Author Topic: 48 volt battery bank- 6 or 12 volt batteries?  (Read 8367 times)

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essej

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48 volt battery bank- 6 or 12 volt batteries?
« on: February 21, 2014, 12:33:58 PM »
Hi all, I'm in the planning stages of an off grid power system and was hoping to get some opinions from those of you who have been there, done that.

I've been leaning toward Trojan T-1275 12v batteries vs the T-105 or similar 6v. I contacted a local distributor to get a quote on the T-1275's and they did give me a quote ($215/ea) but are trying to convince me that I would be better off to go with 6v batteries.

I understand that the 6v batteries have higher ah capacities, but all things considered I'm not sure they would be best for a 48v system. I'll use the T-1275 12V and T-105re 6V batteries as examples in my comparisons below:

T-1275: 12V-150ah-$215

T-105re: 6V-225ah-$160

6v T-105re @ 48v=8 batteries in series=225ah=$1280=$5.68/ah
                                       
12v T-1275 @48v=4 batteries in series=150ah= $860= $5.73/ah
                                       

Comparing costs per amp hour, there's not a real significant difference. Yes, I can get a significantly larger storage capacity with 6v batteries, but that comes with a significantly higher initial cost as well as a significantly higher replacement cost... not to mention the additional cables/connections that would be required for 6v batteries vs 12v.

I'm planning to live completely off-grid, so realistically I'll be looking at at least a 450ah battery bank capacity:

12v T-1275 @ 3 series/parallel strings=450ah=12 batteries=$2580

6v T-105 @ 2 series/parallel strings=450ah=16 batteries= $2560

OK, here I could actually save $20 giong with 6v batteries,  but I still have a lot more cables/connections with the 6v vs 12v, and with the 12v batteries, I have 3 series/parallel strings vs 2 series/parallel strings with 6v... if I'd loose one 6v battery, my bank would be limited to 1 series string and my capacity would be cut to 225ah... if I loose one 12 volt battery, I still have two strings and a 300ah capacity.

Now, If I need to expand the battery bank:

12v T-1275 @ 4series/parallel strings=600ah=16 batteries=$3440

6v T-105 @ 3series/parallel strings=675ah=24 batteries= $3840

12v batteries represent a $400 savings in the above scenario, at a loss of 75ah.

I don't know, maybe I'm just trying to convince myself here, but I'm just not seeing a great advantage of 6v batteries in a 48v system... what I see more than anything is added complexity and many more possible points of failure. The sales rep said that the 6v batteries have a much longer life, now if that's true it would surely be something to consider... but how much different can they be than the 12v batteries (from the same manufacturer)?

Thoughts?

Thanks in advance,

Jesse





HiddenMountain

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Re: 48 volt battery bank- 6 or 12 volt batteries?
« Reply #1 on: February 21, 2014, 02:47:38 PM »
While far from being an expert here, I have some questions.

You don't say how you plan on filling these batteries. If you're using wind or solar, storage is a huge part of the equation. The importance of every amp hour will become apparent rather quickly when the sun isn't shining or the wind isn't blowing. Micro-hydro, on the other hand, is constant except for minor interruptions.

There are so many variables to consider here. What is your projected daily electrical consumption? Will you be needing 110AC for everything or can 12V suffice for lighting and other minor loads? Are you able to maintain your system yourself or will you need outside professionals to do that for you?

Personally, I would be leaning towards the 6V option. The extra up front cost is worth it for the extra storage capacity alone. Also, 12V batteries have 6 cells, 6V have 3 cells. If a cell goes bad you only have to replace 3 with a 6V battery vs 6 for a 12V.


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hydrosun

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Re: 48 volt battery bank- 6 or 12 volt batteries?
« Reply #2 on: February 21, 2014, 04:29:55 PM »
I would recconmend the 6 volt batteries. They have thicker plates and should last longer. And better able to handle multable deep cycle dischages without falling apart. The 12 volt batteries are not true deep cycle batteries but a hybrid between deep cycle and starting batteries.
Chris

Mary B

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Re: 48 volt battery bank- 6 or 12 volt batteries?
« Reply #3 on: February 21, 2014, 05:43:19 PM »
I went 6 volt in my 24 volt system, as mentioned 3 cell sis a cheaper replacement plus I gained a lot of AH capacity. Don't overlook the US battery 2200xc either, I paid $109 each for mine.

madlabs

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Re: 48 volt battery bank- 6 or 12 volt batteries?
« Reply #4 on: February 21, 2014, 05:46:34 PM »
As the others have said, the 6 volters are the way to go.

I bought cheap Sam's club batteries labeled "Energizer", 10 batts in series/parallel for 1200aH at 12V. I will have 6 years out of them in June. I plan to replace them this fall when I switch over to a 48V system. So at that point the cost of them will have been $13.88/month. I think that is pretty cheap and they have done well for me. I haven't always been very good about the absorption stage of charging but I try to as a rule never discharge to less than 80% SOC, although we have had a few incidents.

I've been meaning to start a thread about cheap batteries VS expensive ones. From my single data point and a few of my friends it seems like my batteries cost me less a month that if I had bought Surrettes or something that are supposed to last 10 years. I also feel sure I could get another year if I wasn't changing to 48V and that they would have lasted longer if properly charged.

JOnathan

essej

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Re: 48 volt battery bank- 6 or 12 volt batteries?
« Reply #5 on: February 21, 2014, 06:52:26 PM »
Thanks for the input everyone, it's much appreciated. Since my original post here a few hours ago I've done a little more digging on Trojan's web site and I see that the 6V T-105RE's are the "premium" grade vs the "signature" grade 12V T-1275's I was looking at... the difference is 1000 more cycles/life at 20% DOD and 400 more cycles @ 50% DOD... now that is significant and it does look like 6V would be a better value (just like everyone told me, go figure ;))

I've got a lot of details to work out yet, but to answer some questions my preliminary plans are as follows:

*2kw solar panels (8 @ 250W)
*1kw(minimum) wind turbine(s)
*3-4kw home brew automatic propane powered back up generator (will start when battery bank drops to a certain level, stop when the bank's fully charged)
*450ah battery bank
*48V dc- 240v ac psw inverter(s) (still debating on which one exactly)

I figure that's all a good place to start  ;D I've got some idea but I really don't know exactly what my daily electrical needs will be, it's difficult to compare my current usage to what I will use when I move out of my present  home into my future one. As it is now, my old house has an electric range, electric water heater, electric clothes dryer, a couple 1.5 kw heaters running 24/7 in the winter, 5kw electric garage heater, too many incandescent light bulbs, an older refrigerator, etc, etc... needless to say, I need a lot more kwh/day now than I will in the future. I plan to cut my electrical consumption drastically; a gas range, tankless gas water heater, gas clothes dryer, and gas furnace will go a long way there... that leaves a refrigerator, well pump, furnace fan, lights, tv, computer, and other misc. things that won't be needed all day everyday; clothes washer, microwave, etc... it should all be manageable... well, it's going to have to be. I plan to design the entire system to be expandable if needed, we'll see I guess. A $15k+ estimate from the utility to install gas and electric service (an expenditure with no hope of any "payback", only more expenses) gives me pretty good incentive to take the plunge and go completely off grid.

Jesse
   

ChrisOlson

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Re: 48 volt battery bank- 6 or 12 volt batteries?
« Reply #6 on: February 22, 2014, 01:41:51 AM »
I don't know, maybe I'm just trying to convince myself here, but I'm just not seeing a great advantage of 6v batteries in a 48v system... what I see more than anything is added complexity and many more possible points of failure. The sales rep said that the 6v batteries have a much longer life, now if that's true it would surely be something to consider... but how much different can they be than the 12v batteries (from the same manufacturer)?

There is an advantage in the number of cells you have to service.  Using 12V batteries is best if you don't mind having lots of cells to service (less series and more parallel connections) and want to be able to run your inverter at full load on straight battery power with minimum voltage sag.  Using 6V batteries will provide less cells to service but the battery bank won't have the amp-delivering capacity to power your inverter without severe voltage sag.

There is no difference in the plates in the two batteries you mention.  Both are toy batteries and not suitable for off-grid systems.  They're golf cart batteries and are notoriously short-lived on off-grid duty.  They might be OK for an off-grid weekend cabin, or a hybrid grid/off-grid system.  But for full-time off-grid they won't last more than 4-5 years before they're down to less than 50% of their original capacity.

You don't get into batteries with decent plates that are suitable for a real off-grid installation until you get into the L-16 size, and even then the plates are pretty light duty, usually around .150-.180" negatives.  If you want a real off-grid battery that you can actually pull down to 80% DoD repeatedly without hurting it, buy a forklift battery with 1/4" plates in it.  If you want even better yet buy Rolls-Surrette 5000's with .265" plates.

XeonPony

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Re: 48 volt battery bank- 6 or 12 volt batteries?
« Reply #7 on: February 22, 2014, 10:26:59 AM »
As said above the 2200XS's are a great starter battery, but for a real serious bank you want 2V cells with a bms and they will be 600 dollars a pop, but worth it as with a bms and good care will out last you! pretty well.
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ChrisOlson

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Re: 48 volt battery bank- 6 or 12 volt batteries?
« Reply #8 on: February 22, 2014, 12:31:30 PM »
As said above the 2200XS's are a great starter battery, but for a real serious bank you want 2V cells with a bms and they will be 600 dollars a pop, but worth it as with a bms and good care will out last you! pretty well.

You can buy individual 2V cells but forklift batteries or Surrette 5000-series are a better buy per kWh.  The forklift battery is merely individually replaceable 2V cells in a steel container.  The Surrette 5000's are individually replaceable 2V cells in a polypropylene container.  A 48V forklift battery will weigh in the neighborhood of one ton for a small one.  The Surrette 5000's weigh about 400-500 lbs each so they're a little easier to move without overhead lifting equipment.