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   I like to have two banks of large batteries to deal with awesome (loud) sound systems. My preference is one bank of two group 27 AGM  in parallel and second bank with one AGM 4D (200 amp hours). Then switch back and forth every time I go out to ensure batteries are full charged,

  However, I am hearing from boat manufacturers that the alternators on the boats cant handle anything more than a single 27D. Which makes you wonder why have a switch that can combine the two together, since all new boats have two batteries ? It may get you out of a bind when you cant start and need to use the "both'" setting but then it may blow the alternator. I think the manufacturers are designing boats with too small batteries and then using disclaimers in user manual to protect themselves.  On my 2007 malibu 23 ft LSV wakesetter, my setup worked fine for the 7 years I had the boat. Never had a burnt alternator. Although I wonder if I blew my protournament300 battery charger  .. as I never realized it shouldnt be used on a 4D....... and it isnt charging my 4D anymore...

 But the boat manufacturers are sticking by their recommendations which dont work for the stereos that are on our boats.

So I am trying to understand from a technical standpoint, how do we determine the maximum size battery we can put on a boat. The only constraint is the boats alternator.  If I have a 93 amp alternator, why would a manufacturer say I cant use a  4D battery? Why am I limited to a 27D battery?  It comes down to how much current a dead 4D battery would consume from the alternator if you got a jump start. We all know you shouldnt use an alternator to charge a dead battery, but how many times do we do this when we get a jumpstart in a car and everything works fine.

How do I figure out that calculation?  I think with large 10 inch subs and eight 7 inch speakers, and three amps, we need a second bank with a 4D or 2 27Ds which are in essence the same amp hours and reserve capacity. 



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The role of any OEM alternator, is to maintain standard loads and replenish whats consumed cranking the engine. Alternators were never designed to be a battery charger and recharge deeply discharged batteries. Large amp hour battery banks and anchored in the party cove cranking the tunes for hours on hours, is above what is considered normal. 

In an ideal setup, your main cranking bank should never get depleted while out on the water, and therefore, never need very much of a recharge when off the water. Only some maintaining from a  smart charger during layup. 

For those with large Ah house banks and a normal weekend on the water is to anchor at the party cove and run the house bank down deep, you are best to keep that house bank isolated from the alternator once you fire up and head back to base. Then, plug in shore charger and let it do the hard work. This limits the load to the alternator, which isnt designed for this anyway,  and likely wont do much recharging on that short run back to the dock. 

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@MLA - What are your thoughts on the Blue Sea Systems Add-A-Battery install. 

I like them because, to your first point, the large Ah banks with party-cove discharging is indeed separated from the starter battery. But, to your second point, when the boat is started back up and heading to the dock, the ACR opens the circuit to both batteries, putting the starter and house battery demands back on the alternator. 

FWIW, I installed the Add-A-Battery system with two blue-toppers and an in-boat shore battery charger a few year ago with zero issues. Usual lake day is cove w/music then sports then dock

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I love the passive operation and simplicity of the DCP switch and ACR. Jump in boat, turn switch to ON. Leaving boat, turn switch to OFF. Its a great option for those with small to moderate size Ah house banks. Those with larger house banks and typically spend the weekends anchored driving the stereo and running those high Ah banks down, the DCP and ACR may not be the best. One reason is as stated. The ACR will try and close, thus introducing that heavy load to the alternator. In some cases, the dead battery will then draw down the supply voltage, causing the ACR to again open. A few seconds later, the supply voltage goes back up because the dead battery is off line, so the ACR closes again. And this bouncing repeats until the house bank comes up enough for the ACR to stay combined.

The manual 1/2/BOTH switch can be a better option for those with large Ah banks. Manually isolate that dead battery bank, and the stereo will still have supply voltage.    

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