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Building a Small System (some problems)

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  • #16
    Originally posted by ctwo View Post
    That GV10 looks like a nice controller and there is a version that supports 11.1V Li-Ion, not that I have to married to that type. I'll have to save some pennies for a while. Thank you.
    It is a nice controller, but I'm not sure it solves any of the problems that you've expressed with the model you own now.

    Problem 1: The load terminals (and USB output) shut off when in voltage recovery mode, and the USB port is always 100% powered by battery.
    The Genasun controller doesn't even provide load terminals, and offers no protection against over-discharge. If you just moved your load on the cheap PWM controller from the load terminals to the battery terminals, you cut all the goofy load control logic out of the picture (as Genasun has done).

    Problem 2: Your controller has no ability to limit charge current to 0.5C, or even limit charge current to within the rating of the controller.
    The Genasun is mppt, and is probably smart enough to limit charge current to the rated output by moving off of the maximum power point with the charge current rating is reached. However, it offers no ability to further reduce the maximum charge current, so that if you are using a battery that can only take 1.4 A, you are still at risk of hitting it with more than that. In fact, the Genasun is slightly worse in this respect, because as an mppt, it would convert your 100 W to 100 / 12 = 8.3 A, instead of the ~5.5A Isc that the PWM controller would pass from a 100 W panel (under STC conditions).


    I think "The Kid" by Midnite Solar offers all the functionality you are looking for (and more), but at ~$300, you'll need a few pennies to get there.
    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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    • #17
      Wow, I need to work on my Google Fu. I concede, this was somewhat an impulse buy, but these devices you all are presenting are in a different class than the lion's share I was able to find.

      1. I had already accepted that I would need to move all my loads direct from the battery and would need my own protection, but there is a little more to it than that...and that is my cheapie does not cut charge voltage and will float. Given regular use and the sun cycle, that might not matter, but would need manual attention or an additional circuit. I suspect the GV10 would handle that properly, but I've already been surprised.

      2. I really had planned on using a number of cells in parallel, so the panel would not be capable of over current. The details were not worked out yet, and I will admit that at the point when that was mentioned here, I had not considered it for what I was doing at the time. Dumb luck perhaps, but I did expose myself to risk with my anxious testing that I presented earlier (it was outside in the grass, so no worries). I also failed to discover the different battery technologies and the best option. My choice was one of opportunity and familiarity. To be completely honest, when my panel and controller arrived on the same day, I first drug out an old 10Ahr SLA I've had sitting for a couple years. I first measured 12.5V and that was the first battery I connected to my cheap SCC. I was disappointed to see only a couple hundred mA, but that was an obvious conclusion once I gave it a thought! So, I then went to the industrial 85wHr "laptop" battery pack and was getting confused about the load switching and the potential of some sort of BMS activating. That's when I went searching and found some videos showing three 18650 cells in series with my controller...I think I'm boring you now.

      I should regroup and properly define what I really want to accomplish - that is to have light-weight, portable solar power when tent camping (not hiking), that was the initial drive anyway. My whole system was t be bolted to the back of the panel for ease of use. I soon realized that would not be a constant use of the system and would like to be able to use the panel at home to supplement power needs (to lessen the cost of the system), maybe to run water pumps, lights, or some appliance within the capability of the 100W panel I have. That was to be an auxiliary use and a learning experience as I consider the longer term plan to install a complete solar home system (with windmill).

      The good news is that my battery packs arrived today and I am not nearly as anxious to start putting them to the test this weekend. No casualties :thumbsup

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      • #18
        Originally posted by ctwo View Post
        To be completely honest, when my panel and controller arrived on the same day, I first drug out an old 10Ahr SLA I've had sitting for a couple years. I first measured 12.5V and that was the first battery I connected to my cheap SCC. I was disappointed to see only a couple hundred mA,
        OK you mentioned in your opening statement you were not observing much current with the 18650 cells, and now saying not much with a Pb battery. That is telling me you have either a bad controller or incorrect wiring. Another remote possibility is you do not have the panel in direct sunlight around noon hours on a bright sunny day.

        OK let's make things simple. All batteries have minimum and maximum charge current requirements. Lithium is not real picky as they can be charged quite slowly, or quite fast up to C/2 is what most manufactures recommend. So how do you control the charge current on a solar system?

        It is stupid simple and when you hear the answer will make you wonder what you were thinking. You just match the panel wattage and battery AH capacity to work together. As an example lets say you have a 3S 40 AH LCO battery and want to charge it at C/2 or 20 amps. You could use a PWM controller if you want to throw a bunch of money away using low voltage battery panels. With PWM Controllers Output Current = Input Current. Makes life simple, you look at solar panels Imp current rating. You have to use 36-cell low voltage 12 volt battery panels like you have. A 100 watt panel has an Imp of 5.5 amps. You would need 4 x 100 watt panels wired in parallel. That would give you 22 amps of charge current. Close enough.

        MPPT Output Charge Current = Panel Wattage / Battery Voltage. Using that formula and rewriting it we can find that the panel wattage needs to Panel Wattage = Charge Current x Battery Voltage. A 3S LCO charge voltage is 12 volts for the calculation so, 20 amps x 12 volts = 240 watts. All you would need is a single inexpensive 240 watt high voltage panels with 72 cells and a 20 amp MPPT controller.

        See it is Stupid Simple. So consider yourself lucky you have a defective charge controller or wiring issue. If it had worked correctly you would hit your 2800 mah cells with 5.5 amps or a 2C charge rate which is 400% over the safe limit.

        I assume you intend to use the 100 watt panel which is fine, but you trap yourself with limits on battery capacity. If you use a PWM controller charge current is going to be 5 amps. That means you minimum size battery is 10 AH. You can go higher, but even that has limits because if it is too large will take forever to charge. If I had to put an upper limit would be C/6 and that would mean a maximum capacity of 30 AH. That would take two full days to recharge from a fully discharged sate.

        With a MPPT controller Charge current is 100 watts / 12 volts = ,8.3 amps, just call it 8 amps. Using the above limits is a battery of 16 AH to 50 AH.

        Now this will piss you off and it should. If you had bought 4 x 40 AH Calb Batteries would have cost you $200 to $240 depending on supplier.
        Last edited by Sunking; 06-10-2017, 12:43 AM.
        MSEE, PE

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        • #19
          Based on how quickly it sounds like the battery voltage was going from 9 to 11.1, I'm not so sure that whatever was monitoring the charge current was accurate when it was reporting only 150 mA.
          CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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          • #20
            I sure hope he did not receive his batteries at 3 volts. I would have sent them right back. They should be around Storage voltage of 3.4 to 3.5 volts. 3.0 volts is fully discharged and you do not ever want to go to 3.0 volts rested.
            Last edited by Sunking; 06-10-2017, 12:30 PM.
            MSEE, PE

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            • #21
              The battery packs all measure 36.x volts. They are 10S2P. They are for an e-bike or such.

              I was using a cen-tech DMM to measure battery current since the controller PV values were jumping around, about on a 1sec interval. I do not know how the DMM would respond to PWM.

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              • #22
                I simply ordered the lead acid version of the same PWM controller I was using and connected a 10Ah sealed lead acid battery. I connected my phone to the USB port with one of those USB power monitors and observed about 1 amp going into the phone, and about 200mA going into the battery at 13.7V. There was partial shade as the sun was behind trees.

                This does what I was expecting. I also expect that, with a larger (30Ah?) battery, this panel will allow me to run a continuous ~15W.

                How I get that number is from PVWatts that says I can expect 450Wh average per day in my area with my setup parameters.
                80% efficiency / 24h = 15W

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                • #23
                  Originally posted by ctwo View Post
                  I simply ordered the lead acid version of the same PWM controller I was using and connected a 10Ah sealed lead acid battery. I connected my phone to the USB port with one of those USB power monitors and observed about 1 amp going into the phone, and about 200mA going into the battery at 13.7V. There was partial shade as the sun was behind trees.

                  This does what I was expecting. I also expect that, with a larger (30Ah?) battery, this panel will allow me to run a continuous ~15W.

                  How I get that number is from PVWatts that says I can expect 450Wh average per day in my area with my setup parameters.
                  80% efficiency / 24h = 15W
                  PV Watts does not work with Battery Systems. It assumes a MPPT grid tied system using avg yearly production. For battery systems you must use worst case winter month, and meet minimum battery charge requirements. At best with a PWM system, efficiency is 50% if you are lucky. With a PWM system to run a gizmo of 15 watts 24 x 7 (360 wh/day) requires a minimum:

                  12 volt 150 AH battery
                  15 amp PWM controller
                  270 watt Battery panel (2 x 135 watt battery panels in parallel)

                  Suprise. Here is what you need to know. Anything you take off grid is going to cost you 5 to 10 times more than the POCO charges you for the rest of your life. That 12 volt 150 AH Lead Acid battery will cost you around $300 you need to replace every few years. All that to generate 3 to 5-cents worth of electricity per day.

                  Want LFP? The battery will cost $500. Want LCO like you have? Try $900. Be careful what you ask for.
                  Last edited by Sunking; 06-16-2017, 12:12 PM.
                  MSEE, PE

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                  • #24
                    Originally posted by Sunking View Post

                    PV Watts does not work with Battery Systems. At best with a PWM system, efficiency is 50% if you are lucky.
                    PVWatts output shows the potential DC power that is available from the panel, assuming mppt. That number is good for both grid tie and mppt charge controllers. What happens to that power from there will be different for grid-tie and battery based systems, but it still provides an essential input into any model of how a system will behave.
                    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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                    • #25
                      Originally posted by Sunking View Post

                      PV Watts does not work with Battery Systems. At best with a PWM system, efficiency is 50% if you are lucky. With a PWM system to run a gizmo of 15 watts 24 x 7 requires a minimum:

                      12 volt 150 AH battery
                      15 amp PWM controller
                      270 watt Battery panel (2 x 135 watt panels in parallel)

                      Suprise. You got a lot of learning to do.
                      But it may give some reasonable estimate of long term average array output that may be useful.

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                      • #26
                        Originally posted by J.P.M. View Post

                        But it may give some reasonable estimate of long term average array output that may be useful.
                        Provided the panels are mounted at the correct angle and compass point and there isn't any shade.

                        As you know, all of those factors change the output calculation and most people that go with a small DIY systems do not calculate for those variables.

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                        • #27
                          Originally posted by SunEagle View Post

                          Provided the panels are mounted at the correct angle and compass point and there isn't any shade.

                          As you know, all of those factors change the output calculation and most people that go with a small DIY systems do not calculate for those variables.
                          That is exactly why PVWatts, which models for specific user input array orientation, is so much more useful than the generic insolation charts that might otherwise be used.
                          CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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                          • #28
                            Originally posted by sensij View Post

                            That is exactly why PVWatts, which models for specific user input array orientation, is so much more useful than the generic insolation charts that might otherwise be used.
                            You beat me to it.

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                            • #29
                              There will be some shade. I connected a watt meter and 3 ohm load direct to the panel and got 367Wh in one day with the panel on the ground. Of course this is the optimal time of year, but not the optimal location on the roof where I'd have less shade. PVWatts is telling me the peak daily output is 520Wh and minimum is 260Wh.

                              If half that energy is wasted and I use the maximum value to size the battery, why would I need anything larger than 43Ah? It seems if the battery was any larger, it would never be able to get a full charge. My system wouldn't even charge a 40Ah battery since I'd be using energy during the day anyway.

                              Since the system is now functioning as I expected, I can proceed with my original plan which was to connect loads and take some actual measurements.

                              Yeah, so I could learn something...

                              thanks

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                              • #30
                                De-rate PV watts by 50% and you will be close for Battery systems with MPPT controllers. (I think)
                                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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