Buss bar use?

Lobo
Lobo Registered Users Posts: 18 ✭
in Solar Beginners Corner #1
I have two parallel strings of 12 volt batteries in my battery bank and would like to tie them together using a buss bar. My buss bar's are rated 400 amps each and wonder if there would be an issue using the buss bar's (one for the positive and the other for the negative) to connect the two strings?

Thanks in advance for any answers.
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  • H2SO4_guy
    H2SO4_guy Solar Expert Posts: 213 ✭✭✭
    #2
    Re: Buss bar use?

    Have you measured the current to see if you are using over 400 amps? It really depends on how much amperage you use.

    Skip
    12K asst panels charging through Midnite Classic 150's, powering Exeltechs and Outback VFX-3648 inverter at 12 and 48 volts.  2080 AH @ 48 VDC of Panasonic Stationary batteries (2 strings of 1040 AH each) purchased for slightly over scrap, installed August 2013.  Outback PSX-240X for 220 volt duties.  No genny usage since 2014. 
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  • jaggedben
    jaggedben Solar Expert Posts: 230 ✭✭
    #3
    Re: Buss bar use?

    As long as your overcurrent protection (fuse or circuit breaker) on the battery output is 400A or less, it should be okay.
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  • Lobo
    Lobo Registered Users Posts: 18 ✭
    #4
    Re: Buss bar use?

    On a good day there is 38 amps going into the batteries from the charge controller and the inverter is fused at 150 amps. The combined amperage of the batteries is much more then 400 amps tho.
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  • Psyfy
    Psyfy Registered Users Posts: 7 ✭
    #5
    Re: Buss bar use?
    Lobo wrote: »
    I have two parallel strings of 12 volt batteries in my battery bank and would like to tie them together using a buss bar. My buss bar's are rated 400 amps each and wonder if there would be an issue using the buss bar's (one for the positive and the other for the negative) to connect the two strings?

    Thanks in advance for any answers.

    Tell me four things -
    1.) The total kW/kVA rating of your inverter(s).
    2.) The surge rating of your inverter(s).
    3.) Your other direct DC loads. (if any).
    4.) The total Ah capacity of your battery banks.

    Regards.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: Buss bar use?

    You could just use the diagonal battery wiring method if you don't have a lot of DC connections. The fewer components the better.

    Otherwise you could also put a couple of the Blue Sea post fuses on the batteries so that the wiring to the bus bars is protected.

    If the current draw is going to be over the rating of those bars you'd have a couple of other problems: no single wire could handle the current and the batteries could only feed it for a moment. It would definitely be "up the Voltage" time if you're in that (400 * 12 = 4800 Watts) neighbourhood!
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  • jaggedben
    jaggedben Solar Expert Posts: 230 ✭✭
    #7
    Re: Buss bar use?
    Lobo wrote: »
    On a good day there is 38 amps going into the batteries from the charge controller and the inverter is fused at 150 amps. The combined amperage of the batteries is much more then 400 amps tho.

    It doesn't really matter what the combined amperage of the batteries is. Your overcurrent protection will prevent that number of amps from flowing under normal conditions. Psfy is right that you need to consider any direct DC loads you have, as well as the inverter; add all those up and if it's less than 400A in fuses and breakers then you're okay.

    Analogy: The utility connection to my house has thousands of amps of available fault current. But since there are 100A fuses at the service disconnecting means, the utility wires to the house are sized for one or two hundred amps and not for thousands.

    If there is a serious fault on the battery side of the fuses, you'll probably have bigger things to worry about than whether these busbars melted. If they do melt, and stop the fault, and that's your only problem, that's probably a good thing.
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  • Lobo
    Lobo Registered Users Posts: 18 ✭
    #8
    Re: Buss bar use?

    720 watts of panels
    My inverter is 1000 watts cont. and 2000 watts surge
    No other DC loads
    I have 8 105 ah batteries
    Psyfy wrote: »
    Tell me four things -
    1.) The total kW/kVA rating of your inverter(s).
    2.) The surge rating of your inverter(s).
    3.) Your other direct DC loads. (if any).
    4.) The total Ah capacity of your battery banks.

    Regards.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Buss bar use?
    Lobo wrote: »
    720 watts of panels
    My inverter is 1000 watts cont. and 2000 watts surge
    No other DC loads
    I have 8 105 ah batteries

    :confused: In the first post you said you have two parallel strings of batteries. Here you say you have eight 105 Amp hour batteries. That makes four per string, or 3 Volts each to get 12 Volts. Something's not right ...

    720 Watts of panel on a 12 Volt system about 46 Amps peak charging. Not a problem for current, but again doesn't jibe with the batteries.

    Otherwise the most that inverter would draw is <200 Amps (maximum surge output and minimum battery Voltage) so bus bars rated for 400 would have not trouble.
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  • Lobo
    Lobo Registered Users Posts: 18 ✭
    #10
    Re: Buss bar use?

    Please excuse my lack of correct terminology. I have 8 batteries, due to space they are situated 4 side by side and then another 4 along side the first.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #11
    Re: Buss bar use?
    Lobo wrote: »
    Please excuse my lack of correct terminology. I have 8 batteries, due to space they are situated 4 side by side and then another 4 along side the first.

    What Voltage are they?
    Standard battery Voltages go in 2 Volt increments: 2, 4, 6, 8 and then 12. If you divide 12 Volts by 8 you get 1.5, and if you divide it by 4 you get 3 et cetera. I can't figure a combination where 8 batteries adds up to 12 Volts and two strings.

    I can envision a couple of nightmares where there are eight 6 Volt batteries wired up for 12 Volts by four parallel strings and an even worse case where all the batteries are 12 Volt and are wired in two 'ladders' of four parallel each and then paralleled again. The possibility of keeping current shared evenly through such a configuration is about zero.
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  • Lobo
    Lobo Registered Users Posts: 18 ✭
    #12
    Re: Buss bar use?

    They are all 12 volt batteries. All the negative terminals are connected together going to a 400 amp buss bar and all the positive are connected going to a seperate buss bar.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: Buss bar use?
    Lobo wrote: »
    They are all 12 volt batteries. All the negative terminals are connected together going to a 400 amp buss bar and all the positive are connected going to a seperate buss bar.

    That describes eight batteries in parallel. This is not a good situation to have. The only chance for them to share current equally is with each connected to the bus bars separate, and every battery's wire length (positive and negative wire lengths added together) equal. Even then it is doubtful it will work out. The differing resistances in the batteries themselves will cause problems. This will be amplified by the wire and connection resistance, and if anything is a bit 'off' the effect on a 12 Volt system is significant.

    But if the 12 Volt batteries are what you've already got, you use them and accept the fact they probably won't last as long as you'd like and that there may be function problems in the meantime. Keep an eye on them. Get a clamp-on DC Ammeter and periodically check current to/from each battery. Watch out for corrosion and loose connections.

    Then when they fail you can replace them with higher capacity individual batteries (two of those 12 Volts in parallel is like two 6 Volt golf cart batteries in series).

    The other issue here is: you've got 840 Amp hours @ 12 Volts to recharge. That will require a lot of panel (about 1250 Watts) and a maxed out FM80 or MidNite Classic charge controller. If you really need that much stored capacity (4.8 kW hours), you should consider a higher system Voltage.
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  • Lobo
    Lobo Registered Users Posts: 18 ✭
    #14
    Re: Buss bar use?

    Thank you for all the time spent on this and you answered my question. Will be going to the 6 volt batteries and run them in series.
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