Aux Battery Dying.... or is it?

Hello,

We bought our first Prius last May, an 05 pkg 6 with 144,000 on it in excellent condition. (our mechanic says it's the cleanest most well maintained used car he's ever seen! we like that kind of news....)

Recently we have tried to start the car with no luck a couple of times, and I have discovered that the aux battery was either dead or very low. Charging it seemed to fix the issue, but at 7 years old it may be done. I think the possible death of the battery may have been quickened by the fact that occasionally our daughter falls asleep on the way home in the car and my wife sometimes sits in the car for an hour or more reading, etc, and since we always drive with the lights on, even though the car is off, possibly the headlights have stayed on that whole time. Also, living on a farm we have been leaving the keys in the car all the time until we started having this issue, as we wondered whether the car may have been left on in what I would call accessory mode at one time. For the time being, I pushed the key button below the wheel, requiring the key to be in the slot rather than just in the car, and we are working at being less sloppy with lights, etc as we get to know our little silver space buggy.

Any one have any thoughts on this situation, and also if I in fact need a new battery, I have heard smart key cars take a different battery than those without, but I see only one battery offered by the Optima folks?

Is there any wisdom to turning the smart key off? (if it's even an option)

Thank you!
Peter.