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Proper Solar Bank wiring?

  1. Nov 24, 2015 #1
    Good day,

    When dealing with AGM deep cycle solar batteries, is there a difference between the following two wiring methods?

    By the way, I found these pictures online but they describe the two scenarios

    Basically the two are almost identical but on one you are grabbing your positive and negative leads from the same end of the battery bank. And the second method, you grab them from opposite sides.

    I've seen these two different methods but I was told that it is better to grab the leads from opposite sides as it charges and discharges the batteries more evenly.

    Thank you
     

    Attached Files:

  2. jcsd
  3. Nov 24, 2015 #2

    berkeman

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    The connectivity looks the same to me. Am I missing something?
     
  4. Nov 24, 2015 #3
    That's the thing lol to me it seems like its the same thing.

    In case you don't exactly get what I am asking, look at this pic and compare it to the first one I uploaded. Would these 2 setups behave the same way?
     

    Attached Files:

  5. Nov 24, 2015 #4

    anorlunda

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    If the cables are big enough, there will be negligable voltage differences between batteries in parallel, and thus negligable unevenness. But if the cables are too small it could make a difference. If there is a problem, use thicker cable, rather than fooling with the connections.

    The cable guage that you use has an ohms/foot (ohms/meter) rating. Calculate the voltage drop over the distance of cable between batteries. If it is more than 0.1 volts, then use a bigger gauge.

    Example, 8 gauge copper cable 0.00063 ohms/foot. Say 2 feet between batteries, so 0.0012 ohms total. Say 100 amps current. The voltage drop is 0.12 volts. In that case, I would upgrade to 6 gauge cable.
     
  6. Nov 24, 2015 #5

    billy_joule

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    That's rubbish.

    Your two pictures are electrically identical.
     
  7. Nov 24, 2015 #6
    The cable I am using is 2 gauge, see image. I can't figure out the ohms per foot. I found a table online that says its .1593 not sure if that's right
    Also all the batteries are right next to each other, all the connections are about 20" long. EDIT**:10"
     

    Attached Files:

    Last edited: Nov 24, 2015
  8. Nov 24, 2015 #7
    That is kind of what I thought lol but I need to make sure
     
  9. Nov 24, 2015 #8

    berkeman

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    2AWG copper wire is 6289 feet per Ohm, so 1/6289 = .000159 Ohms per foot.

    c289_pocket_ref_3rd_ed_inhand.jpg
     
  10. Nov 24, 2015 #9

    berkeman

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    BTW, are you using an inverter to make AC Mains voltage from the 12V battery bank, or is your whole system 12V? :smile:
     
  11. Nov 24, 2015 #10
    My bad, I meant to say 10" long. Its short cables
     
  12. Nov 24, 2015 #11
    Yes sir, the whole system stays at 12V. Its powering a mobile surveillance unit. A few IP Cameras, IR sensors, sirens etc
     
  13. Nov 25, 2015 #12

    sophiecentaur

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    That is not a 'schematic' diagram. The connecting leads and contacts have a finite (possibly measurable) resistance. Re-draw the diagram with little Rs instead of ideal connecting wires and the two layouts are different. The point is, how significant are the Rs? If the Rs are significant then the problem can be solved - for free - by changing the connections. With a decent Hall Effect Tong Ammeter, you could, perhaps, measure the difference between the currents from the three banks before making any changes. The characteristics of the PV cells may well compensate, in any case.
     
  14. Nov 25, 2015 #13

    nsaspook

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    Absolutely correct and because batteries are dynamic a small imbalance in the charge/discharge rates of each battery on the parallel string will cause the imbalance to increase over time as one or more battery discharges and charges before the others. The very small interconnect wire resistances serve as equalization resistors at high current levels and using the opposite string ends as the string connection point makes the resistance to each battery closer to being equal if all the batteries are at the same SOC.
     
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