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Old 04-21-2010, 11:29 PM
samtechwaterski samtechwaterski is offline
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How Long can I run my Stereo? - Math Equation?

Hey everyone Im trying to figure out how to calculate how long I can run my stereo and not run it too low that it damages it or the alternator cant recharge it properly? I have a dual battery setup. The Stereo is running off of a Optima blue top battery.

My headunit is pulling power from the original harness so basically it is using the original starting battery. The only thing running off of the blue top optima is the amplifiers.

I have 2 JL amps. One is a 6 channel the other is a 4 channel. The 6 channel runs the 4 inside speakers and the last 2 channels are bridged for the subwooffer.

The 4 channel amp runs the 4 tower speakers.

I cant remember the exact model numbers. I believe the 6 channel is a M6450 and the 4 channel is a M4300, but im not certain.

Thanks so much

Sam
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Old 04-22-2010, 04:37 AM
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I have 2 yellow tops that 3 amps run off of. They last about 6 to 8 hours. I have a blue top that is not used and used for a starting battery and to get back to the dock or to recharge the other 2 batteries. I never run dow the blue one. Every time I put the boat back into the garage I hook up the battery charger and leave it on the 2 yellows over night and it is ready for the next day. I do start the boat every 30 to 45 min and let it run a song or two during the day to keep the 2 yellows form being completely dead. Optima deep cycle batteries are ment to go from full charge to empty and back to full charge lots of times. Mine are 3 years old now.
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Old 04-22-2010, 01:21 PM
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Shaun315 Shaun315 is offline
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The short answer: it depends on a lot of factors... the current draw of the system, temperature, battery age, battery type, internal battery resistance, wiring of the system, etc.

Most batteries are rated at 20hr A*hr capacities, which is capacity (current*time) of the battery when discharged over 20 hours. A*hr rating / current load = time or A*hr rating / time = current available. So a battery rated at 105Ahr (@20hr) / 20 hours = 5.25 A continuous output. Not really practical for a stereo system since 5.25A*12V is only ~60W.

A better gauge might be RC or reserve capacity of the battery, usually how many minutes a battery can provide 25A until reaching end of discharge (or max depth of discharge... whatever they use for pb-acid). 25A*12V is 300W more inline with stereo demands... I think the group 34 blue top is rated at 120minutes RC, so you should get at least 2 hours out of it with a 25A load, probably more because I doubt your stereo pulls 25A constantly...

If you want a more accurate answer you'll have to figure out your average system load on the batteries and go from there...
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Old 04-23-2010, 06:13 AM
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Jesus_Freak Jesus_Freak is offline
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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?.?
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Old 04-23-2010, 07:57 AM
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Originally Posted by Jesus_Freak View Post
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.
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Last edited by CruisinGA; 04-23-2010 at 08:02 AM.
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Old 04-23-2010, 11:47 AM
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Quote:
Originally Posted by CruisinGA View Post

Best thing to to in this case is empirically determine.
second this... You'll get a real answer and there's no harm in running down your battery. The JL Amps shut off automatically at low battery voltage, plus the Optimas recover just fine from deep discharge.

As far as the exponential decay, yes there's a formula that uses Peukert's number (basically a battery efficiency constant) for calculating run time, but overall lead acids have a linear discharge profile. Don't really think that would be useful though, because there's no way you'll be able to figure out the current draw on the battery because your stereo is a dynamic load.

Also intermittent loading of a lead acid battery helps capacity allowing the chemical process to recover so your stereo probably isn't that hard on your battery, wouldn't hurt to start your boat up every once in a while either.
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Old 04-23-2010, 12:34 PM
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I would have the house battery and the blue top run to a perko switch to where you can completely shut one battery off and use the the other. If you are just listening to music you want your house battery to have nothing drawing off of it. I think that if your head unit, PP, and other factory electrical is running off of house and your amps are on the blue you will drain both batteries. The last thing that you want is a boat that will not crank after a day of listening to tunes. I wired mine up to a perko switch and have full control over the batteries and suggest that in your case. If not carry a jumper box because one day you will need it.
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Old 04-23-2010, 12:37 PM
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Quote:
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... 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).
When do you hear your EIT results? How long before you take the big one?
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