Trouble Shooting for 2006 Prius Red Triangle Warning Light

Need any advice you might be able to give to troubleshoot a Red Triangle light / check engine warning - I have a 2006 Prius 180,000 miles. I have owned it for 3 years. Two weeks ago I left an interior light on overnight. The next morning when I drove the car to work with no problems. A day later I left on a trip for 5 days and the Prius sat in the garage without being driven. When I get back I drive the car to work with no problems and then on the drive home the red triangle light and check engine light comes on. I took it to the dealer and they tell me the battery pack has 3 bad cells and needs to be replaced for $3200. I felt a little uncomfortable about the high pressured pitch I was getting and I decided not to do the repair (they came down to $2800 and when I said no they tried to pitch me a refurbished battery for $2100- the sales guy admitted he only makes $ off the labor end of the job!) So the dealer also refused to give me the error code of the warning light so I took it to an auto parts store and the code came up as POAO8 - replace battery pack. The interesting thing is that when I drove the car home the warning lights did not come back on.

Is it possible that the 12v battery could be the cause of the problem because I left a light on overnight and not the hv battery?

Is the car safe to drive now that the red triangle light is off?

Any comments or advice would be greatly appreciated.

 

Great info- thank you. Just curious- what would make the red triangle light come on and then go off ? Does the battery recover during driving and make this happen?

 

So I had to replace the HV battery- I opted to use a refurbished battery from greentec auto. The mechanic came to my driveway yesterday and replaced the battery. This morning 5 miles from my house the red triangle light comes on again. I get to work and the car will not start. The engine would not come on, but I had power to the car. I called greentec and they said it could be the inverter that is bad. What a nightmare! I am going to try to get the code(s) from the latest problem, but does the inverter sound like the culprit now? Are there any possible other issues that could be happening ? The horror...