Wiring diagram question

diego96
diego96 Registered Users Posts: 19 ✭
in Solar Beginners Corner #1
I'm working with an Outback Solar Charger, FlexMax 60. There is a wiring diagram in the manual that shows how to connect everything.
What is that component "Shunt"? I think I know what a shunt is (a jumper of short-circuit) but in this wiring diagram I don't know what the point of this component is. Why not just connect the negative of the battery to the negative bus bar? And are they calling out a component to install there? If so, I don't know what component to use...


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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: Wiring diagram question

    Shuts, in this case, are precision high current/high power resistors. You would place a meter across the shunt to measure the current, or install a Battery Monitor to report real-time current and keep track of the Amp*Hours into and out of the battery bank:

    wind-sun_2191_37947493Deltec 500 amp, 50 millivolt current shunt
    Price: $27.00




    I like Battery Monitors--They are much easier and more accurate (on average) to keep track of your battery bank's state of charge. And less messy than measuring specific gravity with a hydrometer (or waiting for 3+ hours with no loads/charging current to measure battery resting voltage). Can help you prevent your battery bank from having an early death. Also, looking at a meter that read XX% is easier for spouse, kids, and guests to watch their loads (i.e., watch loads if <75% state of charge; turn on genset if <50% state of charge; turn off everything if <20% state of charge and call you for help).

    If you are using a sealed cell type battery (like AGM or GELL), then you cannot even measure specific gravity.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • diego96
    diego96 Registered Users Posts: 19 ✭
    #3
    Re: Wiring diagram question

    But according to the Outback FlexMax user manual, the charger already displays output current on the LCD panel so why would I need to measure it again?
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  • diego96
    diego96 Registered Users Posts: 19 ✭
    #4
    Re: Wiring diagram question

    And is there a reason for measuring current from the negative terminal instead of the positive terminal of the battery? I would have put it at the positive terminal right before the 30 Amp breaker...
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #5
    Re: Wiring diagram question

    Outback does have a Battery Monitor option (I think it is called the Outback Power Flexnet-DC System Monitor)--but assuming you have the "bare" controller--It's displayed value is only the output current/power from the charge controller itself. Loads+battery bank current--So you really do not "know" what is happening with the battery itself.

    A battery monitor is kind of "totalizer" that measures battery current and performance. If the battery is 100 AH at 12 volts. And you draw 1 amp for 10 hours (or 10 amps for 1 hour), the battery is drained to 90 AH or 90% state of charge (meter counts down). When you recharge the battery, you pump 10 AH * 1.25 efficiency, or 12.5 AH, then the battery is back to 100% full (meter counts up with a "fudge factor").

    Of course, they can do a lot more with the battery monitor. Include voltage so they can read in Watt*Hours, adjust capacity base on battery temperature, and as the battery ages, automatically adjust the efficiency factor (say 1.10x when new, and 1.2x when battery is old/worn). Some battery monitors can be set to turn an alarm on if the state of charge is too low, turn off an inverter, etc.

    I like to describe the battery as the "heart" of your off grid power system. You should know the approximate state of charge at all times. If you cycle too deeply, you damage the battery. If you don't recharge back >90% state of charge several times a week, you age the battery faster. If you take the battery to <20% state of charge, you may kill one or more battery cells, etc...

    You can monitor your loads and figure out how long until you need to recharge the battery. Same thing with charging.

    The "gold standard" for estimating battery state of charge is using a Hydrometer to check the Specific Gravity of the cells. With AGM and other sealed batteries, you cannot use a hydrometer and are left with measuring resting voltage (3+ hours of no load/no charging) or using a battery monitor (which is not perfect itself--but is much easier to use for most people--and will display state of charge continuously).

    A couple of Battery FAQ's:

    Deep Cycle Battery FAQ
    www.batteryfaq.org

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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