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Estimated charge current at different terminal voltages?

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  • Estimated charge current at different terminal voltages?

    What kind of charge current will a pair of inline GC2's pull at 50% SoC with alternator maintaining 14.8V?

    Those 6v 220ah golf cart batteries they sell at Costco for $80+- , commonly known as GC2 batteries, flooded lead acid deep cycles.

    Touring and living in an SUV. I don't have a vehicle yet, maybe get a late 90's Expedition/Explorer/Suburban/Tahoe/Yukon/Denali, or maybe a Forester/Outback, or a small Honda/Nissan/Izuzu SUV. I want to put a 300W panel on the roof, an MPPT, or PWM if I can't get a decent MPPT for under $200. If the starting battery is bad then I might just run 2g or bigger wire from the alternator/starter area to the 2 inline GC2's somewhere in the vehicle, and use the starter battery as a core at Costco. Or if the starting battery is fine then I might just isolate the 2 systems with some way of intermittently connecting the deep cycles to the alternator with or without a charge controller.

    Once I choose a vehicle I can get into analyzing the alternator field control circuit behavior, that's something I am curious about. Link literature if you have any on alternator field control circuits and their typical behavior tables.

    What amount of current will 2xGC2's typically take at various voltages and states of charge? I know 20hour rate is ideal (11 amps), but when charging from the engine I would want to go faster, say 5hr charging rate (44 amps). Ideally I'll charge from the panel and not use the alternator, but sometimes I may need to use alternator, let's say gaming on my laptop all night after a cloudy day.

    Let's say it's night time, no panel current coming, I turn on the engine and connect the deep cycles to charge for a few minutes, 2x GC2 6v 220ah batteries, I'm wondering what kind of draw there will be on the alternator with the batteries charging, the engine control unit or some charge controller integrated into the vehicle's wiring controls the alternator field with a PWM circuit I think, and it's programmed for a single cranking battery, not a 220ah deep cycle arrangement. I don't want to burn out the alternator, and I don't want to be running the engine just to get 2 amps from the alternator and wasting gas either.

    Throw me some numbers on a pair of inline GC2's, about how many volts to charge at a 5hr rate at 50% or 80% SoC will the alternator need to maintain? You want to charge fast you need to pump up the voltage, but I'm not sure if 14.8v is enough to charge at 44 amps, or does it need to be 20v? 20 is way too much voltage and will damage the batteries and electronics. 44A will strain a small alternator with lights and dash fan and radiator fan and other unnecessary power suckers, so depending on which way the system is set up I will need to not overload the alternator. Some alternators might have thermal probe safeties inside, or designed to be short circuited and ran hard, I'm not sure.

    What kind of charge current will a pair of inline GC2's pull at 50% SoC with alternator maintaining 14.8V? If it's 20A were good, but if they suck up 60A then the alternator might burn out, I'll have to get a field controller or something to pulse the alternator on and off to avoid overloading it.
    Last edited by copymepls; 02-03-2017, 02:49 PM.

  • #2
    It's kind of a question about RV solar living (but in an SUV instead), but mostly about GC2 battery charge current consumption.

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    • #3
      Originally posted by copymepls View Post
      What kind of charge current will a pair of inline GC2's pull at 50% SoC with alternator maintaining 14.8V?.
      Simple. As much as the alternator can provide. Voltage means nothing. Your alternator cannot maintain 14.8 volts. Voltage will fold back to current limit.

      When you buy a 10 amp charger means it can supply up to 10 amps. Voltage is just a set point.

      All batteries and battery chargers work the exact same way. It depends on the battery Internal resistance, battery OCV, and what the current limit of the charger is, or in your case is alternator. Alternators like any battery charger has a Current Limit.

      Example if I have a 50 amp charger set to 16 volts, and connect a pair of GC batteries that have a OCV of 12 volts with internal resistance of .01 Ohms when I connect the charger I have 50 amps of charge current. If I measure the voltage of the charger/battery I am only going to see 12.5 volts, not 16 volts. I will not see 16 volts for a couple of hours down the road. When you see charge current taper off the voltage will be 15.5 volts. When the battery OCV is 16 volts there will be NO CURRENT to speak of.

      Now if I had a 400 amp charger, as soon as I connect the battery I will see 16 volts just before the battery explodes.

      Try reading the Stickies, this one would have answered your question in detail .
      Last edited by Sunking; 02-03-2017, 04:10 PM.
      MSEE, PE

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      • #4
        If I have a 120A alternator with a field control circuit that pulses on and off if the voltage goes above 14.8 then the battery won't explode right? And the controller is probably set to reduce the voltage to 13.6V float when it somehow detects the battery is getting full based on observations of current or voltage or time estimated or whatever algorithm it uses, but those tables are probably set for a starting battery in the vehicle, not GC2's?

        Voltage is pressure, current is flow, if the battery is low on charge it will have a lower resistance and take more current, that I understand. I just don't know the internal resistance of the GC2's at various charge states and temperatures and all that. So with the charger hooked up and 14.8V terminal voltage with say .3 ohms internal resistance the GC2's will be pulling 49.6 amps, at .2 ohms it's 74 amps, at 1 ohm it's 14.8 amps. The terminal voltage will be based on the power of the charger and won't exceed the open circuit voltage of the charger or whatever the controller is limiting the voltage to.

        Am I wrong?

        I don't know the internal resistance figures at various charge states of the GC2's, that's my question. Have any charts?

        Thank you for responding. I stopped drinking when I moved to Florida 14 days ago.

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        • #5
          I'm thinking of the batteries as pressure tanks, say you have a tank with 1 atmosphere of pressure inside, and a compressor hooked up to it that can generate 10 atmospheres at a theoretical unlimited flow rate, but the piping and tank orifice size connecting the tank and compressor has resistance, the flow will start off quick since there is less resistance or impedance? And as the tank fills up the flow will taper off till the tank is 10 atmospheres then the flow stops, because the pressure difference between the two tapers off. The piping and tank orifice resistance is what I am thinking of as battery internal resistance, it goes up as the tank gets more full. I'm using pressure as voltage and flow as current.

          I'm afraid I'm just confused about language, impedance-resistance, whatever it is, the way the battery resists current flow coming into the battery as the battery gets to a higher SoC.

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          • #6
            Originally posted by copymepls View Post
            If I have a 120A alternator with a field control circuit that pulses on and off if the voltage goes above 14.8 then the battery won't explode right? And the controller is probably set to reduce the voltage to 13.6V float when it somehow detects the battery is getting full based on observations of current or voltage or time estimated or whatever algorithm it uses, but those tables are probably set for a starting battery in the vehicle, not GC2's?
            Your Alternator or any battery charger does not know if it is a golf cart battery, deep cycle battery, starter battery, or beer farts.

            Originally posted by copymepls View Post
            Voltage is pressure, current is flow, if the battery is low on charge it will have a lower resistance and take more current, that I understand.
            Then you would be wrong. As the battery discharges, internal resistance goes up. But for purposes of explanation let's say it stays constant. If I have a battery with 12 OCV, internal resistance of .01 Ohms, and a 10 amp battery charger set to 100 volts and connect it I will have a voltage of 12 volts + [.01 ohms x 10 amps] = 12.1 volts. As the battery charges the OCV rises. So as not to damage the battery I lower the charger voltage to 14.6 volts. As the battery OCV approaches and reaches 14.5 volts, current will start to taper off to 0 amps. When th evoltage reaches 14.6 volts all current stops. Battery still has .01 ohms internal resistance.



            Originally posted by copymepls View Post
            I just don't know the internal resistance of the GC2's at various charge states and temperatures and all that. So with the charger hooked up and 14.8V terminal voltage with say .3 ohms internal resistance the GC2's will be pulling 49.6 amps, at .2 ohms it's 74 amps, at 1 ohm it's 14.8 amps. The terminal voltage will be based on the power of the charger and won't exceed the open circuit voltage of the charger or whatever the controller is limiting the voltage to.

            Am I wrong?
            No you are wet behind the ears, and worried about things you need not concern yourself with.

            You do not need to know the battery internal resistance or worry about anything. If you have a 100-amp alternator, just make sure the wire can handle 100 amps. The battery and alternator will work things out for themselves. In a vehicle you want 14.2 volts.

            All chargers and alternators are Voltage Sources when load current demand is less than Current Limit. Once current limit is hit, the voltage folds back so the current is limited to Current Limit aka Current Source.

            Example if you have a 100 amp charger set to 10 volts, and apply a 0.1 Ohm load you will see 100 amps at 10 volts. Increase the load to 0.05 Ohm and you will see 100 amps @ 5 volts. A current source is limited.

            You did not read the link I gave you.
            Last edited by Sunking; 02-03-2017, 05:26 PM.
            MSEE, PE

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            • #7
              It sounds like you are trying to say the resistance always stays low, like .01 ohms. 14.8V charger with unlimited current makes 1480 amps flow and the battery goes boom. I thought it was higher resistance than that. Resistance based on charge, or some kind of impedance based on the state of charge.

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              • #8
                Originally posted by copymepls View Post
                It sounds like you are trying to say the resistance always stays low, like .01 ohms. 14.8V charger with unlimited current makes 1480 amps flow and the battery goes boom. I thought it was higher resistance than that. Resistance based on charge, or some kind of impedance based on the state of charge.
                You are NOT getting it.

                a 50% SOC voltage is 12.2 volts. If you connected a 14.8 volt charger with unlimited current in a battery with .01 Ohms would deliver [14.8 volts - 12.2 volts] / .01 ohms = 260 amps.

                Again you did not read the link.

                By your logic if I connected a 14.8 volt charger to a battery with an OCV of 14.8 volts and internal resistance of .01 ohms would be 1480 amps when in fact would be ZERO AMPS Math Proves it 0 volts / .01 Ohms = 0 Amps.

                Charge Current = [Charge Voltage - Battery OCV] / Battery Ri

                Battery Under Charge Voltage = [Battery Ri x Charge Current] + Battery OCV
                Last edited by Sunking; 02-03-2017, 05:42 PM.
                MSEE, PE

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                • #9
                  I'm reading the link and trying to comprehend this.

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                  • #10
                    The battery internal resistance is resistance, and the voltage difference between the battery and charger is impedance? 12v battery OCV and a 12v charger OCV means no current flows when they're hooked up. It's that impedance or whatever between say a 12v OCV battery and a 13v OCV charger I am trying to visualize.

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                    • #11
                      If the batteries were like pressure tanks they could be charged way quicker, and that is not the way it works, I thought they just had alot of resistance and if you tried to defeat it with too much voltage it would not charge the battery, just destroy it.

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                      • #12
                        Originally posted by copymepls View Post
                        The battery internal resistance is resistance, and the voltage difference between the battery and charger is impedance?
                        Huh? Where did you come up with that?



                        Originally posted by copymepls View Post
                        12v battery OCV and a 12v charger OCV means no current flows when they're hooked up. It's that impedance or whatever between say a 12v OCV battery and a 13v OCV charger I am trying to visualize.
                        Again 2 very simple math problems:

                        Charge Current = [Charger Voltage - Battery OCV] / Battery Ri

                        Battery Under Charge Voltage = [Battery Ri x Charge Current] + Battery OCV

                        If the charger voltage is say 14.6 volts and the battery OCV is 12 volts there is a difference of 2.6 volts. Quit making this harder than it really is. 2.6 Volts / .01 Ohms is 260 amps, not 1480.
                        Last edited by Sunking; 02-03-2017, 06:02 PM.
                        MSEE, PE

                        Comment


                        • #13
                          I'm hoping my confusing crap is helping you make a better teaching visualization and not just irritating you. I'm picturing a pressure tank that does not have electrons go in and stay in, because they have to flow in a loop, move all the electrons into a battery and the charger is just a bunch of protons and neutrons and that is obviously not what happens. The electron flow is being used to move lead atoms into and out of the acid and water mix? So when I would read electrical examples based on water or gas flow it is not ideal for teaching electricity, but illustrates some points.

                          I feel like if I saw some kind of animation I could learn this in seconds. I thought I understood resistance, but not impedance. Salt bridge examples are confusing.

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                          • #14
                            Forget impedance. It is all simple Ohm's law.

                            Current = Voltage / Resistance
                            Voltage = Resistance x Current

                            That is all you need to know.

                            If I have a current source of 10 amps, and connect that to a battery that is 12.2 volts with a internal resistance of .01 Ohms. So 10 amps x .,01 Ohms = .1 volt. So the battery voltage goes from 12.1 volts to 12.2 volts. So I set the charger to 14.6 volts. As the battery charges, the OCV goes up over time. When it finally reaches 14.5 volt, current will taper to ZERO AMPS. When both the battery reaches 14.6 volts, Current stops because the electric pressure is equal. It is not water, air, or lead. .

                            You are assuming the battery voltage is 0 volts, and/or the resistance is in parallel with the battery. Neither is correct. The resistance is in series like all the wiring and connections. The wiring and connections has higher resistance than the battery.
                            Last edited by Sunking; 02-03-2017, 07:56 PM.
                            MSEE, PE

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                            • #15
                              Originally posted by copymepls View Post
                              ... the flow will start off quick since there is less resistance or impedance?......
                              Forget the word impedance. It only applies to AC circuits. Batteries are DC and have internal resistance.



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