wondering if anyone runs 2 batteries,i just put a second 1 in ,for a/v stuff and was wondering how much more run time with the vehicle off can i run some stuff.i know it depends on all the draw but say just the in dash and the 2 12's subs with the 600 watt amp.

 

You can get a rough idea by disconnecting both batteries and then connect an ampmeter in series between one of the batteries and the disconnected cable. Turn on the equipment you wish to operate while the engine isn't running and note the amount of amps on the meter. Your batteries will have an amp hour rating so total that rating of the two batteries and then divide by the amps that you read on the meter and that will tell you approximately how long you have until the batteries are completely discharged. Since you'll need to have power in the batteries to start your engine, I wouldn't operate more than 1/4 the number of amp hours before starting and charging the system.

You may want to consider making the second battery a deep cycle battery, connect your equipment to the deep cycle battery and install an isolation relay on it. A deep cycle battery is designed to better handle charge/discharge cycles and will deliver full voltage to your equipment longer. The isolation relay will keep you from drawing from the vehicle battery while you're using your equipment without the engine running. Then, I'd figure roughly 50% of the amp hours of the deep cycle for operation of your equipment.

Also remember that stock alternators are designed to replenish a battery after normal starting of the engine and to keep up with normal electrical loads while the vehicle is in operation. If you deplete the battery too much, then you risk overloading your alternator and damaging it. That's why it's always recommend to recharge a depleted battery before installing a new alternator. It's possible that you may need to look into a high output alternator if you want to operate your equipment without the vehicle running.