do I have enough Battery?
 

so I'm upgrading my stereo system this year with some new amps and small subs (1500W total) and I'm trying to figure out if I've got enough power for the boat. The boat is currently equipped with two group 24 deep cycles. I plan to at least replace one with a starting battery, and switch back and forth for when I'm running or at anchor. I also added an on board charger for while I'm at dock (20amp/bank). but I'm trying to figure out if 2 group 24's are really enough for a big stereo and twin 7.4L.
you read enough articles on line and you borderline convince yourself you need 5 group 31's :D
I'm not a "blast music all day at anchor" kind of guy. typically I'll just run the stereo for an hour or two while at the beach. and mostly run the stereo in the no wake zones. Sadly due to limited space (272's are tight in the bilge!!!) I can't find any good place to mount a 3rd battery or trust me I would and be done with it.

Can you add one in parallel or perhaps totally isolated (charges only when plugged in) under the back seat?

Depending on how much sub amperage you are going to be running, you may want to add a capacitor.

2200 old school watts in the ole wellcraft. Had to upgrade to a 150 amp alt .
1500 watt class d arc audio 6 channel run beyond the max in the malibu isnt a problem with 2 deep cycle and a 90 amp alt.
So many variables from boat to boat and system to system ,no way to play armchair q.b. on that one
Your really going to need to monitor what you have with a voltmeter or 2 while in use and decide from there

2 deep cycle are always going to be better than one for running it for any amount of time. What is the RMS for the amps? What is your total amp output from both engines?
Now the 1500w amps are only going to draw that on big hits like subs, so it won't draw that all the time?
Now for some math... hope i do this right
@12vdc your amps will draw a max of 125amps, so with two group 24s @ 80 amp hours each will bring you to 160. So without knowing the RMS of your amps I would guess 2 hours cranked to the max and 4-5 hours at reasonable levels.
With the boat running 2x50amp alternators you'll have power all day. (50A is a guess)

DC watts to amps calculation
The current I in amps (A) is equal to the power P in watts (W), divided by the voltage V in volts (V):
I(A) = P(W) / V(V)

http://www.allaboutcircuits.com/text...ttery-ratings/

thanks that's actually very helpful. the 4 channel sub is rated at 225 RMS and the monoblock for subs is rated at 500 RMS. that said the amps are rated higher than the speakers so I guess in theory they'll only be running about 60-70% of that and I doubt I'll be cranking it to the max very often. I only plan to run the stereo one battery while anchored out though. I don't think I'd relax very well knowing that as I'm anchored and listening to tunes that I'm draining both batteries.

https://www.amazon.com/NOCO-IGD140HP.../dp/B001DKRF2M

Adding a battery isolator will allow you to charge both batteries without switching

https://www.bluesea.com/products/765...ery_Kit_-_120A
Take a peek at this

I'd combine both deep cycles to run the stereo and get a separate starting battery.

Deep Cycles are made to be able to be discharged and recharged frequently.
Starting batteries are made to put out more CCA's. They will not last as long if they are not kept at close to full charge.

I have one of those $15 ebay 4 segment led voltage displays on the amp rack of my wellcraft.
When it goes from 12.5 to 11.5 , im done jamming until i charge the 2 aux deep cycle batteries back up. And it takes a while , even with a monster alternator.

Just removed the isolator and replaced with the ACR. Wasn't having any real issues, just wanted to get rid of the 0.7V voltage drop and have a more level charge