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How can a charge controller be accurate under load?

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  • How can a charge controller be accurate under load?

    My RV has the fridge and other small loads running constantly. Since the voltage drops from 12.7 to 12.4 with a full charge, give or take depending on current draws, how can you ever trust your solar controller to tell how much charge you have left in your bank? Do you have to turn off everything and wait for the battery chemistry to settle before getting an accurate voltage? Does the charge controller take into consideration the load.

    I have this controller.
    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

  • #2
    The charge controller is NOT a battery monitor. If it has solar power at hand, and it can maintain Float Voltage, all is fine, otherwise you wait for sun or start the generator. The Operator has to learn how the battery system behaves, it's not a simple set of rules. Some controllers can remember low voltage levels and extend the next Absorb cycle, some can be programmed to skip a day of charging, in order to deeply cycle the battery every 6 months, to refresh plate material,
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

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    • #3
      Originally posted by JKFlipFlop View Post
      ...Do you have to turn off everything and wait for the battery chemistry to settle before getting an accurate voltage? ...
      Yes, I believe this is the only way. Some folks install an ammeter with shunt like a Trimetric so they can have an SOC gauge to monitor. Personally, I just installed a volt meter that reads to the hundredths. While it is capable of being calibrated, it is taking a voltage reading far from the batteries so I know it isn't the true battery voltage. But I can surmise what charging state my solar is in based on the voltage reading and that is good enough for me.
      I'm an RV camper with 470 watts of solar

      Comment


      • #4
        I started to read an article recently, but didn't finish where its stated you could make a chart knowing what the voltage was and then when you turn on your load you know that's how much it drops. So if it's consistently 12.7, and it dropped to 12.4 is it safe to assume that that is the correct battery level if it has set for 5 or 6 hours with no load on the battery?

        Comment


        • ewarnerusa
          ewarnerusa commented
          Editing a comment
          If it sat overnight with no loads and read 12.4 V the next morning, that is only like 50% SOC. 12.7 V the next day would be more like 100% SOC. Dropping 0.3V with only small loads applied is quite a bit. I would suspect weak batteries or poor connections.
          Last edited by ewarnerusa; 10-11-2016, 09:05 PM.

        • ewarnerusa
          ewarnerusa commented
          Editing a comment
          Getting back to your OP, the controller doesn't care about loads on the battery. But it does consider them in a sense like Mike90250 alluded to. Controller lets current pass from the panels to the batteries, which causes the battery voltage to rise as it accepts the charge. Once the battery voltage reached a set point, the controller then starts throttling back the current so that the battery voltage remains constant and holds at the set point. If there is a load on the battery, then the current from the solar supplies the load first and any remaining current contributes to battery charging. This means battery voltage will either continue to rise or stay constant at the charging set point. If there is not enough solar current available to supply the load, then all solar goes towards the load and the battery makes up the difference. This will cause a voltage drop.

      • #5
        Originally posted by Brian53713 View Post
        I started to read an article recently, but didn't finish where its stated you could make a chart knowing what the voltage was and then when you turn on your load you know that's how much it drops. So if it's consistently 12.7, and it dropped to 12.4 is it safe to assume that that is the correct battery level if it has set for 5 or 6 hours with no load on the battery?
        It is all 5th grade math. Voltage Sag say dropping from 12.7 to 12.4 volts is a result of current flowing through the battery Internal Resistance (Ri) aka Ohms Law. 3 of the 12 Laws are

        Voltage = Current x Resistance.
        Current = Voltage / Resistance
        Resistance = Voltage / Current.

        No harder to figure than wages. You make $4/hour you know it takes 10 hours to make $40. Simple 5th grade math.

        So lets say we have a battery and with nothing connected with a Open Circuit Voltage (OCV) of 12.7 volts. We connect a load that draws 10 amps and the battery voltage sags to 12.4 Volts we will call Vloaded. So Ri = [OCV - Vloaded] / Amps

        or


        [12.7 volts - 12.4 volts] / 10 Amps = .03 Ohms.

        One other thing this tells you is the battery is over severely over loaded. On a 12 volt system between the battery Terminals and Load Terminals of say your Inverter only has 0.24 volts loss to work with That 0.24 volt sag or loss includes wire resistance and battery Ri. At 0.3 volt loss your battery loss alone exceeds the budget. You get .12 volt sag on battery and ..12 volts for the wire for a total of .24 volts. A battery that can supply 10 amps with no more than .12 volt Sag must have no more than .12 volts / 10 amps = .012 Ohms or less. Put another way, a battery 3 times larger than you have.
        Last edited by Sunking; 10-11-2016, 09:12 PM.
        MSEE, PE

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        • #6
          Ok, so if it is only possible to tell the SOC of your batteries by removing all loads and letting the chemistry settle and measuring the voltage, it would be safe to argue that a solar controller will incorrectly apply the 3 stage profile to charge your batteries if they are constantly under load due a voltage sag?

          Then some people would argue that the battery bank would accept less and less current as it reaches fully charged, therefore the voltage readout on the solar controller is again meaningless.

          If both of these statements are true then it would seem that the 3 stage charging profile is worthless if it is solely triggered off programmed voltage points, on a system under continuous load.
          Last edited by JKFlipFlop; 10-13-2016, 05:52 PM.

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          • #7
            Battery voltages are a poor way to determine SOC period. Voltage varies with temperature, load current, charge current, age, and especially if any cells are weak or failing.
            BSEE, R11, NABCEP, Chevy BoltEV, >3000kW installed

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            • #8
              Ya, I do not see how it could be possible to design a solar controller that correctly applies a 3 stage charge at the correct soc voltages, given that load will always be variable.

              If it was based off current flow to the battery bank, then the battery bank size would vary and current to the load would vary.

              Comment


              • #9
                Originally posted by JKFlipFlop View Post
                Ok, so if it is only possible to tell the SOC of your batteries by removing all loads and letting the chemistry settle and measuring the voltage, it would be safe to argue that a solar controller will incorrectly apply the 3 stage profile to charge your batteries if they are constantly under load due a voltage sag?
                No. Charge controllers simply maintain voltages that batteries need to charge effectively. The only "intelligence" they need for a 3 stage charger is when to switch to float, and that's pretty easy; wait until current goes below a threshold at absorb voltage and then switch. Or just wait X hours and switch. Or a combination of both.

                Then some people would argue that the battery bank would accept less and less current as it reaches fully charged, therefore the voltage readout on the solar controller is again meaningless.
                You seem to be equating "voltage readout on the solar controller" to "accurate state of charge." They are not the same thing.

                If you want to measure your battery's voltage (which is important to monitor battery health and for diagnostic purposes) then get a controller with a good voltage monitor, or better yet get a separate meter.

                If you want to measure state of charge, get a coulomb counting battery monitor.

                Two different functions, two different pieces of equipment.

                If both of these statements are true then it would seem that the 3 stage charging profile is worthless if it is solely triggered off programmed voltage points, on a system under continuous load.
                Not at all. If your goal is to charge a battery from a generator or grid powered charger, a 3 stage charger will do a better job than a 2 stage charger (which is really just a CV/CC power supply.) If you want to measure remaining capacity, get a battery monitor.

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                • #10
                  Originally posted by jflorey2 View Post

                  You seem to be equating "voltage readout on the solar controller" to "accurate state of charge." They are not the same thing.
                  No, in fact one of my main arguments is they are not the same.

                  Originally posted by jflorey2 View Post
                  No. Charge controllers simply maintain voltages that batteries need to charge effectively. The only "intelligence" they need for a 3 stage charger is when to switch to float, and that's pretty easy; wait until current goes below a threshold at absorb voltage and then switch. Or just wait X hours and switch. Or a combination of both.
                  Ok so we are now agreeing that charge controllers are actually making decisions based off current levels and not voltage set points?

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                  • #11
                    Originally posted by JKFlipFlop View Post
                    Ok so we are now agreeing that charge controllers are actually making decisions based off current levels and not voltage set points?
                    Lousy charge controllers just use time and voltage. When voltage reaches the absorb limit a timer starts, and after that time the voltage switches to float.
                    Better charge controllers (i.e. MPPT controllers) use current. When current drops to a threshold AND voltage is at the absorb limit, then it switches to float.

                    So both, really.

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