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Old     (1fasdzl)      Join Date: Jun 2012       07-08-2012, 11:35 AM Reply   

Is there a simple formula to figure out how much battery you need in your bank?

Currently have (2) Interstate 24's
(1)JL Audio HD 600/4 (4- tower Bullet 7.7's)
(1)JL Audio XD 700/5 (4- boat Kicker 6.5's and 1-JL 12W3)

Thinking about changing to 1- JL 13W7 and HD1200/1 (instead of the sub channel on the 700/5)

I want to have enough to play 2 days with only the alternator topping them off and not deplete and kill the batteries (i.e. get more than 1 season out of them). I also don't want to have more than 3 batteries total, IF possible. I've always been the type to slightly overkill the situation, so I want to have some reserve still, even after 2 days of no shore charging.

I'm thinking of running (2) Group 31's for the battery bank and (1) group 27 for starting the boat. Am I going to need more? I understand the value of golf cart batteries, but really don't want to go there. I'd rather spend a bit more and keep it simple.


Last edited by 1fasdzl; 07-08-2012 at 11:38 AM. Reason: clarification
Old     (Houstonshark)      Join Date: Jan 2011       07-08-2012, 12:51 PM Reply   
I don't think there is any formula that would help you determine how many Ah you need with X Watts or what your play time would be with X Ah. Music is dynamic and so will be the current draw. I'd say just get as many Ah as you can out of the 2 stereo batteries you want to install. There are several manufacturers who sell a 12V GC battery that will be around 150Ah for less than $200. In my opinion, they are the best bang for the buck.
Old     (1fasdzl)      Join Date: Jun 2012       07-09-2012, 7:37 AM Reply   
There has to be some way to approximate though. It can't be a complete guessing game when it comes to battery setups. I know music is dynamic and there are many factors that determine current draw, but there has to be some type of formula.
Old     (Midnightv10)      Join Date: Feb 2012       07-09-2012, 8:49 AM Reply   

The system you have now will likely draw about 65 to 70 amps when you are playing it hard.
If you add the 1200-1 to your system you will be around 120 amp draw..

I agree with Houstonshark on the GC batteries.. They will allow you to drain much deeper and cycle many more times.
They are not any more complicated than 12 volt batts.
Old     (chpthril)      Join Date: Oct 2007       07-09-2012, 1:30 PM Reply   
There is no real point in calculating how many batteries are needed to play for 2 days with minimul charge from the alternator since you have already limited yourself to a 2-battery stereo bank. I think the focus needs to be on what the largest battery in terms of Ah's and physical dimensions you can fit, then choosing the best battery that will fit that.

Why are you looking at Group-31's for the stereo bank? What Group-31, LAWC or AGM? You need to reconsider 6V batteries. Cant get any simpler then wiring 2 together in series to make a 12V bank. It actually takes one less cable then wiring two 12V together in pareallel to make one large 12V bank.
Old     (polarbill)      Join Date: Jun 2003       07-09-2012, 1:53 PM Reply   
2 8D's should work

You can wish in one hand and $hit in another. Which one do think gets filled first?

In all honestly it sounds like you are going to be asking a lot of your alternator/battery bank. So amps=watts/voltage. 2100/12=175 amps. No your amps aren't going to actually be pushing that many watts the entire time so the amp draw could be a lot less but I have no idea how much. 50% less? 25% less? 10% less? I think the type of music you listen too, how well your stereo is tuned, how much head room you have, what ohm load your amps are running at all a variables I think.

Now lets think about the alternator for a second. How are you going to separate the banks? Perko, ACR, a combo, etc??? If you use an ACR like a sure power you may have problems ever getting the switch to combine long enough to even charge your stereo bank assuming you plan on sitting with the boat off jamming like most seem to do a lot of. If you use a perko style switch you will have to leave the switch on both or the stereo bank. If you do this for long you are asking your alternator to go full blast for too long(if you actually want to charge up the depleted stereo bank) which is going to seriously shorten the life of the alternator. Also, if you think you are just going to start the boat and let idle for a30 minutes and it is going to fully charge the batteries you are expecting too much and abusing the alternator. Remember the rated voltage of an alternator is max and not at idle. Your 70-90 amp alternator might only be putting out 40 amps at idle. Not near enough to keep up with your stereo let alone charge the batteries.

When cmaping for an entire weekend I think the best bet is to have an onboard or portale 20 amp+ charger and using a portable honda generator for a few hours in the evening when not using the boat. Then just leave the stereo bank separate from the starting bank all weekend.

Another option is you could buy a 150 amp Balmar alternator from me for ~$750. The problem there is you will have to upgrade your cables as well and you need to have at least a serpantine pulley. A single V belt won't work.
Old     (david_e_m)      Join Date: Jul 2008       07-09-2012, 2:03 PM Reply   
There is a formula to calculate wattage to amp/hours at rest. Total output power divided by amplifier efficiency divided by voltage divided by the dynamic state of music program material and the fact you are bi-amped. With 600 watts plus 300 watts plus 1200 watts of Class D power you might be pulling an average of 75 amp/hours continuously. Your alternator strength becomes part of the equation and how much of the drive time is at decent rpms. But from there it can be all over the map depending on the music, EQ, good or bad system design and execution efficiency, tuning efficiency and especially how you play the system. For example, one guy plays his system 3 dB down from compression and he gets better than twice the play time at rest as another guy who plays his system at the threshold of clipping or higher. After 10 minutes of playtime the volume won't be perceived as different but the run time is very different.
In any case, two group 31s might hold up for a long weekend intially. However, with deeply discharged cycles the batteries are unlikely to perform close to the original specs for very long. If you want to go that long and have the batteries last for multiple seasons you need to have enough reserves that you are seldomly discharging below 12 volts. Golf cart batteries are a little more tolerant of deeper cycles but you still do not want to make a habit of discharging consumer grade batteries below 50 percent.

Earmark Marine
Old     (1fasdzl)      Join Date: Jun 2012       07-09-2012, 8:14 PM Reply   
Originally Posted by polarbill View Post
2 8D's should work

You can wish in one hand and $hit in another. Which one do think gets filled first?

That's absolutely brilliant! I'm so going to use that!!!
Old     (1fasdzl)      Join Date: Jun 2012       07-09-2012, 8:21 PM Reply   
Sounds like I'm asking for the (near) impossible.

I was thinking of using group 31's for simplicity and cleanliness of wiring. Also, I thought when wiring up an on-board charger it would be simpler. Again...this was all theory.

Maybe I should rephrase my question: given my setup and requirements and going the simplest route with an ACR, what is your recommendation for a setup?
Old     (1fasdzl)      Join Date: Jun 2012       07-09-2012, 8:24 PM Reply   
Also, we are almost always are riding. I very rarely park in a cove with the boat off. When I do, music is not blaring (can't with a family) so music is only loud when we are moving.
Old     (chpthril)      Join Date: Oct 2007       07-10-2012, 4:27 AM Reply   
Originally Posted by 1fasdzl View Post
Also, we are almost always are riding. I very rarely park in a cove with the boat off. When I do, music is not blaring (can't with a family) so music is only loud when we are moving.
If this is the case, then in all honesty, a single Group-24 deep-cycle for a stereo bank would be all you need. Batteries are just voltage storage devices, they do not create the voltage. So if 90% of you play time is with the engine running, then the system will in effect be running off the alternator supply. You will not need a large reserve of battery.
Old     (mikeski)      Join Date: Aug 2003       07-10-2012, 8:38 AM Reply   
I have a spreadsheet you can use to determine play time. It is a simple math tool so garbage in = garbage out. If you want me to shoot you a copy send me a PM


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