Lithium ion battery life

you can't. you can scan the batteries under load (while driving) with the correct toyota diagnostic equipment for a current health reading, but it is impossible to predict lifespan. just like you and me.

 

Note that existing regular Prii have nickel metal hydride batteries, not lithium ion.

 

good point, same problem.

 

Make sure you get your battery fan cleaned especially if you have pets...

Luscious Garage | Blog | Prius battery cooling fan cleaning, P0A82

It also depends on what type of climate you live in. I live in the Interior of British Columbia and our dealership which services a city with about hundred thousand people, including many Pri taxi drivers, they have only had two gen2 hybrid battery failures total. One was a high mileage taxi, and the other was from water damage. I would imagine this is a battery failure rate of well under one per cent over more than a decade! Keep your battery cool and dry and you shouldn't even have to be concerned Imo....

 

What state are you in Titusville?
CARB warranty is 10-yrs in some states.

 

Should I take my Prius 2006 to a Toyota dealer or private mechanic to be fixed? It has been idle in my garage for two years.

 

if you know a good hybrid mech, that's fine. otherwise, a dealer. you probably need a 12v battery, and then maybe a hybrid battery.