+ & - battery cable -> should they be of same length

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Roil
Roil Solar Expert Posts: 25 ✭
in Off Grid Solar & Battery Systems #1
I have got 2 strings of 6 2V 1000Ah AGM batteries connected together as a 12V 2000 Ah battery.
The two strings is connected together with 4 2/0 AWG (70mm2) cables 70 length, 2 on the plus side and 2 on the minus.

I have not yet connected them to my inverter, a Victron Multiplus 3000W, 120A 220V.

Since the battery is of considerable size 110cm long and 90cm height and my inverter is located on the plus side of the battery I have two options when it comes to cable length between inverter and battery.

Should I:

1) Make them as short as possible to reduce resistance

or

2) Make them of the same length and accept increased resistance

Your views is appreciated


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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #2
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    Re: + & - battery cable -> should they be of same length

    As short as possible. Run them together if possible once they exit the battery bank to minimize EMI and inductance.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #3
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    Re: + & - battery cable -> should they be of same length

    balanced length between + & - is not required. But having the proper gauge cable to reduce voltage loss at surge demands (starting fridge motor, could be 900w AC, or about 40A @ 24V.) is required.

    The surge power is added to the other power that is already being consumed, so you may see a peak of 80 amps DC, if you have several things on, and the fridge starts up.
    Powerfab top of pole PV mount | 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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  • Roil
    Roil Solar Expert Posts: 25 ✭
    #4
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    Re: + & - battery cable -> should they be of same length

    On my current batteribank I draw peeks of 300Amps when the inverter support my oven while the systems waits for the genset to kick-in. This is max for a minute. The more Normal loads are below 100A

    The plan is to use a set of 2/0 on each side. They are a bit lengthy though - 9' on the negative and 7' on the positive side.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #5
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    Re: + & - battery cable -> should they be of same length

    2/0 is to small if the were a foot long. 267 Amps NEC , #4/0 AWG/107 mm2. The longer they are the more voltage sag you'll get especially at 12 V. Using 2 cables work, unless one fails unless they are both fused.
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #6
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    Re: + & - battery cable -> should they be of same length

    Work out the v drop and see what she looks like.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #7
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    Re: + & - battery cable -> should they be of same length

    Definitely short and thick.

    Is there any reason why you use 12V? You could configure them as a 24V 1000AH battery, which would be easier on cables.
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  • Roil
    Roil Solar Expert Posts: 25 ✭
    #8
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    Re: + & - battery cable -> should they be of same length

    Yes,

    I have had a 12V system for some time, and the most expensive part - the inverter - is a 12V unit, I would have liked to upgrade it to a 24V 5000W unt it's on my xmas whish list :-)
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #9
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    Re: + & - battery cable -> should they be of same length

    I think you want to do some math re wire size needed to feed the inverter to generate 5000W AC@ 110v, this would be ~ 45A, at 48V it is ~= 105A and at 24v ~= 210A, that is a huge load and will need massive wire to get it to the inverter,...

    NOTE this quick math does NOT include any surge loads...
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • Roil
    Roil Solar Expert Posts: 25 ✭
    #10
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    Re: + & - battery cable -> should they be of same length
    westbranch wrote: »
    I think you want to do some math re wire size needed to feed the inverter to generate 5000W AC@ 110v, this would be ~ 45A, at 48V it is ~= 105A and at 24v ~= 210A, that is a huge load and will need massive wire to get it to the inverter,...

    NOTE this quick math does NOT include any surge loads...

    Well, just to clarify

    Current inverter to be connect is a 220V 3000W 12V Victron Multiplus, the 5000W - 24V is just a wish making it possible to rewire the battery bank to 24V and reducing the wire size issue.

    Using this tool http://www.bulkwire.com/wireresistance.asp and the numbers:
    6 meters
    12.8V
    150A

    I get a 1.8% loss on a single 2/0 AWG wire. A double cable size will consequently give the same loss with 300A

    Since 300A is peak load and "normal" loads are more in the region of 100A and lower I guess I am prepared to accept the 1.8% loss for these peak loads.

    100A on a dual 2/0 AWG will give less than 0.6% loss and thats ok.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #11
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    Re: + & - battery cable -> should they be of same length

    I'd run those numbers again, 12.8 maybe on a full battery after you charged for 6-8 hrs with a generator, you'll be lucky if you have 12.5 with any load. I used your calculator and numbers, I couldn't get what you came up with. ( 9 meters , 150 amps, 12.8 V = 2.754 % on 2/0 )

    Only you know what you want to live with. Where you run into issues is lets say you want to get up in the morning and try to cook breakfast and your batteries are at 12.1 V. You'll have more than just the stove on and those loads do add up. You can always start your generator manually to support the load in advance of using it. The Inverter you listed has " Generator Support " it is a wonderful feature to have.

    We all have the same issues. I am a bit confused on the Inverter you have and it's output. Is it 230 V @ 50 HZ ?? or 120 V @ 60 HZ ?? 6000 w Surge.

    You'v got plenty of battery to back up your loads, you never mentioned the size of the Interconnects on them.
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  • Roil
    Roil Solar Expert Posts: 25 ✭
    #12
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    Re: + & - battery cable -> should they be of same length

    Hi Blackcherry

    Soory for being unclear.

    The distance is 6meter, 3.5m negativ cable and 2,5m positive. Agree on 12.8, will recalculate with 12.5

    The inverter is 230V 50Hz 3000 Watt and 12V - European model that is - i live n Norway

    I use the genset startup. I have programmed the Victron to start amoung others when the loads exceed 2000W. It's a Honda EM5500CXS, and it takes about a minute to warm up and for the inverter to accept the source.

    This is an off grid cabin with moderate loads. Typically weekend consuption is 2-300Ah. The huge 2000Ah battery set is to support many days in winter with limited sun.

    Sorry for missing out on the interconnections. I have used 2 x 2/0 AWG between the two series, length 65cm (2'), i'll see if I can find some pictures of this.
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #13
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    Re: + & - battery cable -> should they be of same length

    I wasn't trying to be nosey, trying to find any weak links so you'll have a good outcome. I have about the same setup you have and used it for many years. You'll learn all the work arounds after you use it for awhile. Good Luck to you....
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  • Roil
    Roil Solar Expert Posts: 25 ✭
    #14
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    Re: + & - battery cable -> should they be of same length
    Blackcherry04 wrote: »
    I wasn't trying to be nosey, trying to find any weak links so you'll have a good outcome. I have about the same setup you have and used it for many years. You'll learn all the work arounds after you use it for awhile. Good Luck to you....

    No problem -
    No offence
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #15
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    Re: + & - battery cable -> should they be of same length

    I'd be watching your use. That size battery bank makes it hard to keep up sometimes. It will take a lot of your charging resources if you have too much, I know it seems like reserve power storage, but it comes at a price if you don't need it. Every time you are charging they are taking 50 % of what you have and they end up as wasted overhead. If you have limited charging they will drag every thing down in a spiral. Just a thought.

    You can always split them apart and come up with two different charging schemes and use. Thats what I do part of the year.
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  • Roil
    Roil Solar Expert Posts: 25 ✭
    #16
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    Re: + & - battery cable -> should they be of same length
    Blackcherry04 wrote: »
    You can always split them apart and come up with two different charging schemes and use. Thats what I do part of the year.

    How do you organize this?
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  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    #17
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    Re: + & - battery cable -> should they be of same length
    Roil wrote: »
    How do you organize this?
    You can use a Battery selector switch, they are used in Marine market ( Boats ). They come two ways Manual or solenoid controlled. With one of these you can put half your bank on maintenance and bring it on line when you need it. ( Remember your AMP load, choose the biggest ) You can set the solenoid type up to do some automatic switching. Batteries are so greedy, they can take you into a dark hole.

    http://www.shop.pkys.com/Battery-Switches_c_9.html

    http://www.shop.pkys.com/Relays-and-Solenoids_c_54.html

    My Choices

    http://www.shop.pkys.com/Blue-Sea-Systems-3002-Heavy-Duty-Battery-Selector-Switch-1-2-BOTH-OFF_p_55.html

    http://www.shop.pkys.com/Blue-Sea-Systems-7701-Solenoid-ML-500A-12V-RBS_p_1217.html
    .
    You have to remember, using a scheme like this is no panacea either. I use Ending Amps in my charging. You can end up with two unequal banks. You can have one feeding the other, it's a balancing act. For me it solves a problem, half the time it is just two of us at home, when my kids come, I never have enough.

    You can also jiggle your charging resources around, once you have one bank in Float Service, you can concentrate it to other bank. It takes effort and battery management.
    .
    .
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