Too many parallel strings with batteries?

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stevend
stevend Solar Expert Posts: 34 ✭
in Solar Beginners Corner #1
I almost exclusively work with Surrettes but recently had a case where another company bought the components and then the customer brought them to us to install. They went with Trojan T105 Pluses.

In a Surrette seminar, when talking about their 4000 series which includes mostly their smaller batteries such as S-460s (460AHr) and S-530s (530 AHr), they said to "do no more than 2 strings in parallel otherwise go with bigger batteries." I wonder if the same goes to the T105 Plus? Anyone know?

In this case I was given 8 T105 Pluses, which are 6V, 225AHr along with an Outback 2012 inverter so I had no choice but to make a 12V battery bank. That meant 4 parallel x 4 series. Wondering if it was a bad thing.
-Steve
PS. I happened to document this one. It's at:
http://rimstar.org/renewnrg/sp_off_grid_solar_power_rv_motor_home_p3.htm#BATTERY
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  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    #2
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    Re: Too many parallel strings with batteries?

    stevend, Surrette is correct, it is best to limit to 2 strings in parallel. Better would have been only one bank. That having been said, I have 6 banks. The more banks you have the more difficult it is to keep all the banks properly charged.
    There are some tricks/rules that help; fuse each bank, all the cables going to and from the banks should be the same length and same gauge, all cables should connect to a common bus. The loads & charging should connect to the bus.

    Some picts of mine: http://smg.photobucket.com/albums/v468/audredger/Battery%20Bank%20Rewire/
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  • stevend
    stevend Solar Expert Posts: 34 ✭
    #3
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    Re: Too many parallel strings with batteries?

    audredger, By "bank" do you mean series string? Your mention of separate fuses for each bank leads me to think you mean wiring the batteries into actual separate battery banks. If so, that's an interesting difference; two busbars for the paralleling instead of a sequential set of parallel cables. Electrically they seem the same unless there's enough resistance to make a difference. But if it works, then that's fine.
    -Steve
    PS. I couldn't see your photo. It asks me for "your" password, even though it shows me as logged in.
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  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    #4
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    Re: Too many parallel strings with batteries?

    stevend,

    Your batteries seem to be wired up OK because the main connections are to opposite corners of the bank. It would be better with the buss bars suggested by audreger as long as the gauges and lengths of the cables are equal. There is good information about parallel battery connections at SmartGauge.
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  • audredger
    audredger Solar Expert Posts: 272 ✭✭
    #5
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    Re: Too many parallel strings with batteries?

    Try this..

    http://smg.photobucket.com/albums/v468/audredger/Battery%20Bank%20Rewire/?action=view&current=DSCN0050.jpg

    http://smg.photobucket.com/albums/v468/audredger/Battery%20Bank%20Rewire/?action=view&current=DSCN0054.jpg

    http://smg.photobucket.com/albums/v468/audredger/Battery%20Bank%20Rewire/?action=view&current=DSCN0051.jpg

    http://smg.photobucket.com/albums/v468/audredger/Battery%20Bank%20Rewire/?action=view&current=DSCN0052.jpg
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  • stevend
    stevend Solar Expert Posts: 34 ✭
    #6
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    Re: Too many parallel strings with batteries?

    Thanks Guys! That SmartGauge page is very illuminating. From my previous post you can see I didn't think the resistance difference would matter. It's nice to see the numbers that show it does - though I wouldn't mind seeing the calcs too but that's not necessary. I learned to do opposite corners a few years ago from a Magnum manual. I guess I'll stick with that since most banks I do have at most 8 batteries with at most two parallel strings. We usually go with bigger batteries rather than more batteries. Thanks for pointing out your photos, Mike. They cleared up what you meant.

    Another way in which I think Mike's suggestion with the common terminal beats all is in resistance due to oxidation at the connections. That can make up a real significant difference in resistance if it happens.
    -Steve
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #7
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    Re: Too many parallel strings with batteries?

    You can read through this thread a bit about wiring details... From one of the posts:

    Well, it is true that adding resistance does make the balance better... As long as the wire is still the correct gauge for the application.

    You are correct that "over sized" wire can make the issue of balancing heavy currents in a bank worse... Because the resistance of the wire "drops out" of the equation and the resistance of the battery and the electrical connections (and fuses/breakers) becomes the major elements in the resistance.

    A 3' length of wire is a pretty fixed resistance. Your connections and battery internal resistance may be more variable...

    To see what kind of resistances we are looking at--Take a look at a typical Optima Lead Acid Battery spec... The internal resistance for a 50-60 AH battery may be 0.0025 to 0.003 Ohms.

    Using this typical copper wire resistance chart--we see that large diameter wire has very low resistance per foot too:
    0 awg = 0.0983 ohms per 1,000 ft
    2 awg = 0.1563
    4 awg = 0.2485
    6 awg = 0.3951
    8 awg = 0.6282
    10 awg = 0.9989

    If we assume a 60 AH battery is around 0.003 ohms, then a 600 AH battery/bank could be around 0.0003 ohms...

    Or, about the same resistance as 1 foot of 4 awg wire...

    So--it kind of gives you an idea of the values you are looking at here... And how the wiring (and electrical connections, over current protection devices) for a large bank actually set the current balance more than the internal resistance of the battery itself...

    So, insuring that the electrical connections around the battery banks is very important to the ability of the batteries to properly share current.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    #8
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    Re: Too many parallel strings with batteries?

    Bill, very interesting points. I hadn't (should've) considered that in a case of diminishing returns (? am I correct) that the battery's internal resistance would become the dominant factor.

    Still, you get what you can, where you can, for what ($ & effort) you can.

    Proper torque on bolted connections might be often overlooked. I know it was for me, until next month, as I mentioned previously.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,439 admin
    #9
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    Re: Too many parallel strings with batteries?

    When new the wiring may be the dominant factor... When the batteries age, perhaps they become the dominant resistance...

    In the end, everything is very low resistance and any, relatively minor, increase in resistance (longer wire, smaller wire, fuse, sulfated battery, corroded lead/joint, etc.) can seriously upset the current sharing of the bank.

    From my design past--I had found many very high quality pieces of test gear (more money than I may have put down on my house) had 5+ wires in parallel to power the different digital logic sections and such in the test gear.

    Turned out that every time I had a piece of test gear go down, I could pop it open and find several burned contacts/connectors...

    My theory--it was not the high resistance path that caused the failures--it was the low resistance path that did it in! 5 wiring/connector paths, one made a better connection than the other 4--it carried all of the current and overheated/failed... Then the next lowest resistance wire/connector path carried too much current---pffftt. And so on.

    In the above case, these were parallel paths between one voltage source and one load sink--and we had improper current sharing.

    In the case of a battery bank with parallel paths--then the "best path" will carry the most current... Cycle battery string more than others--and potentially over heat the wiring (if you assumed 400 amps divided to 100 amp across 4 strings--you will probably be disappointed if you took a DC clamp meter to the strings).

    For me--I never tried to parallel more than 3 runs--and even then, I only would assume the three parallel runs would carry 1/1 + 1/2 + 1/3 = 1 5/6 more current safely--not 4x the current. At least as far as I knew--I never got any burned harness back... (crossing fingers).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Kamala
    Kamala Solar Expert Posts: 452 ✭✭
    #10
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    Re: Too many parallel strings with batteries?

    Yeah, reminds me of when I got my first MSW inverter. 750 it was. Held 2 fuses. I thought, OK, why 2 fuses? Then did math. Needed, from DC analysis only, a 63A fuse. Inverter had two 25A. OK, close enough. Device went to thermal before fuse. But, yeah, current sharing! In this case, fuses. Not wire. Still, one half goes, remaining carries twice. Blown!

    Sorry for broken English. Just finished Heinlein's The Moon is a Harsh Mistress. TANSTAAFL!
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