Battery Interconnect Gauge

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idahoboondock
idahoboondock Registered Users Posts: 5 ✭
in Solar Beginners Corner #1
I have two 6v golf cart batteries (220ah each) on a trailer. I want to connect them in series for 12v. All of my power needs are 12v (no inverter) with the largest being the furnace that draws about 8 amps/hr when running. I also have a solar panel and controller that will put in around 4 amps per hour.

I have a question about the interconnect cable. it needs to be 14 inches long. All I can find locally is a 6ga battery cable. Is this too small? If so, would it work to put two of these together to connect the two batteries?

Thanks for your help
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  • lorelec
    lorelec Solar Expert Posts: 200 ✭✭
    #2
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    Re: Battery Interconnect Gauge

    6-ga will be fine. At that length (14") and those currents (4-8 amps), you'll have only a few millivolts of drop. And yes, you can parallel multiple wires for higher current-carrying capacity.

    Marc
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #3
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    Re: Battery Interconnect Gauge

    And make sure you use fuses/breakers for the "+" wires that leave the battery common point. You want the fuses to open instead of a dead short causing your wires to melt/catch things on fire.

    Regarding paralleling of cables for increased load capacity... In my humble experience (DC power supply harnesses and connectors for computers)--it is not a simple 1 wire carries 10 amps, 2 parallel wires carry 20 amps. The sharing of current depends on the resistance between the two end points. And since copper wire has (reasonably) low resistance--any slight differences in resistance (connections, crimps, wire length, even wire temperature) will generally cause one wire to carry a majority of the current. And since resistive heating is non-linear (P=I^2 * R), a doubling of the current in one wire (because it has the lowest resistance of the set) will increase heating by a factor of 4x.

    I have repaired many pieces of computer equipment where (typically) the wire connections/plugs that had, say 4 connections; where one connection turned browned, then failed. The the next lowest connection browned, and failed--until finally something smoked or there was not enough current for the device to function (the heating obviously accelerating the failures).

    Also--NEC (which does not affect you) does not (if I recall correctly) allow paralleling of wire runs to increase current capacity.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • lorelec
    lorelec Solar Expert Posts: 200 ✭✭
    #4
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    Re: Battery Interconnect Gauge

    It's very common in electronics to parallel wires for greater current-carrying capacity. The cross-sectional area of the conductors will determine that rating. For example, you can take eight 18AWG wires in parallel to make one 10AWG wire. After all, this is the principle behind stranded wire and is used all the time. In electronics it's often done to make a conductor more flexible and easier to work with (when winding coils, for example), and also to mitigate the skin effect at higher frequencies (i.e. Litz wire). The "electricity follows the path of least resistance" rule of thumb that we often hear doesn't aptly describe what's going on. If you have a very low-resistance conductor in parallel with a very high-resistance conductor, naturally the one with the lower resistance will conduct more current. But the higher-resistance one will still conduct its share, as well. To observe otherwise would violate Kirchoff's laws. In a situation of paralleled conductors, they will all be very close to the same resistance (assuming they're the same length), so they will all share current nearly equally.

    Marc
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #5
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    Re: Battery Interconnect Gauge

    Marc,

    I agree in principle--but in practice... With terminations and connectors--because of the variable connection resistance at those points, current sharing on separate conductors tends to be a common point of failure in many pieces of equipment that I have worked on over the years.

    I have been successful in using multiple conductors to share current (when I have too, and I cannot "split the loads" and run separate wires to each)--but I did my own derating... As an example:

    1 wire = 10 amps
    2 wire = 15 amps
    3 wire = 18.3 amps
    etc.

    Basically, I used current capacity = 1/1 + 1/2 + 1/3 ... + 1/n as my own derating formula for connectorize wiring harnesses. As far as I know--none of my designs failed (perhaps, it was over designed).

    But--as you can see, in my own system, adding more than two or three wires in parallel becomes costly and bulky. At that point, I bite the bullet and change the design (re-architect or use heavier wire/connectors).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #6
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    Re: Battery Interconnect Gauge

    a further derating of wire is unnecessaty, but it doesn't hurt anything going oversized.
    i think i may know the nec's thinking on the matter for if one has say, a light circuit, but there's 2 sets of wires of #16 feeding it. i know #16 isn't an allowable size, but 2 of them is better electrically than a single #14 wire. anyway, it can have a set of wires going up, down, and through 2 seperate walls and yet are still paralleled. one set may end up much longer than the other too and compromise the paralleling effect by overloading one set. electricians and the nec don't want to have to trace this either and the smaller wires are physically weaker than one of the larger proper and equal size wires. they just carried over this way of thinking for dc circuits.
    there are just too many ifs ands or buts for the nec to want to deal with paralleling, but properly done it is electrically identical. no derating should be needed either if done properly and if the connections are the reason for derating then it is the connections that are faulty and not the wire and need to be addressed.
    now i feel low voltage dc is a special case they should make some exceptions for as the need to carry more current is present for the same comparitive wattage levels of higher voltages and isn't always easilly achieved with a single wire. they may have need to address wire placement, length, and connection into the rules, but it should be allowed for low voltage dc imho.
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  • lorelec
    lorelec Solar Expert Posts: 200 ✭✭
    #7
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    Re: Battery Interconnect Gauge

    Secure terminals and connections would be essential to keep things balanced between parallel conductors. NEC 310-4 does allow paralleling for conductors 1/0 and larger, with a number of exceptions for smaller conductors, too. I agree that further exceptions should be made for low voltage DC because of the larger currents that one usually has to manage -- provided that approved and reliable means of connection are made.

    Marc
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  • idahoboondock
    idahoboondock Registered Users Posts: 5 ✭
    #8
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    Re: Battery Interconnect Gauge

    Thanks for the help, I think I will run with the 6ga cable and either upgrade or add a second one if I ever decide to use an inverter.
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