Dead Battery

Yes, thanks for catching my mistake. I was thinking along the lines of accessories, etc. But the 12V must also power all the various electronics that are always 'on', if even in a sleep mode. I have no idea what the SKS draws, but it's probably one of the more significant drains on the system.

There is not only the drain from those things that any car would have, but other items unique to the Prius, such as the coolant pump (though that's a one-time thing when you shut down). If I go into my garage when my Prius is just sitting, it's not unusual to hear it make various whirring and clicking sounds. One of these I understand to be the brake actuator, which usually can be caused to kick in if you step on the brake pedal with the car off. The manual mentions a pump in the back that checks for leakage from the batteries. And there are any number of other various noises which I'm at a loss to explain.

So yes, that tiny 12V battery has a lot to do while the car sits idle.

It would, IF the dashboard power outlet were live.

It's not live until the system is in Ready--and that requires the 12v battery...

Most common cause for finding a dead NHW20 Prius 12v accessory battery is from the dome light in the cargo area being left on, usually from the rear hatch not being closed properly.

Always lock your Prius either by the Smart system or the remote - the Prius will not lock the car if a door is left open, and the open door (usually the rear hatch) will cause the 12v battery to deplete faster than just the Smart system or the alarm system's parasitic drain...

Another comment for the OP... If this ever happens again, you don't need to "find and uncover" the battery. Toyota provides a jump start terminal near the fuse enclosure. ...Pat

It is simple to modify one of the two outlets so that it is always live and therefore can be used for a jump. Of course then it is also always available to drain the battery through an attached accessory.

Thanks. I remembered the front terminal after I did the jumper on the battery thing. It's worth a dry run though as I found that removing the cover on the front terminal took some fiddling.

TKM

Shame McGoofus can't turn off his Dumb Key Self...