What AWG to interconnect batteries

shift
shift Solar Expert Posts: 48 ✭
in Solar Beginners Corner #1
What AWG should i be using to interconnect my batteries?

I was thinking of using 4awg welding cable to connect 4x 147AH 12v batteries in parallel.

Right now its just being used for outdoor landscape lights (expanding may capacity).. not sure of future use yet.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: What AWG to interconnect batteries

    How much current do you want to draw from the battery bank?

    Roughly, the largest surge current "practical" from a flooded cell lead acid battery is C/2.5 (capacity at 20 hour rate).

    147 AH * 1/2.5 = 58.8 amps maximum draw per string (single battery in this case)
    58.8 amps * 1.25 NEC derating = 73.5 amp wiring/fuse/breaker

    From the NEC table, roughly #6 to #4 AWG cable minimum.

    And use the Smartgauge wiring suggestions for paralleling the battery bank.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • oil pan 4
    oil pan 4 Solar Expert Posts: 767 ✭✭✭✭
    #3
    Re: What AWG to interconnect batteries

    60 amps is on the edge of what 6 gauge can handle. So if its only surge current it should be fine.

    Solar hybrid gasoline generator, 7kw gas, 180 watts of solar, Morningstar 15 amp MPPT, group 31 AGM, 900 watt kisae inverter.

    Solar roof top GMC suburban, a normal 3/4 ton suburban with 180 watts of panels on the roof and 10 amp genasun MPPT, 2000w samlex pure sine wave inverter, 12v gast and ARB air compressors.

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  • 706jim
    706jim Solar Expert Posts: 523 ✭✭✭✭
    #4
    Re: What AWG to interconnect batteries

    4 or 6 guage will be fine.

    One thing that is often ignored when I see these enormously sized interconnects, is the voltage and the possible amperage numbers.

    As an example, my 1500 watt Trace has a maximum surge of about 80 amps going in and this would only be for a short time.

    Originally, I used 1 gauge wire for the interconnects on three series/parallel strings, but if you think about it, the maximum current the interconnects would see is only about 80/3 or 26 amps or so. And this on really short wires, maybe 16" maximum length. When the interconnects became corroded (yes this can happen when you don't maintain them for 6 months at a time due to accesibility), I replaced many of them with 4 guage wire.

    So, until the actual current being drawn reaches epic proportions, smaller guage wires WILL work and are a lot cheaper to replace when the ends get corroded.
    Island cottage solar system with appriximately 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing due south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter which has performed flawlessly since 1994. Outback Flexmax 80 MPPT charge controller four 467A-h AGM batteries. Insignia 11.5 cubic foot electric fridge 1/4hp GSW piston pump. My 31st year.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #5
    Re: What AWG to interconnect batteries

    Personally, For small numbers of paralleled strings (2-3), I would suggest that the parallel connected wiring be sized to carry 100% of the inverter load for each parallel string. If you want a "peak number" otherwise, It probably does not do much to size the wire over C/2.5 for flooded cell batteries. They cannot carry any more additional current for very long before the

    With paralleled battery strings, it is not difficult for a weak cell or wiring connection to drop that string's current to near zero amps, and the other one to two strings to carry the rest of the current (I have seen lots of failures in low voltage computer power supplies where one high current 5 VDC or similar output paralleled connection goes "low resistance" (compared to the rest of the parallel paths)--That connection cooks (I^2R=Power for heating, 2x current is 4x the amount of heat). And then the next lowest current resistant path cooks, and the whole parallel connection mess "unzips".

    So while large AWG cables reduce voltage drop (especially critical for 12 VDC/high power/high current systems)--The resistance of the smaller AWG cables helps "ballast" the current distribution through the wiring.

    For example a standard storage battery may have 0.001 to 0.005 ohms per battery. And the interconnect wiring is usually around the same resistance. All is takes is a week cell or electrical connection to double the very small resistance and cut the current flow by 1/2 or more.

    What am I saying... I don't know. It has always a been a big issue for power systems and parallel current paths and reliability. That is why I generally suggest that the single string current path be rated to carry the full current demanded by the AC inverter/load. Then a single path failure does not "take out" a good path and leave you with more stuff to fix.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: What AWG to interconnect batteries

    This is why when four parallel connections are used the best method is to size the cables for expect current and put a fuse on each one. That way the fuse pops if for any reason the power from one or more of the strings is lost shifting the additional current to the remaining.

    So if you want: divide the maximum system current by 4. Use cable & fuse capable of handling that to connect each to the bus bar/common connection point. From there use heavier cable with its own fuse/breaker to inverter.

    Note that four parallel batteries on a 12 Volt system is far more likely to encounter problems than four on a 48 Volt system, and especially more likely than using one string of higher capacity batteries. 147 Amp hours * 4 = 588 Amp hours. You could do this with two parallel strings of, say, 305 Amp hour 6 Volts (610 Amp hours). That would simplify things in the wiring department and give you half the number of cells to check.

    And if you want to try working the max current backwards from battery capacity (not recommended) with 25% as an upper limit you get 152 Amps which would need 1 AWG minimum.

    But it does all depend on how much current is expected.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #7
    Re: What AWG to interconnect batteries

    For four or more parallel strings, I would probably go with a minimum of 50% of system current flow.

    And as Marc types, fuses/breakers per parallel connection (technically 3 or more parallel strings) is a very good safety measure--Although it is an expensive measure.

    And another reason why we try to suggest larger AH batteries/cells rather than larger numbers of parallel strings.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • shift
    shift Solar Expert Posts: 48 ✭
    #8
    Re: What AWG to interconnect batteries

    Thanks Guys, I will go with 4AWG I just ordered some connectors of the net and will probally pick up some welding cable for the wire. I really should install some sort of a fuse system in this. Right now it just controls all of my outdoor lighting.

    I'm going to upgrade my battery bank from 2-4 deep cycle batteries.. which will leave my old 2 for future projects.. Maybe the storage shed will get some solar this spring! :)
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