12V vs 6V AGM

brulaz · July 2014

This has probably been discussed but Search isn't doing it for me. So a link(s) would be fine.

My trailer has two 6V, 216Ah, FLA golf carts, and I want to add another, switched battery bank inside the trailer. AGM's sound like just the thing. More FLAs outside on the tongue or elsewhere is not possible.

In the FLA world, 6V deep-cycle golf carts seem to have it over the 12V marine, dual-purpose, group 24, 27,29 31's. At least for most people. So that's what I got.

But does the same hold for AGMs?

Sam's Club is selling Duracell AGM's (East Penn/Dekka is the manufacturer, I think). The 12V group 31 (210Ah with two in parallel) and 6V GC2 (190Ah with two in series) are going for exactly the same price (US$180 ea). Buying two of each you would end up with about the same weight. The 31s would have 20Ah more, but there would be twice as many cells with the 31s, if that's an issue(?).

Over on another forum, people have had good luck with the 12V AGMs, much better than with 12V FLAs, and so they didn't hesitate to recommend the Duracell 12V AGM, Group 31, in part because the bank would have 20Ah more than with the two 6V AGMs.

What say you?

Also, up here in Canadian Tire country, there's the Canadian Tire Nautilus AGM 31 with 100Ah, rebranded Exide Nautilus Megacyle 200 I believe. But no mass-market 6V AGMs that I can find. Another possible 31 AGM?

More expensive AGMs are available (like Lifeline), but right now only the mass-market AGMs work for me economically.

Cariboocoot · July 2014

Re: 12V vs 6V AGM

You know why Canadian Tire sells so many batteries? They're open on Sunday.

Anyhow, as nice as the "FLA's outside, AGM's inside" idea sounds it is not so simple. The two battery types will not have identical charging parameters, so they could not be charged from the same source (without adjusting every time).

How much do you want to mess around?

brulaz · July 2014

Re: 12V vs 6V AGM

Cariboocoot wrote: »

...
Anyhow, as nice as the "FLA's outside, AGM's inside" idea sounds it is not so simple. The two battery types will not have identical charging parameters, so they could not be charged from the same source (without adjusting every time).
...

Ya, I know, it's a pain. But really have no choice weight wise. My trailer's tongue weight is already pushing the truck's limits. Will probably get a separate solar controller for the AGMs. Cheaper than a new truck.:cool:

Anyway, thinking again about the AGM 6v vs 12v, I keep coming back to the 12v batts having the same weight and volume, but twice the # of cells as the 6v. So the 6v plates must be about 2x as thick (increased longevity?), but the 12v has twice the plate surface area (increased CCA?).

Maybe these differences are not really important for mass-market AGMs in the typical RV application. I dunno, guess I'm still leaning toward the 6Vs even though they're 20Ah less. (Will have to wait until we get to a Sam's in Florida though.)

AuricTech · July 2014

Re: 12V vs 6V AGM

The general consensus around here is that it's best to avoid parallel connections to the maximum extent possible, which would support your leaning toward the 6V solution.

inetdog · July 2014

Re: 12V vs 6V AGM

brulaz wrote: »

Anyway, thinking again about the AGM 6v vs 12v, I keep coming back to the 12v batts having the same weight and volume, but twice the # of cells as the 6v. So the 6v plates must be about 2x as thick (increased longevity?), but the 12v has twice the plate surface area (increased CCA?).

Keep your comparisons matched at all times, otherwise you will end up comparing tw0\o 6V batteries to one 12V battery or vice versa.

1. For your usage you do not care about CCA, which is increased by multiple thin spongy plates. If a battery quotes CCA, then it is not a deep cycle battery.
2. Suppose that both the 6V and 12 batteries are equivalent deep cycle designs (probably not completely because of the difference in watt hours per battery between the two models.)
Then each 12V battery has exactly the same total plate area as the 6V but spread over twice as many cells.
Each cell in the 6V has twice the plate area of one cell in the 12V, but since the two 12V will be in parallel, you can consider that each cell of the 6V corresponds to two cells in the 12V pair.
3. Twice the number of caps for FLA would be a maintenance factor, but irrelevant for sealed AGM.
4. Simpler wiring for the 6V in series. But for 12V using opposite diagonal, not really an issue except that it requires one more jumper wire total.

brulaz · July 2014

Re: 12V vs 6V AGM

inetdog wrote: »

Keep your comparisons matched at all times, otherwise you will end up comparing tw0\o 6V batteries to one 12V battery or vice versa.

Yup, talking two 6V in series or two 12V in parallel

1. For your usage you do not care about CCA, which is increased by multiple thin spongy plates. If a battery quotes CCA, then it is not a deep cycle battery.
East Penn, the maker of Duracell AGM sold by Sam's Club, publishes CCA for all its AGM batteries, 24-31, 4D,8D and GC2. They claim they are all Deep-Cycle and market them in Canada, at least, as their "Solar" brand: "Solar Voltaic Batteries for Renewable Energy Applications"
2. Suppose that both the 6V and 12 batteries are equivalent deep cycle designs (probably not completely because of the difference in watt hours per battery between the two models.)
Then each 12V battery has exactly the same total plate area as the 6V but spread over twice as many cells.
Each cell in the 6V has twice the plate area of one cell in the 12V, but since the two 12V will be in parallel, you can consider that each cell of the 6V corresponds to two cells in the 12V pair.
So you're saying there's no difference in thickness/plate, but in area/plate. For example, perhaps 12V AGM cells take up only half the battery width, whereas 6V cells take up the full battery width. OK I hadn't considered that possibility. And really, if that's the case, there would be little fundamental performance difference in the two batteries.
3. Twice the number of caps for FLA would be a maintenance factor, but irrelevant for sealed AGM.
4. Simpler wiring for the 6V in series. But for 12V using opposite diagonal, not really an issue except that it requires one more jumper wire total.

Thanks for that. You've now got me leaning toward two 12V with the extra Ah.;)

brulaz · July 2014

Re: 12V vs 6V AGM

Cariboocoot wrote: »

...
The two battery types will not have identical charging parameters, so they could not be charged from the same source (without adjusting every time).
...

Actually, maybe I could use the same charging specs for both battery banks: US2000 GC2 and EastPenn (Duracell 31?) AGMs

Found some charging specs for both Lifeline and East Penn AGMs. Both say 14.3V absorb at 75C. But East Penn adds that 14.3 is optimum and 14.6V is the max. My US Battery GC2s want 14.75V absorb at 75C. So maybe I could get by with a single controller at 14.5V? Not charging the two banks at the same time, but just with the same parameters.

Lifeline says to continue absorb voltage until amps reach 0.5% of rated capacity. East Penn says nothing that I can find. US Battery gives totally different instructions (down to 3% of rated then hold for 2-3hrs) which my controller doesn't do. So I'm currently doing the GC2's at the absorb V down to 1% of rated capacity as controller manual suggests. Maybe 0.5% would be better, especially if I reduce the absorb V to 14.5. The two banks would have about the same Ah, so that would work out.

US Battery also gives the exact same bulk and absorb charging instructions for both its FLA and AGM batteries. But the float voltages are different: 13.4V AGM, 13.1V FLA. East Penn and Lifeline AGM also give higher float V: 13.4 and 13.3V at 75C.

But when camping and actively using the batts, I raise the float for the golf carts to 13.5V anyway, because the solar controller doesn't go into mppt/bulk until battery V drops to 0.5V below float. 13.4V would be fine too. Only in long-term storage do I reduce float to 13.1V. Looks like I would have to come up with a separate storage solution for each bank if I don't want to dry out my golf carts at 13.4V.

Why do AGMs require so much float? Thought they had low resistance and little self-discharge. Or maybe it's the US Batts that have a low float? The controller's default float is 13.6V.

Equalization is another issue, and as I do it manually anyway, when necessary, any difference between banks is not that important.

brulaz · July 2014

Re: 12V vs 6V AGM

inetdog wrote: »

...
2. Suppose that both the 6V and 12 batteries are equivalent deep cycle designs (probably not completely because of the difference in watt hours per battery between the two models.)
Then each 12V battery has exactly the same total plate area as the 6V but spread over twice as many cells.
Each cell in the 6V has twice the plate area of one cell in the 12V, but since the two 12V will be in parallel, you can consider that each cell of the 6V corresponds to two cells in the 12V pair.
...

Just realized that all 12V 31s have the 6 pressure release (or fill) caps strung out in a row along the length of the battery.
Only the 12V 4D and 8D have two rows of 3 caps. It's this 2 rows of 3 cells that I thought you were saying the 31s had. Doesn't seem to be the case.

So if the group 24-31s do have 6 cells in a single row along the length of the battery, unlike the 4D and 8Ds, I don't see how the plate area could be the same as a 3Cell, 6V battery of the same dimensions and weight. If the layout of 6 cells was like that of 8D and 4D (2 rows of 3 cells), yes, I can see it.

So I'm back to the 12V 31 having twice the plate area but half the thickness of the 6V with same dimensions and weight.

inetdog · July 2014

Re: 12V vs 6V AGM

Well, the plate area per volt of the 6V battery will be twice that of the 12V battery, but you need two of them in series to get the same number of watt hours that you get for two 12V in parallel.

brulaz · July 2014

Re: 12V vs 6V AGM

Thanks, that helped. Maybe I get it now. Somehow was thinking there's only one plate per cell. But there's actually quite a few. One brochure has a picture with what looks to be 8 plates/cell.

So if there's, for example, 8 plates/cell in 6V battery (with one row of 3 cells per battery), but only four identical plates/cell in a 12V battery (with one row of 6 cells), both batteries of the same dimensions and weight, then you would have same number of identical plates/battery (24), each battery with the same plate surface area, approx. same weight and dimensions. And the same applies to different banks, one with 2 6V in series and the other with 2 12V in parallel. The two banks should be approx. identical.

Maybe a pair 12Vs with their slightly more Ah are a better deal. (Appreciate everybody's patience as I go back and forth on this)

BB. · July 2014

Re: 12V vs 6V AGM

The downsides:

2x more cells to check water level/specific gravity in.
For parallel battery connections, I highly recommend a DC Current Clamp DMM (like this inexpensive one).
You should wire per Smart Gauge's suggestions for balanced current flow.
If you wire like Smart Gauge suggests, pick a way that allows you to measure per battery current flow with clamp meter.
There is a slightly higher risk of battery bank failure with more parallel connections.
If you have more than two parallel strings, you should consider a fuse/breaker per parallel string connection ($$$).
Personally, I would suggest no more than three parallel string connections unless you have to.
Parallel strings tend to "hide problems" if you are not doing weekly/monthly inspections with your Current Clamp DMM.
There is a possibility that strings can become unbalanced (a hot string, lower charging voltage, gets hotter, more charging voltage yet).
I like to check individual battery/cell voltage levels weekly/monthly to find early problems. If you have 12 volt batteries in parallel for a 12 volt bank--You cannot check for battery voltage problems (all batteries are in parallel and read the same voltage).

The upsides:

Your selection of battery models/prices may be limited--You have what you have for best price/performance.
Sealed AGM batteries do not require per cell water level maintenance.
Some folks like two parallel strings--If one battery/string fails, disconnect it and the other string(s) will allow you to limp along.

None of the above are absolute reasons for A or B type connections. You can make either work well for you. In my humble opinion, single strings are easier to monitor/maintain. And banks with up to 3 parallel strings are OK (to a bit of a pain) to maintain and reasonably reliable.

My personal opinion--I would much prefer two 6 volt batteries in series vs two (or more) 12 volt batteries in parallel (for a 12 volt bank). Battery problems sneak up on you and because you cannot measure individual 12 volt battery voltages in parallel 12 volt strings--It makes quick voltage checks impossible. You are always looking for "differences" among your bank of "identical batteries". If you find a difference, you need to figure out what that difference is caused by.

And, even if you do the single string system--A DC Current Clamp DMM is still a nice tool to have in your bag of tricks. Makes debugging/finding problems a lot easier and safer (vs having to disconnect wires to find current flow problems).

-Bill

stmar · July 2014

Re: 12V vs 6V AGM

I had 4 12 volt AGMs for years, they were 256 ah which gave me 512ah @ 24 volts. I recently replaced them with 4 6 volt 400ah AGMs. Less ah but I am on a reliable grid so can go with a smaller simpler battery bank. So far I think I made the right choice, will find out in the years to come.

12V vs 6V AGM

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