Is it possible to listen to your car audio system without running ur battery down...

[Natedogg]

...while your car is off if you? if your have a power capacitor

[glebo]

nope without some sort of generator going, which is a running engine.

[N2Audio]

Any electrical accessory running w/o the engine running will eventually run the battery dead, and as it's been clearly pointed out a capacitor will have no effect on that.

What is important to understand is it is the current draw of the accessory in question that will determine how long your battery will operate and still have enough current available to crank the engine.

Just a head unit will run for hours w/no problem.
Add a small amp at low/medium volume and you're probably still going to have a few hours of engine off listening time.
A few hundred watts worth of amp power at med/high volume -- maybe an hour.
1000w rms at high volume -- 15 min, maybe a half hr.

[Kevin]

I have a 2 farad cap and I unhooked the battery once and the radio kept going for about 10 seconds before it faded out.