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Thread: Batteries series parallel vs series for longevity

  1. #1
    Arkansas Tom Guest

    Default Batteries series parallel vs series for longevity

    Hi, I was in the process of ordering 4 Concorde sun extenders today size 8D. They are 12 volt batteries and I was planning on running them series parallel for a 24 volt battery bank. The company I was ordering from recommended I re think that plan. I was told that in their experience a system with batteries run in series only usually see about twice the life over systems with batteries running parallel in it.

    In my case they recommended I run 12 Concorde 2 volt batteries. Any one have any comments on this advice.

    For any that are interested these are the 2 volt batteries -

    PVX-5040 Concorde 2 Volt, 504 Amp Hour, valve regulated sealed lead acid battery. 57 LBS


    PVX-6480 Concorde 2 Volt, 648 Amp Hour, valve regulated sealed lead acid battery. 65 LBS



    Tom

  2. #2

    Default Re: Batteries series parallel vs series for longevity

    Tom,

    The company’s recommendation is valid, but I think it’s a stretch.

    A bank of a batteries wired in series will have all charge- and discharge current running through the one series. That’s a good thing.

    A bank of batteries wired in parallel will “share” the current among the batteries. Due to manufacturing differences, connection quality and other factors, some batteries will receive more current and others less. Neither is a good thing.

    However, this usually isn’t a problem in a well connected and maintained bank of quality batteries with a low number of parallel circuits. Two, three and four parallel paths are not uncommon.

    In your case, four 8D batteries wired in series/parallel to make 24 V x ~500 Ah will work fine. Just use good interconnection cables between the batteries, and make sure you connect your main battery cables at electrically opposite corners of the bank (i.e., the (+) cable to the positive terminal on series pair #1, and the (-) cable to the negative terminal on series pair #2). This technique provides for essentially equal current paths for each series pair.

    This is how my four 4D AGM batteries are now connected, and I expect for them to do fine.

    HTH,
    Jim / crewzer
    120618: System off-line for a while...

  3. #3
    Join Date
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    Default Re: Batteries series parallel vs series for longevity

    there are disadvantages technically to both. on the one hand paralleled batteries can develope a difference to one another, for the lack of a better word, because of the way the batteries may be fed. they are fed via heavy gage wires with connections. one series string may have a slight difference in the electrical resistance of the other through differing lengths of wires or differing connection resistances. these are to be made as small as possible in order to minimize this as over time this causes one to be more favored and used than the other. how can this be you may ask? it's simple as each battery has an internal resistance that is very low. the more capacity the lower this resistance. example: 12v 100ah battery. 12/100=.12ohms and for your battery 12/255=.047ohms. now wires have resistance at the common temperature of 20 degrees c. this is the standard temperature and is for 1000 feet of the gage of wire in question. that means if a 1000ft resistance is .003ohms then 1ft is .003/1000=.000003ohms. as the temperature is increased however the resistance also increases. 2 main things tend to influence this temperature and they are the ambient or room temperature and passing a current through the wire as the resistance causes power to be dissipated as heat. solar radiation can increase the temperature too, but batteries and their wires usually aren't in the sun. you can get a condition that when the wire can't rid itself of this heat as fast as the heat is produced causing it to produce steadily more heat by the resistance dissipating more power. this continues until something gives and is termed thermal runaway. i'm starting to runaway here too by writing a book. anyhow, with the parallel batteries the additive resistances(because the wire resistance is in series with the battery resistance) may differ with each battery before the common points to each battery. connections are equally subject to resistance variations so good heavy wires along with good heavy connections are good in insuring long battery lives. anything that will preserve or create balance to all cells involved (in this case batteries) preserve battery life.
    as to the series arrangement the wires and connections still do as in the parallel arrangement in creating resistances and a series imbalance, but between the cells and although it is not as pronounced as the parallel arrangement, it has more connections and more interconnecting wires thus reducing the capacity of said cells in a battery though more in balance than parallel. differences in individual cells/batteries cause series imbalance. the series cells are good in preserving this balance, but can be more expensive to buy individual cells and if doing seriesed batteries or cells i'd recommend switching them around once in a great while to allow a more even balance as outer cells tend to take more of the brunt than inner cells.
    overall either way is fine and to do parallel arrangements is not that unusual, but go extra the for those wires and connectors to help preserve the balance.
    another way is the use of a bussbar system, but even then make everything even and extra heavy. in any case, series or parallel, when one cell is damaged or gone in the battery bank unless you can do without that battery the cell is in or do without the cell with individual cells your entire bank is now shot as it is not good to mix batteries in any way other than the same original batteries in the bank. a different type battery will setup differentials causing premature failures along the lines of the balance i was refering to. even if you have a bank of batteries(all the same) and one battery goes bad and you replace it with another battery(same kind, but just newer) this too sets up differentials as the old batteries are technically different than the new one. this means the new one won't be any better than the old ones it's placed in with and in any battery bank it will only be as good as the weakest battery or cell in it.
    my conclussion for you is the individual cells can do better for you in lifespan, but may be more expensive or difficult to work with. you weigh which is better for you because paralleled systems can last a long time done correctly.
    i see my buddy crewzer has posted at the same time and we do concur. i just got carried away.
    NIEL

  4. #4
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    Default Re: Batteries series parallel vs series for longevity

    Probably several pros and cons here (speaking as an engineer--not as somebody with a lot of experience with these batteries)...

    Series Parallel of 12 volt batteries:

    Pro:
    • Fewer battery interconnects for you to maintain. Easier wiring.
    • If one battery/cell fails, you still have 1/2 a bank to run on until replacement arrives


    Cons:
    • If one cell dies, have to replace 1/4 of your battery bank.
      Batteries need balanced current flow for proper charging and discharging--Series Parallel could favor (or deny) a weak cell in part of string--leading to over/under charging of just a portion of a bank.
    • ~161lbs per battery--bit more difficult to service.


    Series of 2 volt cells:

    Cons:
    • Many more battery interconnects to make and maintain.
    • If one cell dies, may have trouble running bank until replacement arrives.


    Pros:
    • If one cell dies, only have to replace 1/12 of battery string.
    • All cells get identical charging and discharging currents. Never out of balance with weak cell or other wiring/balance issues.
    • ~65lbs per cell. Easier to move around.


    IMHO, I would probably tend towards the single string of series 2 volt cells if price of 12 volt bank is not much different. Also look at which type(s) are available locally from stock to determine how quickly you can bring a replacement on-line.

    If the company has good records of replacements based on series/parallel installations and finds series last 2x longer--would be a difficult data point to ignore.

    And others have already posted similar comments as I was typing...

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  5. #5
    Join Date
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    Default Re: Batteries series parallel vs series for longevity

    "Pros: (of the 2v cells in series)

    *If one cell dies, only have to replace 1/12 of battery string.
    *All cells get identical charging and discharging currents. Never out of balance with weak cell or other wiring/balance issues.
    *~65lbs per cell. Easier to move around."

    the second sentence of the second asterisk can be missleading as those weak batteries or cells will drag down the others to equal it. i had replyed recently to somebody that thought by inserting 1200ah cells between 1000ah ends it would extend the lifespan. this does cause an imbalance of sorts and draws things downward in performance.
    as i stated individual cells can be pricy, but if the price where one that it was equal for each method i would pick the series. your wiring and connections in both are very important in any setup and we can all agree upon this.
    NIEL

  6. #6
    Arkansas Tom Guest

    Default Re: Batteries series parallel vs series for longevity

    Thank you all for the well thought out replies.

    Tom

  7. #7

    Default Re: Batteries series parallel vs series for longevity

    Quote Originally Posted by Arkansas Tom
    ...The company I was ordering from recommended I re think that plan. I was told that in their experience a system with batteries run in series only usually see about twice the life over systems with batteries running parallel in it.
    Tom
    I would suggest that company does not have much experience then.*

    We see no difference either way - but we have only been around for 30 some years, so what do we know.

    The only time parallel can be a real problem is when the loads are way out of balance - and that is usually easily fixed by using the right size cables to cross connect, and tying the inverter or other heavy loads to opposite corners for load balance.

    Most of the problems that we have seen with large parallel banks stemmed not so much from being in parallel, but because over time some connections would get high resistance. That only happens in flooded batteries though, where you get corrosion.
    Northern Arizona Wind & Sun Forum & Website Administrator

  8. #8
    Kristofer Guest

    Default Re:Batteries series parallel vs series for longevity

    Although I am presently tinkering with a portable RV sustem, this topic is of future interest to me, since I have some remote mountain property that I may want to "improve" some day. For stationary domestic power PV systems, the issue of wiring, especially as the price of copper drastically escalates, is significant. Running current thru and from the PV panels in and thru the battery array, and on to the inverter, requires much heavier conductors for lower voltages because of the amperage, so it seems that it would behoove one to go with as high a voltage as practical, say 48v as opposed to 12v. Assuming one can more easily and quickly connect and rotate a bank of 12v batteries versus 6v or 2v, and the cost and complexity of the termination is thus reduced, doesn't the overall picture more or less favor the combined series parallel approach to acheiving a target capacity? Are the battery companies making these recommendations so that you will spend more money on their products? I realize the topic of this thread is "longevity", but the overall cost of that increased longevity could negate the "benefit"...

    K.D.

  9. #9
    Join Date
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    Default Re: Batteries series parallel vs series for longevity

    Generally, carrying heavy currents are a pain in the butt... So, higher voltages are almost always better for wiring and efficiencies.

    I an not the battery expert, but 12 volt batteries are supposed to be better at preventing corrosion between cells than having external jumpers...

    Be that as it may, vary large batteries are probably going to be single 2 volt cells and if your choice is twelve 100 lb x 12vdc batteries in series/parallel versus 12 100 lb x 2 volt batteries in series, I would go with the 2 volt cells. You will have just as many battery connections and you will not have the issues of trying to balance charge/discharge currents between parallel strings.

    Also, with the 12 vdc battery installation, you will have 72 cells to maintain water levels in and the possibility of one of those 72 cells failing.

    With the 2 vdc battery bank, you will only have 12 cells to fill/maintain and only 12 cells that can risk failure...

    So, my two cent summary, if you are building a large battery bank and have the choice to build either a series/parallel out of 6vdc of 12vdc batteries, or a pure series bank out of 2vdc batteries--I would choose the series bank. You will have way fewer cells to maintain, easy to measure voltages (and current, as there is only one series connection), more balanced charging/discharging, and just as many connections to maintain.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  10. #10
    Kristofer Guest

    Default Re: Batteries series parallel vs series for longevity

    Well, let's use an example of a 48 volt system, to minimize the wiring amperage load. It will take 24 series-wired 2v cells to get 48 volts, versus four 12v batteries. Since you are doing away with 20 external connections, that is significant in itself. I realize that you will double up on these "legs" to achieve higher capacities with 12v batteries, but below some crossover point, these other considerations may be equally or even more important. As I understand it, replacing one battery in an array isn't a good idea anyway, so if four 12v batteries will meet your demands in such a system, the cost of installing and/or replacing a bank might be a consideration as well. BTW, when you are wiring individual batteries in an array, do you use the same gauge wire to connect between individual batteries as you do to connect to the inverter input?

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