A Prius won't show any signs of life unless both the main & accessory batteries are in good working condition. The petrol motor apparently starts off the traction battery & the accessory battery is only used to run airconditioning, instrumentation etc. Furthermore, I've been in a few high-mileage Prius cabs and can verify that they all ran on battery power until road speed exceeded the normal 40kmh, at which speed the petrol engine started up. Toyota Australia advised there have only been three Prius battery replacements in Australia. My conclusion for what its worth is that the batteries don't get heavily discharged and that results in the extremely long life.