Originally Posted by Jesus_Freak
I am not an EE, but I think I remember an exponential decay. Since the supply voltage drops as it is discharging, the load is also varying?.?
For a given power output, as the voltage drops, the current must also increase. P=IV. I'm sure you knew that equation, but I just took the EIT so I'm feeling smart.
I'm not sure of exponential decay. Since we're talking batteries, that's almost more of a chemists arena than EE or ME (what I am).
The short answer is that there are too many variables to reliably use an equation to find out how long you can run your stereo on your battery. If the load was more constant like a blower or light etc then you could probably get a useful result.
Best thing to to in this case is empirically determine.
You stereo will shut off at a battery voltage that is not low enough to damage your alternator.
IMO you should set up your stereo so that it all draws off the 2nd battery, or use a Perko switch and set it up so that selects which battery is used for every draw. If you don't do either one, you are playing with fire.