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  #21  
Old 11-20-2012
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Re: Battery Sizing - am I crazy?

Quote:
Originally Posted by CaptTom View Post
Agreed. The advantages to the 6V GC batteries are high AH rating for their size and cost, full deep-cycle capability, a smaller footprint that gives you some mounting flexibility, and an easier job installing and removing them. Sam's club also sells GC batteries, as does Batteries Plus.

I agree you would do fine with a conventional 12V battery, but at least spec out a pair of GC batteries, you may find that works better for you.
They do have lots of Ah's but of course it takes two before you can compare those Ah's to a 12V battery. In terms of energy density by weight you can compare two 60lb batteries on Deka's website:

21.35 Wh/lb for the Group 31 (59 lbs, 105ah)
21.32 Wh/lb for the Golf Cart GC10 (60.5 lbs, 215ah)

Basically a rounding error in terms of the difference. I've run the numbers on density by volume and again it comes out the same. So there may be advantages for the GC's but it's not density.

http://www.dekabatteries.com/assets/...MasterGolf.pdf
http://www.dekabatteries.com/assets/base/0194.pdf
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Old 11-20-2012
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Re: Battery Sizing - am I crazy?

Quote:
They do have lots of Ah's but of course it takes two before you can compare those Ah's to a 12V battery. In terms of energy density by weight you can compare two 60lb batteries on Deka's website:

21.35 Wh/lb for the Group 31 (59 lbs, 105ah)
21.32 Wh/lb for the Golf Cart GC10 (60.5 lbs, 215ah)

Basically a rounding error in terms of the difference. I've run the numbers on density by volume and again it comes out the same. So there may be advantages for the GC's but it's not density.
Not sure what this means.


Of course you need two 6- volts to get 12 volt current. The advantage of them is that the footprint for the 2- 6 volt is almost the same as 1 12 volt. So for the same footprint you get twice the ah. NOt sure what density has to do with it. The advantage is more for the SIZE...irrespective of weight.
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Re: Battery Sizing - am I crazy?

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Originally Posted by chef2sail View Post
Not sure what this means.


Of course you need two 6- volts to get 12 volt current. The advantage of them is that the footprint for the 2- 6 volt is almost the same as 1 12 volt. So for the same footprint you get twice the ah. NOt sure what density has to do with it. The advantage is more for the SIZE...irrespective of weight.
I didn't make it clear enough. I'm quantifying size and I think the statement "GC batteries have more capacity for their size" is misleading. It implies they have some inherent advantage. But it isn't true in terms of size and weight which are the only ways to truly quantify size. I also doubt they're cheaper but I didn't run those numbers.

Sure they may have more capacity for their footprint because they're taller. But it's not like taller is universally better. My two batteries are both in height contained locations where "size" most definitely includes consideration of the 3rd dimension. So in my case, 12V's give the most capacity possible for their "size".

I'm not knocking GC's at all. If they fit better in my boat I wouldn't hesitate to use them. But in terms of the numbers its basically a complete wash between GC's and regular 12V's.

Last edited by asdf38; 11-20-2012 at 01:57 PM.
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Re: Battery Sizing - am I crazy?

Quote:
Sure they may have more capacity for their footprint because they're taller. But it's not like taller is universally better. My two batteries are both in height contained locations where "size" most definitely includes consideration of the 3rd dimension. So in my case, 12V's give the most capacity possible for their "size".
Not necessarily so. My 4 Lifeline AGM 6 volts are the same height as most standard group 31. They deliver almost ah per volume than 1 group 31. I beleive the reason they weigh more is a heavier lead plate. The 4-6 volt batteries + 2 inches in length fit into the exact same space as the 2 group 31 I used to have.

Heres what you need to compare to compare apples to apples 2 - 6 volt GC vs 2 Group 31. Both will have approx the same ah. The two group 31s will take up substabtially more volume than the two golf carts. The weight difference is minimal ( 2X59.9 or 2X66)
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Re: Battery Sizing - am I crazy?

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Originally Posted by chef2sail View Post
Not necessarily so. My 4 Lifeline AGM 6 volts are the same height as most standard group 31. They deliver almost ah per volume than 1 group 31. I beleive the reason they weigh more is a heavier lead plate. The 4-6 volt batteries + 2 inches in length fit into the exact same space as the 2 group 31 I used to have.

Heres what you need to compare to compare apples to apples 2 - 6 volt GC vs 2 Group 31. Both will have approx the same ah. The two group 31s will take up substabtially more volume than the two golf carts. The weight difference is minimal ( 2X59.9 or 2X66)
Ok

GC10 = 794 square inches = 1.62 Wh per square inch
G31 = 833 square inches = 1.511 Wh per square inch

Ok so that's a 7% advantage for the GC battery in this case in terms of energy per volume. I'll agree that's not nothing. Although I know I've run these numbers in the past, for a different pair of batteries and seen the 12V's come out ahead like it does on weight in this case. I'll try to find that.
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Old 11-20-2012
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Re: Battery Sizing - am I crazy?

"I know Deep Cycle batteries are not supposed to be starting batteries"
A common mistake. Deep cycle batteries are supposed to be deep cycle batteries. Starting (SLI) batteries are designed to be CHEAPER batteries with a MORE LIMITED PURPOSE and one special trick, high impulse discharges.
When you build a battery with thick plates, that physically limits how much impulse power those plates can put out, probably due to internal resistanace and heating. When you build lots of thin plates, you can put out lots of impulse power--as an SLI battery does. The problem with thin plates is that traditionally that compromises their physical strength, so they tend to fall apart and break down faster IF they are allowed to weaken (by discharging) in any significant way.
New result, a typical SLI battery can be destroyed by less than 6 deep discharges.
But if all you are doing is SLI (Starter, lights, ignition) service, an SLI battery may weigh and cost half of what a deep cycle battery does.

If your starting loads are small, as they are for most recreational marine engines, the average deep cycle battery can easily handle then without problems. Hell, I've started a Volvo MD7 with a 17AH "alarm and exit" type battery and neither one protested about it.

But if your engine is a 600hp Cummin Diesel, then sure, you may be better off paying closer attention to battery types and ratings.

Any deep cycle battery WILL have discharge ratings, and if you can't find them a quick call to the maker will tell you just how many amps that battery can safely provide in a surge for a starter motor. To put it in perspective, even a Group24 car battery will put out some 3000 Amps in a dead short, but the starter motor it is paired with usually draws one kilowatt or less (~100Amp) for all of 2-3 seconds.

So does the OP need deep cycle batteries? YES. Remember that SLI batteries have thin plates that easily take physical damage from discharge. They are designed, according to the folks I spoke to, for a MAXIMUM discharge of not more than 10%. At that point they start taking damage no matter how soon you recharge them. Ascompared to deep cycle batteries, that are designed for a30-50% typical discharge cycle.

Take an SLI battery, run the VHF all day, come home late and use the running lights, stay out for dinner and use the cabin lights, gee, doesn't the stereo sound great? and yes, you can and will damage an SLI battery with just day sailing, if you use the electronics. If you're not using much power, either one will certainly last the season, or two, maybe three. But the deep cycle won't cost much if anything more, and if will give you that extra margin for the weekend trips and longer days.
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Re: Battery Sizing - am I crazy?

My suggestion is to go back to first principles and calculate your daily 12v requirements, take account of your onboard charging facilities (engine alternator, and maybe solar panels and wind generator) and derive your battery requirement accordingly. Take a look at Understanding Boat Batteries and Onboard Electrics and it will guide you through the process.
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Re: Battery Sizing - am I crazy?

Quote:
Originally Posted by asdf38 View Post
Ok

GC10 = 794 square inches = 1.62 Wh per square inch
G31 = 833 square inches = 1.511 Wh per square inch

Ok so that's a 7% advantage for the GC battery in this case in terms of energy per volume. I'll agree that's not nothing. Although I know I've run these numbers in the past, for a different pair of batteries and seen the 12V's come out ahead like it does on weight in this case. I'll try to find that.
One thing this does not take into account is the cycle life of a 12V flooded battery vs. a 6V flooded battery when put in the same deep cycling application. The 6V batteries will outlast the 12V. I find on average 20-40% more life.

Beyond that on about 70% of the boats I work on "foot print" is the limiting factor and on about 30% height is the limiting factor.

GC2 / T105 size golf cart batteries are closer in foot print to a group 24 than a 27 or29/31 and with foot print usually being the constraint limit more so than height one must consider foot print comparisons vs. weight or box volume...

GC2 / T105 = 10 3/8" Long by 7 1/8" Wide by 10 7/8" Tall
Group 24 = 10 3/4" Long by 6 13/16" Wide by 8 11/16" Tall
Group 27 = 12 9/16" Long by 6 13/16" Wide by 8 3/4" Tall

If we compare the Deka / Duracell brand from Sam's Club (same as West Marine)

EGC2 6V = 230Ah for 2 batteries
Deep Cycle 24 12V = 150Ah for 2 batteries

The 6V batteries, in the same basic foot print of a group 24 have 80Ah more capacity for two, or nearly an entire group 24 battery... You also chose to compare weakest GC2 battery Deka makes at 215Ah/.. They also make that same case size in a 230Ah and 235Ah battery.

On boats where height is the limiting factor then 12V are often the only way to go unless you want to get spendy on Lifeline GPL-4CT's... But on boats where foot print is the limiting factor 6V batteries win hands down..

The OP really does not need much and a couple of group 24, 27 or 31's will do just fine. I would advise against the current crop of Wal*Mart batteries. Firstly they are labeled incorrectly and the labeling means NOTHING. It was clearly drawn up by a marketing department who knows squat. Secondly the new batteries are more of a dual purpose than the original Maax 29's that were actually built by US Battery and were an excellent product. I would instead look towards Wal*Marts sister store Sam's Club where in about 25 states they sell the Deka / East Penn brand of batteries.
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Last edited by Maine Sail; 11-20-2012 at 04:37 PM.
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Re: Battery Sizing - am I crazy?

Quote:
Originally Posted by Maine Sail View Post
One thing this does not take into account is the cycle life of a 12V flooded battery vs. a 6V flooded battery when put in the same deep cycling application. The 6V batteries will outlast the 12V. I find on average 20-40% more life.
Right and I don't have any numbers to quantify life. And while I've heard people say that GC's last longer I've remained skeptical for three reasons:
1) It's said that because they have less cells the plates are thicker. Fine but you could realize the same benefit by moving to larger 12V batteries. I've never seen/heard evidence of that.
2) The weight numbers I just ran through came out nearly 100% identical energy density by weight for the GC vs the 12V. That suggests, though doesn't prove, identical technology internally. (The volume numbers differed but volume factors in non-functional aspects like case design).
3) The applications are largely the same - why would Deka engineer better cycle life into their GC batteries than their marine batteries.

So do you have any data to point too on this?

Quote:
Originally Posted by Maine Sail
You also chose to compare weakest GC2 battery Deka makes at 215Ah/.. They also make that same case size in a 230Ah and 235Ah battery.
Good point. I didn't notice that..If I have a chance I'll change my numbers previously. Obviously that helps the GC a lot in terms of volume. The weight of those larger capacity batteries does go up however.

Last edited by asdf38; 11-20-2012 at 04:46 PM.
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Re: Battery Sizing - am I crazy?

Quote:
Originally Posted by asdf38 View Post
Right and I don't have any numbers to quantify life. And while I've heard people say that GC's last longer I've remained skeptical for three reasons:
1) It's said that because they have less cells the plates are thicker. Fine but you could realize the same benefit by moving to larger 12V batteries. I've never seen/heard evidence of that.
Actually you can't the plates are still thinner, on average 0.040 - 0.065 vs. .100 - .125, and footprint is again one of the biggest limiting constraints.

Quote:
Originally Posted by asdf38 View Post
2) The weight numbers I just ran through came out nearly 100% identical energy density by weight for the GC vs the 12V. That suggests, though doesn't prove, identical technology internally. (The volume numbers differed but volume factors in non-functional aspects like case design).
Again, energy density is a totally meaningless figure because it does not take life cycles into account. You can have a 100Ah battery rated at 200 cycles and a 100Ah battery rated at 2000 cycles... No comparison..


Quote:
Originally Posted by asdf38 View Post
3) The applications are largely the same - why would Deka engineer better cycle life into their GC batteries than their marine batteries.

So do you have any data to point too on this?
Actually the "applications" are not the same. For years the marine industry has used "hybrid" dual purpose style batteries that by true deep cycle definition are not really deep cycle batteries at all. They do make true deep cycle 12V batteries but your not buying them at WM or most marine stores.. Do they work? Sure, are they as optimal as 6V or 2V cells for a marine deep cycle application in terms of longevity, absolutely not..

So as for data yes I have hundreds of boats I work on, in the "real world", that show a 20 - 40% longer average life with 6V vs. 12V. I use the best analyzers money can buy, one is $2000.00 and the other was about $800.00 plus I have many other means of testing too. My Midtronics tool is the one battery manufacturersuse and supply to distributors like West Marine, Wal*Mart & Sam's Club for warranty claims.

Beyond that data you can always call Trojan and ask them for the cycle life at 50% DOD for the marine SCS series of 12V batteries vs. the Golf Car line. What they will tell you is that the SCS at 50% DOD is rated by Trojan at 600 cycles and the golf car batteries like the T105 are rated at 1200 cycles!!! Of course these are "lab" numbers and you'll never see that in the "real world" but you will see longer life out of 6V golf cart batteries vs. 12V in the same application.

Again that is DOUBLE THE CYCLE LIFE, per Trojan battery.... So yes the thicker plates, case design etc. do make differences. Remember that 6V battery has 3 cells and weighs 66 pounds vs. the 12V battery having 6 cells and weighing the same. Which battery has the thicker plates the one with 3 cells or the one with 6 cells?

I don't see the 50% differences in the "real world" but I do see 20-40% longer life "on average" out of true deep cycle golf car batteries vs. 12V "marine style" batteries.I measure this with industry standardized test equipment so this is not just a shoot from the hip thing... The differences are real, quantifiable and measurable differences and also backed up by companies like Trojan battery in their own in-house testing.

You can choose to believe or not... I see, touch, feel & measure so I believe....
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