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Old 14-07-2017, 06:54   #31
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

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There is nothing wrong with batteries in parallel. It all depends on having proper connecting cables.
Until they become unbalanced, that is. This has already been discussed at length on other threads. Parallel wiring should be avoided if possible. If it must be done that way for some reason, then do it. If there is a viable alternative, then do that. A very small imbalance you will likely never notice, though it will actually be robbing battery performance. A short cell on one parallel branch can cause problems. Two or more isolated banks avoid this, though Peukert effect will be greater, at high discharge rates. Simply having cells big enough to carry the load avoids it. Best practice IMHO is two isolated series banks (a 12v battery is still a series array... 6 cells in series) with cells big enough to do the job.
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Old 14-07-2017, 16:44   #32
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

After all the input (thank you) and further research I pulled the trigger on the Duracell GC2 batteries. I bought 4 of them which I'll run in both series and parallel to achieve 12V and 430ah of capacity. So now the question becomes - what gauge wires to connect the batteries? And then what gauge wire from the bank to my panel given its about a 7' cable run away?
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Old 14-07-2017, 20:45   #33
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

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After all the input (thank you) and further research I pulled the trigger on the Duracell GC2 batteries. I bought 4 of them which I'll run in both series and parallel to achieve 12V and 430ah of capacity. So now the question becomes - what gauge wires to connect the batteries? And then what gauge wire from the bank to my panel given its about a 7' cable run away?
Keep in mind that the 4 GC batteries are also an emergency start bank. I would not use wire any smaller than the start battery has for inter-connects and run to the switch. When I wire such a system that is either 1/0 or most often 2/0 cabling. All battery cables should be the same gauge.

From the switch to the panel for house loads I would use a chart such as Blue Seas has on their site for a voltage drop of less than 3%, figuring the panel is at max. This depends on the loads but for say 50 amps I would use 4 awg minimum.
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Old 14-07-2017, 22:36   #34
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

I like big fat wire. 2/0 is what I use. I am using welding cable which is cheap and very flexible and easy to handle. Tinned cable would be better, yeah, but cost more and be less flexible. Tinned cable won't corrode at connections like untinned copper will. Of course my batteries power not only house loads, but propulsion as well, and on occasion I might run up to about 140a for a short time and up to 100a for several minutes. But too big a cable size won't hurt anything. Too small and you increase resistance and therefore losses converted to heat in the wires. Once again you are in a balancing act or a compromise situation. All I can tell you is I use 2/0 in a fairly demanding application and it works sweet. YMMV.
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Old 15-07-2017, 00:12   #35
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

I am all for using 6 volt GC's if you need the capacity but that's a big if. Replacing one 60# 12 volt battery with two 60# 6 volt batteries in series is not an improvement if the capacity is not needed. It adds a lot of weight and increases your charging requirements. I would much rather add solar and reduce loads (LED's) to avoid adding battery capacity. Thanks.
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Old 15-07-2017, 06:42   #36
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

Charging requirements aren't by battery capacity, but by use. Use 100 amp hours from a small bank, or use the same from a large bank. It is still 100 amp hours. The interconnections are the most important in building a bank. Use 2/0 for battery to battery. Then size wiring by the length and amps for the panel feed. Use a voltage drop table.
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Old 22-07-2017, 17:44   #37
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

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Intuitively, this makes no sense. Maybe I missed the point but simple geometry would indicate a wider 12v battery has more space for 'free flooding' effect than a smaller 6v battery in which the fluid is better confined.

As to your other contention that if one 6v goes bad, you lose two - while true, the net result is equivalent to losing one 12v battery. No advantage.

Did I miss something?
Yes you missed something,

1. A 6Vdc is slightly larger than a Grp 24 and slightly smaller than a Grp 27 12 Vdc battery.

if one has:

A) 2 x 6Vdc batteries in series, and loses one, they lose two, and 100% of their capacity because nothing on the boat will run on 6 Vdc of the remaining battery.

B) 2 x 12Vdc batteries, being the similar volume, weight, and capacity as A), if they loose a battery, they have only lost 50% capacity, because the remaining
12 Vdc battery can still operate all boat systems.
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Old 22-07-2017, 18:37   #38
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

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if one has:

A) 2 x 6Vdc batteries in series, and loses one, they lose two, and 100% of their capacity because nothing on the boat will run on 6 Vdc of the remaining battery.

B) 2 x 12Vdc batteries, being the similar volume, weight, and capacity as A), if they loose a battery, they have only lost 50% capacity, because the remaining
12 Vdc battery can still operate all boat systems.
my attempt at the Socratic method didn't work it seems...

By your example, if you 'lose' half of 2 x 6 volt bank, you lose the entire bank is equivalent to what happens when you lose one 12 v battery. No net difference making it a dubious reason to suggest one option is preferable.

Plus, a larger size rectangle has more free flooding effect which means a larger 12 v battery's plate are at greater risk of drying while heeled for prolonged periods.
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Old 22-07-2017, 18:51   #39
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

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my attempt at the Socratic method didn't work it seems...



By your example, if you 'lose' half of 2 x 6 volt bank, you lose the entire bank is equivalent to what happens when you lose one 12 v battery. No net difference making it a dubious reason to suggest one option is preferable.



Plus, a larger size rectangle has more free flooding effect which means a larger 12 v battery's plate are at greater risk of drying while heeled for prolonged periods.


RamblinRod got it right, if you only have room for 2 or 3 batteries, 12v are the way to go.

With 2 6v batteries in series if one battery fails you are left with 6v which will not run anything on a boat except maybe a fan.

With 2 12v batteries you still have 12v but only the total amp-hr of one battery and will need to recharge more often, reduce consumption or some of both.
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Old 22-07-2017, 19:03   #40
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

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my attempt at the Socratic method didn't work it seems...

By your example, if you 'lose' half of 2 x 6 volt bank, you lose the entire bank is equivalent to what happens when you lose one 12 v battery. No net difference making it a dubious reason to suggest one option is preferable.

Plus, a larger size rectangle has more free flooding effect which means a larger 12 v battery's plate are at greater risk of drying while heeled for prolonged periods.
OK, I'll try again.

If contemplating replacing a GRP 31 with 2x6Vdc batteries, also consider replacing same with 2 x Grp 24's or Grp 27s, as space permits.

Again, a Grp 24 is slightly smaller than a 6 Vdc battery, and a Grp 27 is slightly larger. 2 x 6Vdc batteries are way bigger than 1 Grp 31.

As far as your free flooding effect theory, this has no bearing, the electrolyte is much higher above the 12 Vdc battery plates to begin with, when flat, so the battery (being approximately the same size as a 6Vdc battery, can be heeled much more before plate exposure.
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Old 22-07-2017, 19:53   #41
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

If you have the footprint for two Group 31 and have a couple inches clearance in height, you can use 12vdc golf cart batteries. Great battery 155 Ah each, for a total of 310 Ah. Saves weight and wiring. I have a pair and the plates are very well covered. US made, only $145/ea and has 50% more power. I first started looking at them after seeing Disney using 4x12V XC in their carts. Figured they could afford whatever they wanted

U.S. Battery | Leader in Deep Cycle Batteries | US 12VRX XC2 - U.S. Battery | Leader in Deep Cycle Batteries
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Old 22-07-2017, 19:57   #42
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

Don't forget to put a fuse in the positive conductor regardless of battery choice.
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Old 23-07-2017, 07:09   #43
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

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RamblinRod got it right, if you only have room for 2 or 3 batteries, 12v are the way to go.

With 2 6v batteries in series if one battery fails you are left with 6v which will not run anything on a boat except maybe a fan.

With 2 12v batteries you still have 12v but only the total amp-hr of one battery and will need to recharge more often, reduce consumption or some of both.
I don't see anything in the OP's question indicating limited space dictating battery type. From where did that come?
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Old 23-07-2017, 07:14   #44
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

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OK, I'll try again.

If contemplating replacing a GRP 31 with 2x6Vdc batteries, also consider replacing same with 2 x Grp 24's or Grp 27s, as space permits.

Again, a Grp 24 is slightly smaller than a 6 Vdc battery, and a Grp 27 is slightly larger. 2 x 6Vdc batteries are way bigger than 1 Grp 31.

As far as your free flooding effect theory, this has no bearing, the electrolyte is much higher above the 12 Vdc battery plates to begin with, when flat, so the battery (being approximately the same size as a 6Vdc battery, can be heeled much more before plate exposure.
If electrolyte is "much higher", that's operator error, not some design parameter distinct to 12 v batteries.

Assuming battery fluid level is as designed, the geometry speaks for itself. As does the free surface flooding potential to dry larger battery plates. The science isn't a matter of opinion.
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Old 23-07-2017, 08:44   #45
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Re: Duracell Deep Cycle Group 31 Lead-Acid - 105 AH

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I don't see anything in the OP's question indicating limited space dictating battery type. From where did that come?
The OP discussed buying 4 Grp-31s, which is not the same as having space for 4 GC2s. GC2s are a different form than Grp-31s and 4 of them may or may not fit in the space available.

RR covered the special case where the OP can not fit 4-6v batteries in the space available. If there is only space for 2 batteries, 12v is probably the better choice.

I forgot to indicate in my last post that I agree that you are correct to be concerned about plates being uncovered as a boat heels. The best solution to this is to orient the batteries with their long axis transverse to the boat. That results in the short axis of each cell also being transverse which minimizes free surface effects leading to uncovering of plates. Taller batteries with more overhead for electrolyte is another way to deal with this.
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