Why 2 x 6V battery over a single 12V?

RybrenRybren Solar Expert Posts: 351 ✭✭
I often see recommendations to use multiple 6V batteries in series rather than a single 12V (or multiple) batteries to build the battery bank, but I've never seen a good explanation for why (aside from the possible cost).

For example, if I wanted a 200Ah 12V bank, I could use 2 Crown CR200 6V batteries - $149ea locally or a single Crown CR210 12V - $349 (they don't have a 12V 200Ah)

Aside from the cost, what is the advantage to using the 6V batteries? There'd be a cost to connect them in series, and I'd have to check & test two batteries. What am I missing?(or perhaps I've misunderstood and there is no advantage :blush: )

I'm not ready to buy anything yet (still working on the AIM) but I am trying to improve my knowledge and basic understanding.

Thanks

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Why 2 x 6V battery over a single 12V?

    This might help clear things up for you:
    http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power

    It's about 12 Volt vs. 24 Volt vs. 48 Volt systems, but the basic principal is the same: having a single string of batteries rather than parallel strings of equivalent power.

    So with the 6 Volt batteries vs. 12 Volt, putting two 200 Amp hour 6 Volt batteries in series is preferable to putting two 100 Amp hour 12 Volt batteries in parallel.
  • BB.BB. Super Moderators, Administrators Posts: 32,741 admin
    Re: Why 2 x 6V battery over a single 12V?

    Here is a thread that goes "oh too deeply for sane people" into why I, and others, suggest trying for a single string of batteries, and never more than ~3 strings in parallel unless you have no other options:

    Series rule of thumb


    Note, there are people with lots of parallel strings of batteries and very happy with them... However, in my humble opinion, lots of paralleled batteries require a fair amount of extra costs (fuses/breakers per string), work (more cells to check/add water), and monitoring (a DC Current Clamp Meter/DMM measuring current sharing per string and voltage per cell/battery when under heavy charge/discharge), etc...

    For folks outside the US/major population centers, their battery choices are severely limited and they have few other options than lots of paralleled connections.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Why 2 x 6V battery over a single 12V?
    Rybren wrote: »
    I often see recommendations to use multiple 6V batteries in series rather than a single 12V (or multiple) batteries to build the battery bank, but I've never seen a good explanation for why (aside from the possible cost).

    For example, if I wanted a 200Ah 12V bank, I could use 2 Crown CR200 6V batteries - $149ea locally or a single Crown CR210 12V - $349 (they don't have a 12V 200Ah)

    Aside from the cost, what is the advantage to using the 6V batteries? There'd be a cost to connect them in series, and I'd have to check & test two batteries. What am I missing?(or perhaps I've misunderstood and there is no advantage :blush: )

    I'm not ready to buy anything yet (still working on the AIM) but I am trying to improve my knowledge and basic understanding.

    Thanks
    In the battery you mention there would be no difference in buying one 12 v or two 6 v, unless the weight ( 131 LB ) of one makes it hard to handle. It is true you have to have a connector cable for the 6 V. Most that talk about a 12 V are talking about a " Wal-mart Marine / Deep Cycle " battery that will NOT hold up under Solar Charging and Discharge.
  • BB.BB. Super Moderators, Administrators Posts: 32,741 admin
    Re: Why 2 x 6V battery over a single 12V?

    BC04 brings up another key point--True Deep Cycle vs "Marine" vs SLI (Starting, Lighting, Ignition) type car batteries. Marine and SLI batteries are not very good at supporting deep cycle (typical off grid power) systems. This is not a 6volt/12volt battery issue--it is the internal design and external wiring/cabling/maintenance issues that tend to favor a single string of high AH batteries vs a lot of "higher voltage" batteries connected in parallel.

    Physical weight of battery bank is a limiting factor for many people. Not everyone has a concrete loading area and a setup for a crane/forklift/pallet jack to move 2,000 lb batteries around. The "solution" is usually finding lower voltage (6 volt, 4 volt, 2 volt cells) with high AH ratings that are still reasonable weight to move around by hand (150-300 lbs maximum for a couple people) and connect in series. And raise the bank voltage (12 to 24 to 48 volt) to keep the AH size down (i.e., a 220 AH battery bank at 48 volts has 4x the energy storage of a 12 volt 220 AH battery bank--and is much "easier" to support than an 880 AH @ 12 volt battery bank with the 4x higher current needed to support charging/loads).

    Read up on Batteries--Lots of information, and opinion (sometimes the old way of doing things still make sense, and others are actually dead wrong):

    Battery FAQ
    www.batteryfaq.org

    After I read the above--I am about convinced that lead acid storage batteries do not work for solar applications :p.

    Obviously they do, and are one of the best choices (cost, reliability, ruggedness, ability to recycle, etc.) of battery chemistry out there.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RybrenRybren Solar Expert Posts: 351 ✭✭
    Re: Why 2 x 6V battery over a single 12V?

    Guys thanks for the quick and informative replies.

    Blackcherry04 hit on what I was trying to get at - is there an advantage to using 2 x 6v versus a single 12V to obtain the same amount of capacity (200Ah in my example above). I guess that the short answer is that there isn't a performance advantage.

    Thanks
  • BB.BB. Super Moderators, Administrators Posts: 32,741 admin
    Re: Why 2 x 6V battery over a single 12V?

    A single 12 volt @ 220 AH battery would weigh ~2x as much as a single 6 volt @ 220 AH battery--So it is your ability to move the battery(ies) around that would be the limiting factor here (and total costs).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • RybrenRybren Solar Expert Posts: 351 ✭✭
    Re: Why 2 x 6V battery over a single 12V?

    Thanks Bill.

    I guess that I was reading too much into all of those battery threads, FAQs, etc.
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Why 2 x 6V battery over a single 12V?
    Rybren wrote: »
    Thanks Bill.

    I guess that I was reading too much into all of those battery threads, FAQs, etc.

    Yep. The other practical difference comes when the AH requirement is so high that you cannot find a reasonable 12 volt battery to supply it and would have to go to putting 2 or more 12 volt batteries in parallel. That would be a big performance problem, in terms of equal sharing of charge and discharge current, etc. So by making the bank up of a series string of higher amp hour, lower voltage batteries you avoid the need to parallel wire.

    This is a different region of size than you are currently operating in.
    SMA SB 3000, old BP panels.
  • VicVic Solar Expert Posts: 3,162 ✭✭✭✭
    Re: Why 2 x 6V battery over a single 12V?

    An additional point, is that when/if one cell fails, the fewer the number of additional cells that would need to be tossed when replacing that battery, the better.

    Some larger batteries have removeable cells that are bolted together, which also helps. IMHO, the fewest number of cells per battery, the better. 2 V batteries are very nice.

    This is a bit beyond the original question, but just belaboring the point, while throwing in a few misspelled words.
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  • MikeekimMikeekim Registered Users Posts: 5
    Re: Why 2 x 6V battery over a single 12V?

    old thread, but the previous poster alludes to the redundancy issue. when off the grid running 12VDC draws, if one cell goes bad in one member in a serial 2X6V setup, you're 12VDC powerless. if one cell goes bad in one member in a paralleled 2X12V setup, you've still got some power available at 12VDC. i'm assuming you're in the field somewhere, and repairing or replacing the failed 6VDC battery/cell isn't an option.
  • BB.BB. Super Moderators, Administrators Posts: 32,741 admin
    Re: Why 2 x 6V battery over a single 12V?

    There are even other options too... We had one person here with a large 48 volt battery bank made from 2 volt cells in series.

    He had one 2 volt cell fail. Just took it out of the string and reprogrammed the charge controller one cell lower until he could get a replacement.

    Parallel strings are a bit of a pain for maintenance. Many more cells to check, more wiring to keep clean. Should have a fuse/breaker per string (more expense), more cells--therefore a greater chance that "something" will fail, etc...

    My own personal recommendation is to have backups... Generally, that would be a fuel driven genset (or two). But, if you need a backup battery bank--Then going high voltage with 2 volt cells, or limiting the number of parallel strings to 2-3 parallel strings maximum--both to reduce maintenance/things that can fail/expenses, and because current sharing for Voltage Sources (e.g., batteries) can be poor.

    Get a "cheap" (or expensive) DC current clamp meter (as well as a hydrometer for flooded cell batteries plus a Battery Monitor of some sort) and use it to monitor shared current (during heavy charging and discharging) to ensure that the batteries are properly sharing the loads (once a week to once a month).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • MikeekimMikeekim Registered Users Posts: 5
    Re: Why 2 x 6V battery over a single 12V?

    re: backups, yes, always. i have a small RV with 2 large 12V AGMs for coach power, and a small generator as a backup. i've been toying with the idea of upgrading the whole package with newer technology for years, but every time i travel and use what i have, and it works flawlessly, it winds up on the back burner until the next trip. and there's no guarantee the RV will outlast whatever i add, so that slows the thought process down a bit, too. my luck, i'd spend a few thousand on it, trick out the off grid power systems, and the engine would blow up the next day.
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Why 2 x 6V battery over a single 12V?

    and the engine would blow up the next day. :cry:

    well then you will have a tricked out PV system for the new coach...8)
     
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  • MikeekimMikeekim Registered Users Posts: 5
    Re: Why 2 x 6V battery over a single 12V?

    i could look at it that way, i suppose.:p
  • salah4luvsalah4luv Registered Users Posts: 1
    edited August 2017 #16
    what is the difference between using 3 X 2v 1200ah and using 6v 1200Ah. and which one is advisable

  • EstragonEstragon Registered Users Posts: 4,496 ✭✭✭✭✭
    The main advantage to the 3x2v is having a reasonable chance of being able to handle them without heavy equipment. A 1200ah 2v cell is still going to be pretty heavy, but you might be able to move them around by hand. The 6v version is just 3x2v cells preconnected.

    It may also be easier to replace a failed cell depending on how the 6v string is constructed.
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  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited August 2017 #18
    It's often better to think of cells which make up a battery, 3×2v cells would make a 6v battery, a 6v battery consists of 3×2V cells, generally speaking a 6v battery  would consist of 3 smaller cells, to form a monoblock, so using larger capacity 2V cells in series is advantageous to overcome the limitations of the smaller cells used in monoblock configuration. Using monoblock batteries generally limits the capacity to what is available, promoting some to go the parrallel route, whereas using larger cells allows series connected, which  is favorable, to obtain a desired capacity. Hope  this makes sense 
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  • Marc KurthMarc Kurth Solar Expert Posts: 964 ✭✭✭✭
    salah4luv said:
    what is the difference between using 3 X 2v 1200ah and using 6v 1200Ah. and which one is advisable

    That is like asking how long is piece of string? Well, it depends......

    What is the application? Are you talking about 12v, 24v or 48v or other systems?
    What are your expectations of the system in terms of daily performance and lifespan?

    Marc
    I always have more questions than answers. That's the nature of life.
  • maplefallmaplefall Registered Users, Users Awaiting Email Confirmation Posts: 1
    why parallel systems are not advised. If one battery fails ,it will drag the other batt down its level destroying both batts. if one batt fails in a series circuit example two 6 volt batts producing 12 volts ,the circuit will break and you can replace the faulty batt. six 2 volt cells or 2 6volt cells making 12 volts is ideal. I run 3 separate 12 volt systems with their own controllers rather than 24 volt.
  • MikeekimMikeekim Registered Users Posts: 5
    edited September 2019 #21
    If you're relying on batteries for power, say while camping in the back country, after dark, a failed 12V member of a parallel configuration can be removed from the configuration, and you've still got some power. If a series pair of 6V batteries die in a 12V configuration, you're sitting in the dark, until you get the replacement member of the pair, and most people would recommend replacing both, to balance the load across both batteries. 6V in series = no redundancy, in that scenario.
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    One significant reason, is AH per pound.  For example, 250 ah of T105s come in at ~124# for the pair in 12 vdc config.  I can handle 62# fairly easily, but 124# in one battery gets pretty heavy.  Carry that through to larger (L-16 batteries) 114# each, 228 for a 12 vdc pair. And it becomes obvious.  

    Tony 
  • EstragonEstragon Registered Users Posts: 4,496 ✭✭✭✭✭
    For flooded, another advantage to lower voltage blocks and fewer strings is also fewer cells to water, etc.

    2 x 12v =12 cells,  2 x 6v =6 cells for about the same overall weight and capacity.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    Eggs in more baskets?  I'm curious as to which of these is true:

    a) with more physically separate cells, one can often catch a problem, replace/omit that cell and get greater life from the entire battery system
    b) more cells is more failure points, reducing average life of the entire battery system

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  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    jonr said:
    Eggs in more baskets?  I'm curious as to which of these is true:

    a) with more physically separate cells, one can often catch a problem, replace/omit that cell and get greater life from the entire battery system
    b) more cells is more failure points, reducing average life of the entire battery system
    Both are true in my experience.
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  • mike_smike_s Registered Users Posts: 147 ✭✭
    Mikeekim said:
    If you're relying on batteries for power, say while camping in the back country, after dark, a failed 12V member of a parallel configuration can be removed from the configuration, and you've still got some power. If a series pair of 6V batteries die in a 12V configuration, you're sitting in the dark, until you get the replacement member of the pair, and most people would recommend replacing both, to balance the load across both batteries. 6V in series = no redundancy, in that scenario.
    Exactly when was the last time you experienced a sudden and unexpected deep cycle battery failure?
  • MikeekimMikeekim Registered Users Posts: 5
    edited September 2019 #27
    Not recently. In 2009 one of the original 12V wet cells in my camper van went bad after dark in a NP in Utah, and started to drag down the pair, lights dimming, unable to start generator, monitor panel showing battery level red (older technology). Checked cells and it was dry due to crack in casing. Removed, and still had some power overnight. Besides, it doesn't invalidate my comment.
  • mvasmvas Registered Users Posts: 384 ✭✭✭
    A shorted cell ( dendrite growth ) is sudden and unexpected.
    Also, in an RV ( mobile environment ) an open cell is sudden and unexpected.
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