After 8 months of no-problems motoring in my 07 prius T4, I have had a dead battery 3 times in the last month. The AA and the toyota service technicians say the battery is charging fine and that there is no drain on it. From reading this forum, I am guessing that I may have, each time, inadvertently put the car in Accessory mode when I started it up, then driven it and not switched off Accessory mode, so draining the battery when I left it? How do I switch off Accessory mode? I assumed that this happened automatically but perhaps not. And - now I have mistreated my 12v battery so badly, will it ever be the same again? Or should I replace it? Thank you for reading and for your advice....
you can't be in accessory mode when you are driving or when you power down. replace your battery with an optima. you didn't do anything wrong, some of these batteries just don't last that long.
You can also add water to the 12v battery and connect a smart charger while parked 1-2 nights/month. That has been my approach.
I've only added water once but I put it on the smart charger about twice a month. I don't think it has quite the same draw-down resistance as a new battery would but it's been perfectly usable. I've never needed a jump-start.
Thanks everyone. I'll look into an optima (direct replacement?) or a smart charger - which smart charger do you use Tanglefoot?
The new Optima with "pencil" posts (you'll easily find it with a search) is a direct replacement. Using a smart charger without replacing the now half-dead battery will just leave you stranded somewhere sooner or later. It has long been noted that the typical dealer battery tester often does not have PASS/FAIL thresholds set appropriately for a Prius battery. As you have found they can test "good" but aren't.
