First - avoid ever running your system when the car is off.

Running or not, when you exceed the output of the alternator, the excess is provided by the battery. Two ways to look at the power you can get out of a battery:

1) Cold-Cranking Amps (CCA) - A rating that is used to define the battery's ability to start an engine under low-temperature conditions. BCI defines it as "the number of amps a lead-acid battery at 0ºF (-17.8ºC) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt lead-acid battery).

ie. A 'standard' 650cca battery can provide about 650amps for 30seconds or so. After that it's dead.

2)Reserve Capacity (RC) - "the number of minutes a new, fully-charged battery at 80ºF (27ºC) can be discharged at 25 amps and maintain a voltage equal to or higher than 1.75 volts per cell" (i.e., 10.5 volts for a 12-volt battery). This rating represents the time the battery will continue to operate essential accessories in the event of a charging system failure.

So you can see that the more you draw, the less time you have. Thus - always have the car running when playing even at moderate volumes.

Oh - I copied that info from Interstate's site.

__________________
Paul Niwranski
1966 Cadillac Superior Crown Royale Limousine Hearse

If you're mirrors ain't shakin...you got taken...
RIP Member #8
Be careful what you wish for...