I have leased a 2009 Prius for 5 years and my lease will be up in Nov. The mileage is very low – approx. 25,000 miles. My payout is 12,300. I would like to keep the car but have been told by friends that eventually when the battery needs replacing it will be very expensive. Is the battery life a function of mileage or of age? I am retired and won't be driving more than 5000 a year. Do you have any advice for me?