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  • #16
    Originally posted by Sunking View Post
    Sure if I were the manufacture and someone said I have 12 volt 85 watt battery panels in parallel I would say you can make it work. Ask them off-line if they recommend that and I bet you get a answer.
    Actually, I would recommend that two 12 volt panels be put in series, rather than parallel for a 12 volt bank.

    The Midnite engineers have found that in some systems with 12 volt panels and 12 volt battery, a switch from a PWM to a Midnite MPPT controller has caused decreased or inadequate charging. This only occurs when the panels are hot, and with batteries that need (relatively) high absorb voltages.

    It occurs because the MPPT controllers need a volt or two more headroom than the PWM controllers, and there may not be enough headroom between Vinput and Voutput when the panels are hot. If the controller is going to sweep and find the max power point, it needs a few volts to sweep through.

    That all said, I tend to agree with you on over all system design... I was just being picky about your use of the word "optimal".

    --mapmaker
    ob 3524, FM60, ePanel, 4 L16, 4 x 235 watt panels

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    • #17
      Originally posted by mapmaker View Post
      That all said, I tend to agree with you on over all system design... I was just being picky about your use of the word "optimal".
      No worries. Back to the OP question higher voltage is always more efficient. With 500 watts on this controller screams to be 24 volt battery. At 12 volt battery is already maxed out with no room left to grow.
      MSEE, PE

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      • #18
        OK so the best thing is 3 strings of 2 panels in series. This will change the imput voltage to 35 volts to the kid, which Midnite says makes no difference. Can I not leave my battery bank as 12 volt...as I have a 12 v inverter?
        The array voltage goes up,amperage down, and the Kidd does the rest.
        Does this sound feasible ?

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        • #19
          Originally posted by opticalmike View Post
          OK so the best thing is 3 strings of 2 panels in series. This will change the imput voltage to 35 volts to the kid, which Midnite says makes no difference. Can I not leave my battery bank as 12 volt...as I have a 12 v inverter?
          The array voltage goes up,amperage down, and the Kidd does the rest.
          Does this sound feasible ?
          Yes, it sounds very feasible. --mapmaker
          ob 3524, FM60, ePanel, 4 L16, 4 x 235 watt panels

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          • #20
            Originally posted by opticalmike View Post
            OK so the best thing is 3 strings of 2 panels in series. This will change the imput voltage to 35 volts to the kid, which Midnite says makes no difference. Can I not leave my battery bank as 12 volt...as I have a 12 v inverter?
            The array voltage goes up,amperage down, and the Kidd does the rest.
            Does this sound feasible ?
            Yes you can leave it at 12 volt battery as long as you understand the system is maxed out with respect to panel power input at 500 watts. If you want to go higher will either require replacing the controller if you desire 12 volt battery, or go to 24 volts with the same controller up to 1000 watt input.

            There is a difference going 3 x 2. Half the fuses and wiring so it makes it less complicated and less expensive. Technically it is superior because it gives you a lot of head room at high temperatures.
            MSEE, PE

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            • #21
              Had I known I was getting a kid I would have bought higher wattage panels and less of them so that I might have an array with higher amperage. These 85 watt panels only output just less than 5 amps. When I rewire for the 3 strings of 2 the system amperage will be less with the 6 panels than it is with the 4 I have hooked up now to the PWM controller. Will the higher Volts make up the difference somehow? If I had my druthers, Id slap a couple of Hanwa 160's onto the kid and piggyback the two systems. That way on cloudy or crappy solar days I would be making more power.or maybe 2 kids!!

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              • #22
                Originally posted by opticalmike View Post
                Will the higher Volts make up the difference somehow?
                Yes, that's the whole point. You can put in 34 volts at 15 amps and get up to 42.5 amps out at 12 volts. Except that the Kid can't handle 42.5 amps, and it's not 100% efficient.

                Think of it as power out = power in minus a small loss due to inefficiency.

                Thus, 34 volts X 15 amps = 510 watts going into the controller.

                Coming out will be about 95% of those 510 watts, or 485 watts.

                When the battery is at 12 volts, you could have 485 watts ÷ 12 volts = 40.4 amps
                When the battery is at 13 volts, you could have 485 watts ÷ 13 volts = 37.3 amps
                When the battery is at 14 volts, you could have 485 watts ÷ 14 volts = 34.6 amps

                Of course, your panels will rarely produce 510 watts, and your controller can't put out more than 30 amps.

                --mapmaker
                ob 3524, FM60, ePanel, 4 L16, 4 x 235 watt panels

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                • #23
                  Originally posted by opticalmike View Post
                  Had I known I was getting a kid I would have bought higher wattage panels and less of them so that I might have an array with higher amperage. These 85 watt panels only output just less than 5 amps. When I rewire for the 3 strings of 2 the system amperage will be less with the 6 panels than it is with the 4 I have hooked up now to the PWM controller. Will the higher Volts make up the difference somehow? If I had my druthers, Id slap a couple of Hanwa 160's onto the kid and piggyback the two systems. That way on cloudy or crappy solar days I would be making more power.or maybe 2 kids!!
                  OK you need to understand how controllers work. With PWM controllers INPUT CURRENT = OUTPUT CURRENT. With MPPT OUTPUT CURRENT = PANEL WATTAGE / BATTERY VOLTAGE. That is very important to understand and know how it works. So you have 85 watt battery panels. That means they have a Vmp of roughly 18 volts and a Imp of roughly 4.7 amps. So you have 6 in parallel and the current adds up to 6 x 4.7 amps = 28.2 amps. So with a PWM controller if you have 28.2 amps input you have 28.2 amps output to a 12 volt battery. Now do the math. You have 510 watts input, and 12 volts x 28.2 amps output = 338 watts. You loose 172 watts.

                  With MPPT 510 watts input = 510 watts / 12 volts = 42 amps.

                  Do you see any conflict yet? Your Kid is to small for 510 watts.
                  MSEE, PE

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                  • #24
                    Originally posted by Sunking View Post
                    OK you need to understand how controllers work. With PWM controllers INPUT CURRENT = OUTPUT CURRENT. With MPPT OUTPUT CURRENT = PANEL WATTAGE / BATTERY VOLTAGE. That is very important to understand and know how it works. So you have 85 watt battery panels. That means they have a Vmp of roughly 18 volts and a Imp of roughly 4.7 amps. So you have 6 in parallel and the current adds up to 6 x 4.7 amps = 28.2 amps. So with a PWM controller if you have 28.2 amps input you have 28.2 amps output to a 12 volt battery. Now do the math. You have 510 watts input, and 12 volts x 28.2 amps output = 338 watts. You loose 172 watts.

                    With MPPT 510 watts input = 510 watts / 12 volts = 42 amps.

                    Do you see any conflict yet? Your Kid is to small for 510 watts.
                    Most of the time, with the conditions not being ideal, ie: shoulder months with less daylite and or cloudy days, the output is a lot less than optimal....therefore this would be a plus to have too much panel wattage ...wouldn't it? Say 350watts input =350/12=29 amps. So for all but a few hrs a day there should be more output.I guess it I were to rejig the wiring of the trojans to a 24 volt config, and get a different inverter, things would all be better.

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                    • #25
                      Originally posted by opticalmike View Post
                      Most of the time, with the conditions not being ideal, ie: shoulder months with less daylite and or cloudy days, the output is a lot less than optimal....therefore this would be a plus to have too much panel wattage ...wouldn't it? Say 350watts input =350/12=29 amps. So for all but a few hrs a day there should be more output.I guess it I were to rejig the wiring of the trojans to a 24 volt config, and get a different inverter, things would all be better.
                      As you mentioned, most of the time conditions are not ideal. If your panels are only putting out 350 watts, switching to a 24 volt system will not increase your panel's output.

                      Of course, on the few occasions when your panels are putting out 500 watts, your controller cannot handle all that power with a 12 volt system, but it could with a 24 volt system.

                      --mapmaker
                      ob 3524, FM60, ePanel, 4 L16, 4 x 235 watt panels

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                      • #26
                        Right now my combiner box has 4 10amp DC breakers if I am doubling up some panels in series will I need bigger breakers?

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                        • #27
                          Originally posted by opticalmike View Post
                          Right now my combiner box has 4 10amp DC breakers if I am doubling up some panels in series will I need bigger breakers?
                          Nope.

                          Wiring them in series will increase the voltage, not the current.
                          Paul

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                          • #28
                            Originally posted by opticalmike View Post
                            Right now my combiner box has 4 10amp DC breakers if I am doubling up some panels in series will I need bigger breakers?
                            What about the feeder cable going to the controller? What is in that one? You are going to need 40 amp
                            MSEE, PE

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                            • #29
                              Originally posted by Sunking View Post
                              What about the feeder cable going to the controller? What is in that one? You are going to need 40 amp
                              I'm just using the #10 multi strand stuff that the solar panels are wired with. The distance is 24" or less.
                              The controller imput terminals won't accept much bigger though, not sure what size wire can be used with the kid.

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                              • #30
                                Originally posted by opticalmike View Post
                                I'm just using the #10 multi strand stuff that the solar panels are wired with. The distance is 24" or less.
                                The controller imput terminals won't accept much bigger though, not sure what size wire can be used with the kid.
                                The largest wire that the terminals will accept, is #10 (5.26mm2). Visit page 15 of the Kid Instruction Manual, for further information.
                                Paul

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