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Unequal battery cable lengths on parallel string

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  • Unequal battery cable lengths on parallel string

    Hi. I installed a 400 watt solar system on a Class C RV. There are space issues. The existing batteries were group 24 under the entry stairs with absolutely zero room for a size upgrade or relocation. So, I left those where they were and added 4 more group 24 12 volt batteries in parallel in a small cubby next to the stairs. All the batteries are connected with 12" 2/0 cable with the exception of the cables between the #2 and #3 battery, which are 4' in length. I know this is not ideal but there was literally nowhere else to put batteries. The only other option would be to drill a hole through a couple steel plates and shorten that 4' run to 2'. Is there a problem with this, and if so how can I ascertain that? I don't have a DC clamp meter to check the current running through the cables. Thank you.

  • #2
    For that longer run, going to the next larger wire size will help with lowering the resistance.
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

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    • #3
      Originally posted by Mike90250 View Post
      For that longer run, going to the next larger wire size will help with lowering the resistance.
      Thank you. I have thought of doing that. Would there be any symptoms from the current configuration to look for? If I disconnected all the cables and checked the individual battery voltage, would that tell me if there was a problem?

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      • #4
        You would need to check battery voltages while charging or discharging. Clamp on DC amp-meters are great, but for subtle mismatches, I think their 10% repeatablilty and interference issues might make it hard to see a small imbalance. They will show a large imbalance.

        You can use an on-line calculator to select a wire gauge and length that will all be equal for the different batteries, and then you have terminations and bolt on differences too.
        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

        solar: http://tinyurl.com/LMR-Solar
        gen: http://tinyurl.com/LMR-Lister

        Comment


        • #5
          Mike's comment about measuring battery voltage while charging and discharging is right. Mike's suggestion of larger wire is a good idea.

          Please forgive me if this is obvious. Your goal is equal battery voltage when charging and equal battery loading when in use. It isn't just equal wire length or equal wire resistance. You need to think about the current flow path.

          You might want to draw a wire map with each length of wire shown as a resistor and see if the voltage drops work out correctly. This sketch shows you what I mean. The top diagram looks nice, but B4 is at the end of the line, so gets the least charging and the least loading. The bottom diagram is better. The series resistance to each battery is the same, so each gets loaded and charged the same.

          This assumes that all connections are excellent, which may or may not be true. That's why Mike's suggestion of voltage testing is so important.

          Batteries.jpg
          7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

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          • #6
            I have tested the voltage at each battery with my multimeter while the array is charging, and they are identical. I do have the ground hooked to the number 6 battery, and the positive lead hooked to number 1, as shown in the above "good" diagram.
            Last edited by fulltimer; 07-10-2020, 10:17 AM.

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