Only three HV batteries. Remember the owner lives and drives around Phoenix, heavy stop and go traffic & summer temps that are often in 117 range. Pavement temp can easily be 140, radiating up at the bottom of the car. 3 batteries in 600,000 miles is pretty remarkable.
If you get to one million miles do you think toyota will give you a new Prius? has anyone travelled one million miles on a prius?
Just to add my 2 cents, 2001, original HV battery. Currently at 120,000 on Sept/2017, 16 years on original battery going through humid summers and snowy winters in New England. I think it has more to do with charge/discharge cycles (milage) than simply time. Weather and how long each trip has been will effect it too, as it effects the temperature of the battery, which definitely will effect it's lifetime (as we obviously know with cell phone batteries)
I bought a parts car this summer for $400. New HV battery in 2013, new 12v battery this summer. 200,000+ miles on the little 02. I've repainted it, and done some other stuff to it, so I've got maybe $2500 in it. Desert cars last forever, but the HV can still be a terminal illness. It morphed from a "parts car" into a daily driver. Anyhow, I took it to Oregon and back in August, well over 3,000 miles. I love this little car! Kind of the ultimate little touring vehicle, good mileage, superior power, and very comfy. I'm keeping it as long a I can. Current mileage is 205k. I'm 65, and it may be the last car I ever own.
When you get to the mountains, it's remarkable how much power they have. They'll climb grades in cruise and not lose any speed. On 2 lanes, they have the get up and go to pass multiple cars in passing zones. Cruise all day at 75 and easily accelerate to 90. They may not have muscle car like power, but the way Toyota set them up, it's like the little 1.4 l engine is supercharged.
I posted earlier that my experience with 4 Gen 1 Prii has been expected life of HV Battery is about 8 years. Time is the limiting factor, not miles. 2001 Prius A needed HV in January 2009 at 197,000 miles. 2001 Prius B needed HV in September 2009 at 97,000 miles. Gen 1 had 10 years, 100,000 miles warranty on Hybrid System. Gen 2 (2004-2009) Hybrid warranty changed to 8 years, 100,00 miles.
My 03 has about 150k and the HV is original (as far as I know) and still going strong. I use the Prolong system to condition and balance the battery and according to Techstream, the cells are strong and balance. My 02 got a new factory battery transplant in lat 13, I don't even worry about it. Both are strong runners and very nice cars otherwise, too.