Is there a way to figure out how long a stereo will play?

I am trying to figure out if I need more battery power. I am wanting my system to play for 3-4 hours straight at a very loud volume. Currently, I have 4 Stinger Electronic SPP1500D batts on the stereo alone. I have two other reserve batts for starting the boat. I have the following amps:

JL 500/1
JL 450/4
JL 300/2
JL 300/2
JL 300/4

I just added two more batts and am trying to figure out if I should add two more. When it had just two batts powering it, it would play for about 1.5-2 hours before it would start to die. Is there a formula or a way to figure it out?

There are so many variables to your question.....if your batteries are fully charged at the beginning of the day (not many people start the day out fully charged) ....at what ohm load the amplifiers are at.......even the temperature of the day (to cold and the batteries don't hold a charge as well / to hot and your amps will heat up and not be as efficient) ....and then the last item on play time is the thermal ability of the Amps. Sometimes your amps will shut down from lack of cooling way before your batteries do! Its sounds like you have plenty of battery power now to achieve the play time. There are formulas to figure expected current draw but real world applications very rarely match formulas.

Each of your batteries is good for 65 Amp-hours when charged. Assuming you are getting 100 Watts RMS out of each amplifier average, which would be insanely loud, and assuming 70% efficiency, you are constantly drawing 60 Amps of current, so on a full charge you could play for 4:20 with 4 batteries and 6:30 with 6 batteries. However with voltage drop factored in I would think actual play time would be lower than this. The real issue is your alternator. You need a beast of an alternator to get those charged. Or I guess you can charge them every time after you use them, but even then you'll need a monster battery charger to get a good charge on that many batteries.

What type of alternator do you have? If it is the factory alternator you should dial back to 2 batteries, I would think you can get as long or longer play times since the batteries will actually reach a full charge

No stock alternator will come close to charging enough to cover that much play time with that much power!! Your run time to play time would be around five to one (one hour of play time to five hours of run time) even though it is hard on the alternator never remove batteries thinking your playtime will increase!! As many "Good none damaged batteries" are the best thing you can do for your system! A good decent charger that you can plug in when you are not using the boat (many brands and options available with multi banks and higher rates of charge) will do the job.... but make sure that they are maintenance chargers as well this means they will power down and not over charge the batteries. This is especially important during the winter months and will help so you don't come back to bulging batteries at the beginning of the new season. A good Balmar Alternator is a wonderful idea as long as you have an extra 5 grand and plan on running your boat at a higher rpm and not sitting listening to the stereo while parked or at low idle.

Thanks Brian,

I have an onboard charger that I plug into the wall at my storage unit after each use. If I understand what you are saying, then my charger is a maintenance charger. It has a green light when it is done charging and shuts down, red means its still charging. To charge all six batteries, I am sure it takes a long time. I appreciate all the info. I think for now, I will save my money and not buy the alternator or add more batts. Thanks again.

Yes if its a maintenance charger it will shut itself off and comeback on every couple of days......