Battery cable length

rgk1
rgk1 Solar Expert Posts: 135 ✭✭✭
in Off Grid Solar & Battery Systems #1
I am making a few changes to my system and in the process going to replace the battery cables with more flexible ones than i have now. I understand the importance of having equal length battery interconnect cables. But, what about the cables from the battery bank to the system - and + tie points? Do those also need to be the same length including passing through the shunt on the negative side and the fuse block on the + side?
4-Risen 320 watt in series/parallel, 8-215ah 6 volt GC2 batteries in series, Exeltech 1100 watt/48 volt inverter, Tristar 45 MPPT controller.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #2
    Re: Battery cable length

    No, those cables should be as large a gauge and short length as possible, but they do not have to be equal length. --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Battery cable length

    Just to be clear, the problem arises when you put batteries in parallel. In such a case unequal wire lengths between the two batteries means they will each have different resistance in the wires. That's when the current becomes unequal between them, and one ends up doing more of the work than the other.

    In a single series of batteries the wire lengths between them do not have to be equal nor do the positive and negative cables to the inverter or controller. But it is always best to do as vtMaps says and keep them as short and fat as possible to reduce the over-all resistance in the circuit.
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  • rgk1
    rgk1 Solar Expert Posts: 135 ✭✭✭
    #4
    Re: Battery cable length

    Thats what I needed. I do have batteries in series and then parallel. The layout would require the negative to be about 2 foot longer than the positive. Just didn't know if that extra, unused length of + was required to keep them equal length. No battery to system cable will be over six foot. Planning on going with 2/0 welding cable. I am currently using 2/0 SAE cable. Just guessing its half as flexible as welding cable. Thought about going with 4/0 but its a bit too expensive to get and then not make the bends i need.
    4-Risen 320 watt in series/parallel, 8-215ah 6 volt GC2 batteries in series, Exeltech 1100 watt/48 volt inverter, Tristar 45 MPPT controller.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #5
    Re: Battery cable length

    There is a new code option now I think, MTW (locomotive cable), flexible like welding, but modern code approved insulation.

    NAWS (our host) carries it @ http://www.solar-electric.com/bainca.html
    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
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #6
    Re: Battery cable length

    Just to be clear,

    MTW is Machine Tool Wire. This does not affect the stranding of the wire. Most of the increased flexibility of multi-strand cable is from using fine stranding -- many more conductors of a finer guage (larger guage number). Some aditional flixibility results from using a more pliable insulation.

    IIRC, many building wire is quad-rated. Like THHN - THW - THWN-2 - MTW. Much of the MTW rated wire that I've seen is fairly inflexible. Have not checked, but it is quite possible that MTW is available as solid wire.

    JMHO, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #7
    Re: Battery cable length

    Apparently, the MTW cables that are sold by NAWS, are fine strands - spec sheet calls out 30 ga strands,http://lib.store.yahoo.net/lib/wind-sun/MTW-specs.pdf which would be as flexible as welding cable. That now leaves the user to obtain "pre-made" cables, or use a hydraulic crimper, cheap hammer crimping will not cold weld the strands, and they will eventually escape the lug.

    1/O cable is over 1,000 strands of #30
    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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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #8
    Re: Battery cable length

    rgk1,
    as long as each string is identical and then each is paralleled identically, they should be fine. if you have a 2ft longer length off of the - of one string then the other string will need this extra wire too. not to try to confuse you here, but if the total length for example of that string is 3ft with 2ft off of the - terminal that the other string needs only to have the same length of the same wire and can be reconfigured to even be 1ft off of the - and 2ft on the + or even 1/2ft off of the - and 2.5ft off of the +. if this confuses you then just keep in mind equality as seen by the loads or charge source and if you can only envision this with both strings to have the 2ft off of the - then that's fine. of course these wires would require to be connected to a single point connection for all strings to keep the equality you have set up with the wire lengths being equal such as a bus or even a nut and bolt arrangement.
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