As it happens, we've just replaced the 12v battery on our Prius. It's on battery number 3 now. First battery went after less than a year, battery number two lasted 5 years or so. We've never done any conditioning charge or whatever in the 2.5 years we've owned it. Although after leaving the car for 2 weeks whilst on holiday, it seems it would help it to give the battery a boost before using the car, as it never seemed to like that much. However, at £90 for a new battery (which I think is fair for 5 years use), I'm not overly concerned with it. Just for the record, you'll know when your 12v is getting too low when the car starts doing strange things, like not going into Ready mode on the first press of the power button, or sections of the digital dash and HUD turning themselves off... Ours was down to 10v when I gave in and bought a new one. I'm amazed it even worked at all at that voltage really. You can also check it's state by pressing and holding INFO-TEL whilst switching the lights on and off three times. On one of the diagnostic screens that pop up, you can see the 12V battery state. Do that when not in Ready mode to get a true reading.