Battery bank

Will2020
Will2020 Solar Expert Posts: 62 ✭✭✭✭
in Solar Beginners Corner #1
What's the recommended size wire I should use for 3, 105 ah agm batteries?

How should these be wired for a 12volt system? Series? Parallel?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #2
    Re: Battery bank

    What is your maximum charging/discharging current?

    What is the bank voltage? (12 volts?)

    What is the battery voltage? (12 volts?)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Will2020
    Will2020 Solar Expert Posts: 62 ✭✭✭✭
    #3
    Re: Battery bank

    This is what it says on the battery.

    Bulk Charge Voltages: 14.20 - 14.60
    Absorbtion Charge Voltages: 14.20 - 14.60volts
    Float Charge Voltages: 13.10 - 13.40

    This may be more informing. http://www.lifelinebatteries.com/rvflyer.php?id=3

    I don't know where to find the maximum charge/discharge current.

    Also, I only have one of them at the moment, but will have the others really soon. I want to buy the wire and the terminals for them now though.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #4
    Re: Battery bank

    How big of loads? How big of charger?

    Or, assume the maximum surge current is C/2.5, then the wiring should be rated to carry:
    • 3 x 105 AH x 1.25 NEC safety factor * 1/2.5 = 158 amp minimum circuit rating
    According to this generic wire awg chart--That would be a minimum of 3 AWG wiring to carry the example surge current.

    How big of inverter would that support?
    • 10.5 volt cutoff * 158 amps * 0.80 inverter eff = 1,327 watt surge
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #5
    Re: Battery bank

    Those are 12v batteries - for 12v wire them in parallel.

    Take care to wire them properly for balanced charging/discharging:

    http://www.smartgauge.co.uk/batt_con.html

    (Method 2, 3 or 4 - Method 1 shows the wrong way to do it.)


    The wire size depends...

    For battery to battery you want the biggest fattest wire you can get, with good solid CRIMPED ON connectors.

    From the charger to the battery...if the charger is 25a or 30a then size the wire (and fuse) for (at least) 30a.

    From the battery to inverter...depends on the inverter - use Bill's example.

    The battery to battery cables don't require a fuse in them - but anything connected to the battery bank DOES require a fuse. The fuse is to protect the wire, so if the wire is rated to 30a, use a 30a (or less) fuse. If the wire is rated to 150a, use a 150a (or less) fuse.

    Distance also matters, for a long run say from the charge controller to the battery, you might want to kick the wire size up to reduce the voltage drop.
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  • Will2020
    Will2020 Solar Expert Posts: 62 ✭✭✭✭
    #6
    Re: Battery bank

    I need a battery meter. What's the proper one to get?

    http://store.solar-electric.com/trtmbamosy.html

    I bought the Morningstar dual purpose charge controller in case I need to charge two separate battery banks and I think this would pair nicely with that.

    Which shunt do I need?
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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #7
    Re: Battery bank

    A lot of people around here love the Bogarts. Most have the TM-2020 and that one in your link is the RV version (different color) of the newer TM-2025.

    That's an excellent choice.

    There are some things to be aware of with amp hour meters. SmartGauge has their usual exhaustive analysis here:

    http://www.smartgauge.co.uk/sgvahrs.html


    EDIT: Missed the part about which shunt. There is a shunt selection guide here:

    http://www.deltecco.com/resources.htm

    "Continuous operating current should not exceed 2/3 ammeter shunt rating. To ensure proper operation the shunt temperature is in no way to exceed 145°C, as a permanent change in resistance will occur.

    For example, if continuous current for an application is 500 amps, the ammeter shunt rating should be no less than 750 amps. This applies to both 50 and 100 millivolt current drop. To estimate minimum standard shunt rating required, divide continuous application amperage by .6666."


    So, you need a shunt 1/3 bigger than whatever your max load amps will be.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #8
    Re: Battery bank

    And don't get too large of shunt (i.e., 10x what you need)... It will give you less accurate readings at the Battery Meter.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Will2020
    Will2020 Solar Expert Posts: 62 ✭✭✭✭
    #9
    Re: Battery bank

    It looks I will be pulling less than 50 amp hours per day. Will a 100 amp shunt do the trick?

    Will the 100 amp shunt handle a 1000 watt inverter and power tools?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,623 admin
    #10
    Re: Battery bank
    • 1,000 watts * 1/10.5 volts * 1/0.80 inv eff = 119 amps
    If there is a surge current, it could exceed 200 amps...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • dwh
    dwh Solar Expert Posts: 1,341 ✭✭✭
    #11
    Re: Battery bank

    I'd go with a 500a shunt.

    500a x .666 = 333a (nominal shunt rating)

    It's big enough to handle whatever surge loads, but no so big as to seriously impact meter efficiency.
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  • mikeo
    mikeo Solar Expert Posts: 386 ✭✭✭
    #12
    Re: Battery bank
    Will a 100 amp shunt do the trick?
    GO with a 500 amp shunt with the TM2020 and you will have plenty of accuracy.
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