive my car not play with a bunch of battery modules just for fun there are guys online who will sell you complete racks of batteries for sales balanced it within three Delta in the 599 range and I'm going to tell you my daughter could change the hybrid battery in a Gen 2 Prius. She just doesn't have the gumption to do so because she has somebody who will ride her around everywhere and she's never driven a car in her life but she works on batteries and computers for a living go figure

 

Techstream is not absolutely necessary to diagnose your issue which is most likely a failed hybrid battery. There are scanners that are Prius literate. Whether your local auto parts store uses one or not is anybody's guess. Techstream is vastly more powerful though and is essential or nearly so for brake jobs, calibrating seat sensors, stuff like that. But a good bluetooth OBD II scanner with a phone app like OBD Fusion, Carista, Car Scanner, Torque, Dr. Prius, etc. will tell you what you need to know. Your battery is not actually over 20 years old, but it is over 17 years and that's gettin' up there.

A lot of people sweat by Greenbean battery. Roughly an equal number of people swear at them. If I planned to keep the car and wanted it to be reliable, I would either get a new battery or a near new one from a totaled Prius. The Gen 3 battery modules will work fine in your Gen 2. It's just a little time consuming to transplant them. But any hybrid shop worth their salt will have used and rebalanced hybrid batteries that they can install for you. If you like diy, it's doable with some help from YouTube. Not many tools needed, but you will need a torque wrench that measures inch/pounds.

But the 1st order of business is to get an accurate reading of the trouble codes.

 

That's rather low, but it should not cause any problems. Using the self test on the display is far from ideal, though. The resting voltage with the car turned off is likely a little higher than that. I've never heard of a low 12V causing a P0A80. It's good to start troubleshooting from a known good 12V because, if it's low enough, it can generate confusing trouble codes, but even 11V won't be low enough for that and you'll get other codes and symptoms before the P0A80 if the 12V is very low.

 

auto zone (and maybe advanced auto) can load test it for you free.
at that voltage, i would replace it, but it may not be absolutely necessary, depending on load test results.
they should also be able to read the codes for a bad hybrid battery at the very least.
as far as replacements go, new oem will give you another 10-20 years.
you can get them at any dealer, but they are competitively priced, so it's worth calling as many dealers as possible.
new modules are available for $1,600. from 'newpriusbatteries.com', but you have to install them yourself, or find a shoppe that will do it.

refurbished are always iffy, because they are made up of old battery cells that have been brought back to life to some extent, but to what extent depends on the quality of the work, which is an unknown.
the most important thing with a refurbishing company is that they have a physical address, even if they will come to you. the warranty they offer, and the strength and longevity of the company to back it up.

 

new prius batteries is owned by a member here. the cells are made in china, but i wouldn't use the term knock off. they are very high quality cells, made in a strict manufacturing environment to the owners specifications.
most users here have been quite pleased, and the owner interacts with them here. he has been a prius owner for many years before deciding to manufacture his own battery cells.
i don't have personal experience, and haven't searched the web for reviews.

 

This is an example of the trouble codes you will get when the 12 volt battery is low. Other possibilities exist as well. Using old-fashioned load tester not the new high-tech ones that check internal resistance of the 12-volt battery.