Two days ago, I went downstairs to the garage to get something from the Prius' trunk. Went back upstairs.<br><br>Next day, I saw that my trunk was still open and that the little light at the top of the trunk was burning all night long.<br><br>I hesitantly got into the Prius and turned it on, hoping that the battery had not drained completely. To my surprise, the battery status was at it's optimally charged status and the Prius started without a hitch.<br><br>Shouldn't have the battery have discharged at least to half? The light was on for about 10 hours.

I suspect the trunk light is powered off of the 12-volt "auxilliary" battery, not the hybrid battery (which is what the battery status in the MFD displays). If you had totally drained your 12-volt battery, you wouldn't have been able to start the car (wouldn't be able to power up the computers) and would need a jump.

What does/is the aux. batt used for?<br>Thanks<br><br>

The auxilliary battery is used to power up the computers when you start the Prius. It is also used to power things like the trunk light, security system, etc. while the car is off.<br><br>--Jeremy

The light is powered by the 12V battery, not the high voltage battery which the state-of-charge indictor displays the charge for. While the 12V is kept charged by the HV system via the converter, that converter does not run when the car is not "ready". The HV battery is completely isolated when the car is off.<br><br>Burns

The 12V runs essentially everything except for the electric motors and associated switches, and the HV-to-12V converter. In particular, it runs the computers, so if the 12V is dead, you can't start the Prius, even if the HV is fine.<br><br>Burns

Well, this has been my main reliability peeve. <br><br>After 100 miles of first-time Prius driving I left my elderly mother in the car to listen to the radio while I went to a doctor's appointment. Of course, the head lights were also on, because I didn't have the subtle interactions of lights/operating mode down yet. Result: Dead battery, roadside assistance, long delay for a jump. (I think this was Toyota's Roadside Assistance:Prius's very first call, late Summer 2000. They had read the manuals and were pumped for for action, and knew exactly what how to perform a 12v jump.)<br><br>A half-dozen times since then (I am at 18k) I have left trunk or dome-lights on overnight. Fortunately always at home. <br><br>I have read that the GM EV1 has the same motorcycle-sized 12v battery, and is vulnerable to the same short-term depletion. <br><br>I now pack a 12v recharge pack in the trunk, about $50. It might be even better if the Prius had an emergency capability to use the traction battery to restore the 12v battery in a pinch. That was my frustration the first time this happened, that I actually had PLENTY of electricity available.