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Old 03-11-2013, 03:17 PM   #1
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Time for new batts, should i go 12v or 6v?

I currently have two banks, each bank consist of 2 12v D8 AGM's.

The time has come to replace one of these banks, my questions goes:
1. Would it be an advantage to use 4 x 6V batts, golfcart type or agm?
Obviously they are easier to carry and install, other advantages?

Recommendations on 6V or 12V deep cycle batt suppliers?
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Old 03-11-2013, 03:35 PM   #2
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The last time I compared flooded cells were definitely cheaper watts. But unless you are prepared to religiously do the maintenance don't go there. I have flooded cells on Gray Hawk and AGMs on my bus. When the flooded cells come due for replacement I will replace them with AGMs - the extra cost is worth the reduced hassle IMO.

6v cells will be easier to handle but you will need many more cables and connections with attendant maintenance to consider.

The big question I would ask is why do you have 2 banks? Everything I know about battery lifespan favours a single bank and the user experience is much more pleasant as well.
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Old 03-11-2013, 03:40 PM   #3
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I think yes. And even better w the Trojan brand GC batts. That's what I plan for my next batts. I changed batts recently and probably pre-maturely as I think I passed a bad/incorrect judgement on their condition. Have your batts checked first. I just drew my own conclusions and ordered new batts. Had Life Line and now have Full River. Same size but mow I'd rather have a 3rd batt. I think one should replace batts as a group not singularly.
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Old 03-11-2013, 03:51 PM   #4
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Originally Posted by bobofthenorth View Post
The last time I compared flooded cells were definitely cheaper watts. But unless you are prepared to religiously do the maintenance don't go there. I have flooded cells on Gray Hawk and AGMs on my bus. When the flooded cells come due for replacement I will replace them with AGMs - the extra cost is worth the reduced hassle IMO.

6v cells will be easier to handle but you will need many more cables and connections with attendant maintenance to consider.

The big question I would ask is why do you have 2 banks? Everything I know about battery lifespan favours a single bank and the user experience is much more pleasant as well.
I did not design this system, but basically the two banks are connected through a bluesea switch, i think it is a pretty good set up.
One bank is mainly for cranking, the other for "house".
But do i really need 2 x 8D's just for cranking? ofcourse through the blue sea switch, the batts are also partially used for "house" purposes.
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Old 03-11-2013, 03:53 PM   #5
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i think i just answered one of my own questions, since both banks are charged from the same charger, i cannot mix AGM and wet cells, they have different charge settings..duh.
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Old 03-11-2013, 04:05 PM   #6
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I'm a big fan of golf cart batteries. More AH for the same footprint, much easier to handle, and cheaper to boot. I don't consider wiring them in to be much of a maintenance issue. The ability to move them around if needed probably negates any extra time bolting cables on.

The big question is AGM vs flooded, and that's simply a money issue. If you can justify the cost of AGM, they're much better. If sticker shock has you thinking checking water levels now and then isn't such a bad thing after all, then there's your answer.
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Old 03-11-2013, 04:32 PM   #7
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I'm in Eric's camp for changing batteries as a group rather than singular. I have no gen so I use 4 x 6Volt golf cart for house batteries and 1 x 12 volt for starting. There is only one extra jumper to use 6 volt batteries, not really a big issue and I check and fill a couple times a year. I would go with AGM as well if you have the $$$$.
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Old 03-11-2013, 04:40 PM   #8
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I did not design this system, but basically the two banks are connected through a bluesea switch, i think it is a pretty good set up.
One bank is mainly for cranking, the other for "house".
But do i really need 2 x 8D's just for cranking? ofcourse through the blue sea switch, the batts are also partially used for "house" purposes.
With respect I don't think its a very good setup and just because the P.O. did something doesn't preclude you improving it.

In a nutshell, battery banks should be large enough that you never discharge them past 50% and ideally less than that. Their lifespan goes up exponentially as you reduce the level of discharge. That's the argument for a single large bank. Multiple smaller banks will tend to be discharged more deeply with consequent drastic reduction in their lifespan.

Your bank can be physically divided into 2 groups of batteries which are then combined through a combiner switch effectively making them function as a single bank. In that event though they all need to be replaced simultaneously. And you need to make some provision for starting redundancy. IOW you want a backup battery that is not part of that bank so that you can always get the noisemakers running again, no matter how deeply you discharge the "house" bank.
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Old 03-11-2013, 05:04 PM   #9
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I don't know if you have twin engines on your boat, but if I did, I would have a separate starting/cranking battery for each. I like my golf cart 6v set-up, but I have it organized such that my inverter cannot tap into my cranking battery. I suggest you do the same so you don't wake up disappointed some morning swinging on the hook. I don't find checking water levels to be difficult, but I have very easy access. I may someday go AGM but as of now, I have no reason to do so and have been very happy with the flooded.
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Old 03-11-2013, 05:49 PM   #10
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I'm a Golf Cart fan (flooded). Can't beat the economy, or the convenience. I set up my house bank with 6 6V. Bought them at Sam's club for somewhere around $78US each...220 AH per 12V pair. Built a special box for them low in the bilge. For starting I have 2 4D's combined in a second bank. Operationally, I agree with the gentleman who suggested combining both banks...with the caveat that you keep a very close eye on your battery level so as not to endanger your starting capacity. Good luck!
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Old 03-11-2013, 05:59 PM   #11
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Our boat has four battery boxes. Originally 3-8Ds and one 4D. When I reconfigured, I was able to fit three Sam's 6V GCs sideways in each 8D battery box...so ended up with 6 GCs in two boxes as a house bank. Dedicated the third box for 2 group 27 start batteries for the engines. Fourth box still has a 4D for the genset, but it will get a group 27 when it gives up the ghost. I water the GC's roughly once per month, although they don't get much use.

I found pre-made "OO" cables of various lengths (starting at about 6") at a farm supply store called Fleet-Farm...similar to Tractor Supply. Made the interconnect of the house bank a snap....and not very expensive.
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Old 03-11-2013, 06:36 PM   #12
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Your bank can be physically divided into 2 groups of batteries which are then combined through a combiner switch effectively making them function as a single bank. In that event though they all need to be replaced simultaneously. And you need to make some provision for starting redundancy. IOW you want a backup battery that is not part of that bank so that you can always get the noisemakers running again, no matter how deeply you discharge the "house" bank.
thats what i have, two banks which combine when charging and combine when using or inverting but only down to a V threshold, then the switch separate the two systems so that the cranking bank is not run down.

i think if i get the same type batts (ie agm), then i can replace one bank at the time, but could i then switch to 6V for that bank? i think for running it is fine, but how about charging?
does 2x6V AGM have the same charging requirements as 1 12V AGM?
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Old 03-11-2013, 07:01 PM   #13
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Its typically better to replace all the batteries that will be charged together at the same time. Obviously that doesn't always happen. Personally I try to have my batteries identical and the same age. YMMV. I'm sure someone will chime in to tell you that what you want to do will work just fine. Life is about the compromises that we are prepared to live with.
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Old 03-11-2013, 08:16 PM   #14
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My batteries are in locations where routine maintenance is very difficult so when they failed, I spent the extra money on AGM batteries. I belive the extra cost is worth in in my case.
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Old 03-11-2013, 08:20 PM   #15
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.............. The big question I would ask is why do you have 2 banks? Everything I know about battery lifespan favours a single bank and the user experience is much more pleasant as well.
In my experience, all but the simplest (entrry level) boats have two battery banks even if each consists of only one battery. These are either arranged with a 1/2/both/off switch or wired as a starting bank and a house bank with a voltage sensitive relay to keep both banks charged.

As for the user experience, the second method I described is pretty much a no brainer. No user intervention is involved for normal operation. This is how my boat is wired.
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Old 03-11-2013, 08:22 PM   #16
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We replaced our two 8Ds with six, 6vdc golf cart batteries (three 6vdc batteries fit in one 8D box). The big advantage for us is that four 6vdc batteries have approx twice the amp hours of one 8D. The other two 6vdc batteries fill the role of start battery and have the same amp hour rating as one 8D.

So with no change to the boat's wiring or physical battery setup we doubled our house power capacity and retained our start battery capacity but with smaller and lighter individual batteries.
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Old 03-11-2013, 08:24 PM   #17
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In my experience, all but the simplest (entrry level) boats have two battery banks even if each consists of only one battery. These are either arranged with a 1/2/both/off switch or wired as a starting bank and a house bank with a voltage sensitive relay to keep both banks charged.

As for the user experience, the second method I described is pretty much a no brainer. No user intervention is involved for normal operation. This is how my boat is wired.
I agree!
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Old 03-12-2013, 12:09 AM   #18
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I'm a big fan of 2 alternators on a single engine boat. Adding the second alternator may require help from a mechanic, but then you have one bank for the house and one battery to start the motor. Each has there own alternator and you don't have to remember to separate batteries when you turn the engine off. No worries about draining the starting battery. On my boat, both banks have their own 110 ac battery chargers. A simple 6 amp maintenance charger for the engine battery, and a 20 amp charger for the house bank to be able to run everything while tied to the dock.

I'm currently running an AGM 8D for the house battery and a wet cell 8D for the starting battery. Jury is still out. 8D wet cells are worthless as house batteries (last about 2 years for me). AGM is heading into it's 3rd season. If the AGM doesn't last 4 seasons, I'm going Trojan wet cell 6 volt golf cart batteries next time.

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Old 03-12-2013, 12:50 AM   #19
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Lots of good advice here. I like the wet cell lead acid, 6 GC battery in the 2 HD footprint, all same age/type/size, separate banks with simple automatic (and foolproof) combining and isolation, quality but inexpensive Sam's Club or Costco GC batteries, and bigger is better bank sizing. The only tidbit of experiential advice I could add is a rerun, but one I've found very valuable and helpful.

After installing these caps, I still check the batteries every 3 months, but hardly have to add any water. Little to no acid venting on the battery tops. I could probably go longer between battery maintenance, but feel comfortable with this schedule. And it's easy to remember...every month divisible by 3, feed the lawn and the boat batteries.



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Old 03-12-2013, 06:24 AM   #20
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I'm a big fan of 2 alternators on a single engine boat. Adding the second alternator may require help from a mechanic, but then you have one bank for the house and one battery to start the motor. Each has there own alternator and you don't have to remember to separate batteries when you turn the engine off. No worries about draining the starting battery. On my boat, both banks have their own 110 ac battery chargers. .............
That's where the voltage sensing relay comes in. It is the "automatic switch".

Many, perhaps most late model chargers are capable of charging seperate battery banks. No need for two physically seperate chargers.
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