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  • Whats wrong with this system??

    I bought a 115W (24V) solar panel from Ebay
    Also an MPPT charge controller (the 20A black and yellow one with the LCD screen)
    and 2 x 105AHr batteries in parallel (they were new from a store).
    And a 1000W pure sine wave inverter

    By my math I should have around 1260 Whr of power - figure on using the batteries to 50%

    I converted a chest freezer to a fridge after reading some posts about the great efficiency and how to implement an external Johnson Controls thermostat set around 35-38F.
    As a freezer, the unit has a rating of 172KWHr per year, which should factor to about 470WHr per day. Even though I am running as a fridge, I am taking the conservative measurement of freezer numbers.

    Strange behavior so far: At the end of every day, the batteries are basically dead (11.7V or less), even when we arrive at the cabin and everything is topped up (>13.1V).

    All voltages checked on my inverter display, and my MPPT controller display and also verified by a hand held reader.

    OK, so you would say that either the fridge is sucking way more power than I figured or I just have bad batteries right?
    But the really weird part is that by the next afternoon (4pm or so) all the readings are back up to 12.4-12.5 volts!
    Which tells me that I am back to almost 50% battery capacity, even though I only have one 115W panel !

    How is this possible, and do you have any advice for me to try this weekend ?

    THANK YOU

  • #2
    The very first step (if you have FLA batteries) is to get a good temperature compensated hydrometer to measure the specific gravity (SG) of the electrolyte in each cell of your battery bank.
    Using voltage as a guide to State Of Charge (SOC) is not reliable, especially if you do not rest the battery (neither charging nor discharging) for three or four hours before you measure the voltage.

    It sounds like you batteries are being undercharged, and this can lead to a permanent loss of capacity from sulfation.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

    Comment


    • #3
      Originally posted by msalari View Post
      I bought a 115W (24V) solar panel from Ebay
      Also an MPPT charge controller (the 20A black and yellow one with the LCD screen)
      and 2 x 105AHr batteries in parallel (they were new from a store).
      And a 1000W pure sine wave inverter

      By my math I should have around 1260 Whr of power - figure on using the batteries to 50%

      I converted a chest freezer to a fridge after reading some posts about the great efficiency and how to implement an external Johnson Controls thermostat set around 35-38F.
      As a freezer, the unit has a rating of 172KWHr per year, which should factor to about 470WHr per day. Even though I am running as a fridge, I am taking the conservative measurement of freezer numbers.

      Strange behavior so far: At the end of every day, the batteries are basically dead (11.7V or less), even when we arrive at the cabin and everything is topped up (>13.1V).

      All voltages checked on my inverter display, and my MPPT controller display and also verified by a hand held reader.

      OK, so you would say that either the fridge is sucking way more power than I figured or I just have bad batteries right?
      But the really weird part is that by the next afternoon (4pm or so) all the readings are back up to 12.4-12.5 volts!
      Which tells me that I am back to almost 50% battery capacity, even though I only have one 115W panel !

      How is this possible, and do you have any advice for me to try this weekend ?

      THANK YOU
      I see possible reasons for the low battery.

      First is one 115watt panel will be able to generate at best about 6 amps for 4 to 5 hours. That is way to little for your 12v 210Ah battery system. You need about 20 - 25 amps for proper charging.

      Second your black & yellow 20 Amp MPPT charge controller may really be a PWM type which will turn that 115 watt panel into one around 75watts.

      Another problem is discharging a battery by 50% is not a good way to get the full life out of it.

      Also a battery that has a 12.4 to 12.5v reading is closer to 60% - 70% charged. It needs to be around 12.7 to 12.8v a couple of hours after it has been charged for it to be at 100%.

      So worst case is those batteries are close to dying and need to be replaced. But before you just replace them you really need to design a battery system that only gets discharged about 20 - 25% max a day.

      If you determine that a 12v 210Ah system is sufficient then you need to add 2 more of those 115watt panels and get a quality 30Amp MPPT CC.

      Comment


      • #4
        Thanks to both of you.

        Regarding the panel not being enough to charge the batteries... I use the cabin a few times during the summer, for perhaps 2-3 days per trip.
        our intention was only to have them charged and 'ready' when I arrive at the cabin, so I plan on having a couple of weeks between visits. This should be more than sufficient to fully charge them. I guess the problem being that as soon as I start using them, there won't be enough power from the panel to replenish them after my 1st or 2nd day of use.

        thanks again

        Comment


        • #5
          Originally posted by msalari View Post
          Thanks to both of you.

          Regarding the panel not being enough to charge the batteries... I use the cabin a few times during the summer, for perhaps 2-3 days per trip.
          our intention was only to have them charged and 'ready' when I arrive at the cabin, so I plan on having a couple of weeks between visits. This should be more than sufficient to fully charge them. I guess the problem being that as soon as I start using them, there won't be enough power from the panel to replenish them after my 1st or 2nd day of use.

          thanks again
          The problem will be that you can't slow charge batteries like that. There isn't enough power to maintain the batteries and they will not last long at all with this kind of abuse.
          OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

          Comment


          • #6
            Originally posted by msalari View Post
            By my math I should have around 1260 Whr of power - figure on using the batteries to 50%
            Your math really sucks. A 115 wat panel will make eon a very good day will make 300 to 400 watt hours at best. A 114 watt panel can only support a 24 volt 60 AH battery. So your panel is way to small for the battery and your math really sucks. Your system is doing exactly what it is suppose to be doing, going dead from poor math skills.
            MSEE, PE

            Comment


            • #7
              Originally posted by msalari View Post
              How is this possible, and do you have any advice for me to try this weekend ?
              MEASURE your actual wattage draw over time with a P3 International Kill-A-Watt meter. Just watts * the hours you plan to run things will do for now. You can measure it running from your household ac first.

              Your batteries - if they were actually in good shape- should provide:

              105 * 2 = 210ah total
              210 / 2 = 105 ah usable for a 50% DOD max.
              105 * 12v = 1260wh available for use before reaching the 50% DOD. This is a simplistic napkin-calc that doesn't take into account peukert or other losses. So lets cut a bit more from the napkin:

              1200wh available.

              To avoid massive peukert effect (drawing too much too fast which drops voltage):

              1200 / 10 = 120wh

              So, for that system, my napkin says that for a continuous draw, about 120w per hour is about the max daily for about a 10 hour run continuous. If your fridge combo doesn't have excessive inductive current spikes, then use a smaller inverter to cut down the loss / self consumption to perhaps a smaller 500w unit. I'm assuming a 12v inverter since you mentioned battery voltages in that range ... Turn OFF the inverter when you can, as that by itself draws current even when not in use.

              I'd LOVE for peers to check my math!

              Comment


              • #8
                Originally posted by msalari View Post
                I bought a 115W (24V) solar panel from Ebay
                ...
                and 2 x 105AHr batteries in parallel (they were new from a store).
                ...
                the unit has a rating of 172KWHr per year, which should factor to about 470WHr per day. Even though I am running as a fridge, I am taking the conservative measurement of freezer numbers.
                Have you actually used a Kill A Watt meter to see exactly how much power the freezer is actually using? That should be the first step. I definitely agree that your panel is way too small to support that size of bank but I'm curious about the actual specs of the batteries too. Are those 105 AH batteries true deep cycle batteries with an actual AH rating on the sticker (rated at the 20 Hr rate)? Sometimes it is easy to confuse that number with a reserve capacity value. Batteries that are not true deep cycle batteries should have an actual Ah rating on them so I was just checking.

                It sounds like you were hoping to run that fridge for 3 days straight with basically very little re-charge capability. It isn't working out that way so either the system is draining much faster than you think and/or you don't have the capacity you think you have, and you definitely don't have a good way to put the power back into the batteries.

                Comment


                • #9
                  Thanks to all of you, except Sunking - wow what a tool that guy is. I've read a few of his posts on this forum so at least I can see that its not just me.
                  Why would people be mocked for lack of understanding?

                  PNjuntion: I am confused about the 10 factor you used and the "peukert effect" - never heard of this before.
                  But it certainly might help explain why they are draining fast. Can you explain a bit more?

                  Also, I can't see evidence anywhere that says that charging batteries with less than 10% amps factor will kill this system so fast. I know <10% charging rate is NOT ideal, and could reduce battery life, but to me it does not explain why my system is draining fast when it is fully charged and I am only talking about 10-12 cycles since startup last July - very low usage here.

                  I have checked the actual ratings on the batteries and we are good there.

                  The fridge could absolutely be pulling more than it claims, so I bought a Kill-A-Watt meter and we will take some real measurements on July4th weekend.
                  Of course this could absolutely explain the problem.
                  The other factor that I'm considering is the STARTUP current that the fridge uses - it is too much for the inverter and so it sends it into a tizzy until the startup surge is done for a few seconds.
                  So I am thinking about a larger inverter, but I don't know if this will really help the situation: The startup current must be more than the 1800W rating of the inverter but could this really be killing the batteries if the startup is only lasting 2-3 seconds?

                  Thanks again guys!

                  Comment


                  • #10
                    As others have said, you are killing your batteries. This is what the trend in the 70's was, when PV panels were really really expensive. Small panel, large battery bank, and hope that on a good day, the PV would slowly recover the batteries. Didn't work that way, the batteries sit too long below 80% full (20% discharged) and sulfate, and die an early death.
                    You have the same thing going on, the batteries have sulfated, lost capacity, and now appear to recharge quickly. And then go dead just as quickly.
                    What's a person to do ?
                    Forget the startup load, it's not long enough to kill batteries, but the inverter sure knows it. Larger inverter = larger standby losses.
                    I'd say you need to rethink the whole system. I'd say you need 3 of those panels, a real charge controller, and a battery bank of 2, 6V 100ah golf cart batteries (wired in series for 12V) The key is to keep the batteries charged. Running them to 50% on weekends is hard on them, but it's a cabin.
                    Do you shut the fridge off while you are gone, or leave it running ?
                    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

                    Comment


                    • #11
                      Thanks very much.
                      I do completely shut down the inverter when I'm gone, just let the batteries charge.

                      OK so if I have "trained" the batteries to charge fast and drain fast, I still don't get why I saw the problem right away - I would have thought this effect would take some time to inflict the damage.

                      But anyway, I will definitely look at more panels, and I guess I need to look at new batteries also. Thanks!


                      Comment


                      • #12
                        As I said earlier your math and understanding really suks. That is just a fact you cannot hide from. I tell it like it is.

                        If in fact your fridge uses 1200 watt hours per day, would require if using 12 volts @ 500 AH battery. You have 210 AH. A 210 AH battery can only supply you reasonable 500 to 800 watt hours per day. Compund that problem with your 100 wat panel can only generate 300 watt hours of usable energy per day. Nothing you have adds up.

                        To support that large of a battery and generate at least 1200 watt hours per day depends on what kind of controller you use and location. If PWM would require roughly 600 watts, and if using MPPT would require roughly 400 watts. with a 40 amp controller. You are not even remotely close.

                        With a 100 watt panel you have with a 20 amp MPPT controller can only support a 12 volt 80 to 100 AH battery. Your 100 watt system can only support one of your batteries, not two. With a 100 watt panel you can only support a 100 to 150 watt Inverter and only give you about 300 watt hours of usable energy per day.

                        Put another way in a way you might understand is your job only pays you $300 per day, but you are spending $1200 per day. How many days until you are bankrupt? Answer less than a day. To live you need a job that pays at least $1200 per day and have $6000 in the bank for rainy days.

                        Bad news is basically you cannot use anything you have, except maybe the inverter, and have to start over.
                        Last edited by Sunking; 05-31-2016, 10:25 AM.
                        MSEE, PE

                        Comment


                        • #13
                          Thanks Sunking. Just so you know, your English skills really suck. That is just a fact, and I tell it like it is.
                          I'm not sure where you got that my fridge is using 1200WHr per day. I said it was 470WHr per day and that is the calculation if I am running it as a freezer.
                          Mike and the others have the capability to explain concepts without being insulting.

                          I understand now that the issue is that I am under-powering the batteries. I think I will also replace the inverter as it does not like the startup current - I will buy a larger one just to stop the annoying screams every time the fridge starts. The few mA of extra drain from a larger inverter I think I can live with.

                          I believe that my MPPT controller is OK and 20A is plenty for my system. I need MPPT anyway since the panel is 24V

                          I will add 2 panels to the one I have and replace the batteries and see where to go from there.

                          Does anyone see a problem with this approach?

                          Comment


                          • #14
                            Originally posted by msalari View Post
                            Thanks Sunking. Just so you know, your English skills really suck. That is just a fact, and I tell it like it is.
                            I'm not sure where you got that my fridge is using 1200WHr per day. I said it was 470WHr per day and that is the calculation if I am running it as a freezer.
                            Mike and the others have the capability to explain concepts without being insulting.
                            I have made posts to this effect... he may be more caustic than you are used to, but he knows what he is talking about. Not to sounds as if I am defending him (or that he needs it) but I am sure that the same questions get asked AND ANSWERED over and over and over and over. I too am guilty of not doing my homework prior to asking questions; I learned that lesson.

                            My advice?? Power through it; you'll end up learning a LOT.

                            As Sun has said in other posts, he may just be saving your life by shocking you with words, before you shock yourself with 100 Amps.

                            And, as the world is changed by your example, not your opinion... I will just say this is MY opinion. YMMV.

                            Comment


                            • #15
                              Originally posted by msalari View Post
                              Thanks Sunking. Just so you know, your English skills really suck. That is just a fact, and I tell it like it is.
                              Hey you can't get better info on battery systems, than from Sunking. Once you get used to the style, you may find him
                              very entertaining.

                              What bothers me a lot more, is all the people who don't know the difference between TO and TOO, including a couple just above. Bruce Roe

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