I can't possibly imagine this killing a battery that's in good shape. I own a sailboat and power consumption is a key consideration when you're spending days on end with very opportunities to recharge the batteries.

The FEH has a standard 12v car battery in addition to the 330v battery array under the cargo floor. The 330v system is really just for running the electric motor -- everything else in the car runs on 12v power just like any other car.

You can calculate the drain if you know the number of amps drawn by the stereo and the amp-hour rating on the 12v battery. I wouldn't imagine that the draw could be much.

Along the topic though... have you read the jump-start procedure in the owners manual? The Hybrid has special jump-start procedures. There's also a switch that lets you couple the 12v battery to the 330v battery so that one can jump-start the other (without the aid of a 2nd car).

If you do fear that you might kill the battery in the darkness of a drive-in theater parking lot, you might want to read the manual to learn the jump-start process (and find the switch that let's it do a self jump-start) while there's plenty of daylight to find things.