My Prius (2008) is into 7 years and 75000+ miles. The motive battery (the big one under the back seat) is doing fine. My local dealership reports they have customers with 150k and 200k who are still on the original battery and those test Okay.
My challenge is my Prius seems to kill the smaller 12 volt battery about annually. (the small 12 v battery is located in a rear fender well.) I used to be in love with the keyless entry/access feature, but have since turned it off since that feature represents a continuous battery drain and will leave the battery dead in about 28 days.
Don't get me wrong, I still love my Prius. I am spoiled by the high gas mileage. (I love telling my fellow commuters my car and my motorcycle both get the same gas mileage.) However, I found the cost, at least locally, of the Toyota 12 v replacement battery to be too high. Especially as the battery seems to fail every 12-15 months. I found folks online who were using a Mazda replacement as an alternative, and for about $35-50 cheaper. The Mazda battery lasts longer for me, but not a lot longer.
I bought my Prius used in 2010 and have replaced the 12 v battery three times already. The present 12 v battery is weak, I put the car on a trickle charger if i know i'm going to leave it parked for more than 5-6 days. However, that doesn't work when I leave the car at the airport long term parking.
Any ideas on what kills the 12 v battery? as I said, I have already disabled the keyless entry feature.....