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  • Absorb times and SG measurements

    Hi guys -

    My off-grid solar power system is working great from what I can tell. Likely due to all the help I had from you guys to help design it and tune the charge times.

    The SG in the late afternoon after a full sunny day is 1.280 or slightly higher. My absorb time is programmed for 160min (was at 180min but slowly reducing).

    I was reading old posts the forum and found this statement from one of the wise members (sorry forgot the name/ID):

    Large lead acid batteries cannot be recharged in 3-5 hours. It takes 3-5 hours to complete BULK, and another 3 for Absorb, 6 hours minimum, So with off grid use, it's always a contest to try to get batteries properly charged at least once a week even in summer.

    My question is: Should I keep adjusting my absorb time to keep the duration to a minimum that will still get me to the battery manufacture's 100% SG reading?
    I am finding I am having to add a small amount of water every 2 weeks but wondering if i am pushing the batteries too hard on absorb... OR given the above quote, I shouldn't worry about it and go for long absorb times?

    I am using Rolls S-550 batteries and the BULK and ABSORB are set to the manufacturer specified 60 volts (For a 48volt off-grid frequently cycled system). Actually I reduced the absorb voltage slightly 59.7 volts just because I was a little worried about over charging and thought I'd maybe slowly reducing...

    I am keen to keep adjusting the system but need to understand what I am shooting for... the specificed SG (1.280) OR just have a long absorb time??

  • #2
    Actually you can forget all about Adsorb times. With Solar you want to set Bulk = Absorb = Float and no Absorb time as it becomes completely meaningless Marketing Term that it is. Your goal or objective is to find the voltage required to get your hydrometer to read 100% SOC at the end of day. In summer months if your system is sized properly might be 54 to 56 volts, and in winter months there is no voltage high enough to ever get to 100 % SOC.

    You have two choices with Pb batteries of overcharged and undercharged. Solar users due to improperly designed systems usually only have one chose to make and that is chronically undercharged. If given the choice you want slightly overcharged. Both are not good for Pb batteries but overcharged is the lessor of the two evils. Undercharge leads to much shorter battery life, sluggish performance, lower capacity, and poor cold weather performance. Overcharge maximizes capacity, performance, cold weather performance, and if not excessively overcharged much longer cycle life. Even if severely overcharged will yield like cycle life as chronic undercharged but wil perform a lot better than undercharged.

    So here is what you do. Forget Absorb time. Turn it off or set to minimum. Set Bulk = Absorb = Float. Monitor Specific Gravity at the end of each day. If Specific Gravity is low, turn the voltage up, if too high or using water turn the voltage down. Right now with days getting shorter wil result it you having to turn up voltages, and in longer days turn down voltage. If your system is properly sized wil not take a lot of adjustment. If like most folks are undersized will find there is no voltage voltage high enough in winter months, and summer is hit and miss.

    Now if your system is grossly oversized a very rare event or part ime weekend users can use Bulk, Adsorb, and Float setting with a 2 to 4 hour Absorb time because the batteries go into Adsorb shortly after Sun Rise each morning and have all day with no loads to Absorb and fully charge the battery. Or if a part time users really wants their batteries last a long can set the controllers up Bulk = Absorb = Float and use the manufactures Float Vootage for all 3 set points. Example on a 48 volt battery set all three to 54 volts and forget about Absorb.
    MSEE, PE

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    • #3
      ok, thanks Sunking. That covers my underlying question. You are recommending it's better to adjust the voltage as opposed to the duration. Currently (summer time), I am easily getting to 100% SG by mid-day.... so that's why I was trying to reduce the absorb time. This was because I was concerned about over charging.

      This is a summer home only and I fully understand that as we move into fall and shorter days and lower sun on the horizon, I will need to adjust and compensate.

      Not that I doubt your advice and strategy, I am curious about the underlying science of reducing voltage verses charge times ect...
      When I read the Rolls manual in the section for Renewable use case; they state BULK = ABSORB (60V) but a lower FLOAT (54V) voltage.

      Can I ask for my own understanding why (in the high PV conditions of summer) that it's better to keep the voltages all the same and reduce the voltage instead of higher & shorter Bulk/absorb voltages and durations?

      Comment


      • #4
        Originally posted by Riley5781 View Post
        ok, thanks Sunking. That covers my underlying question. You are recommending it's better to adjust the voltage as opposed to the duration. Currently (summer time), I am easily getting to 100% SG by mid-day.... so that's why I was trying to reduce the absorb time. This was because I was concerned about over charging.
        Understand your concern. The object of the game is to reach 100% SOC before the you run out of sunlight. Otherwise you loose the game. Make sense? There are a few ways to go about that. Some better than others and with a limited power source like solar, you have to make some compromises. So the goal is 1005 SOC and the only way to know with any accuracy is with a hydrometer. Pick a Pilot Cell, one near center mass, and keep a Temperature correcting hydrometer around. I prefer a separate thermometer and leave it in the cell. If you have not reached 100% an hour before sunset, you are not going to make it. Can extending Absorb Time work? Yes if you have enough panel wattage to start with.

        So if you have the voltages set to manufacture spec, maxed out Absorb Time, and still not getting to 100% guess what? You gotta change something because what you are doing is not working. Being Summer your troubles only get worse as days are getting shorter. So before you run out and buy more panels, crank up the voltages.

        This is a summer home only and I fully understand that as we move into fall and shorter days and lower sun on the horizon, I will need to adjust and compensate.

        Originally posted by Riley5781 View Post
        Not that I doubt your advice and strategy, I am curious about the underlying science of reducing voltage verses charge times ect...
        When I read the Rolls manual in the section for Renewable use case; they state BULK = ABSORB (60V) but a lower FLOAT (54V) voltage.
        I understand, and if you hit 100% SOC before sunset, yes you want to roll back Voltage to 54 volts they call Float. That misses the Point. What do you do if you do not hit 100%? Last thing you want is a chronically undercharged battery.

        Originally posted by Riley5781 View Post
        Can I ask for my own understanding why (in the high PV conditions of summer) that it's better to keep the voltages all the same and reduce the voltage instead of higher & shorter Bulk/absorb voltages and durations?
        Sure you can ask. When I have more time and if I remember will try to answer.

        MSEE, PE

        Comment


        • #5
          Originally posted by Sunking View Post

          Understand your concern. The object of the game is to reach 100% SOC before the you run out of sunlight. Otherwise you loose the game. Make sense? There are a few ways to go about that. Some better than others and with a limited power source like solar, you have to make some compromises. So the goal is 1005 SOC and the only way to know with any accuracy is with a hydrometer. Pick a Pilot Cell, one near center mass, and keep a Temperature correcting hydrometer around. I prefer a separate thermometer and leave it in the cell. If you have not reached 100% an hour before sunset, you are not going to make it. Can extending Absorb Time work? Yes if you have enough panel wattage to start with.

          So if you have the voltages set to manufacture spec, maxed out Absorb Time, and still not getting to 100% guess what? You gotta change something because what you are doing is not working. Being Summer your troubles only get worse as days are getting shorter. So before you run out and buy more panels, crank up the voltages.
          Thanks again for the advice and mentoring. I am getting to 100% SOC (according to my temp compensated hydrometer) and even with backing off the absorb time (now 180 min), it's still getting to 100%. That's my concern and purpose behind my questions. for summer time, my panels are able to fully charge and more....

          My goal is to optimize and reduce the over charging... which I believe is happening and over the next few weeks anyway.... I fully understand as I get into fall, I will need to charge the settings.

          Comment


          • #6
            Originally posted by Riley5781 View Post
            Can I ask for my own understanding why (in the high PV conditions of summer) that it's better to keep the voltages all the same and reduce the voltage instead of higher & shorter Bulk/absorb voltages and durations?
            OK have a few minutes now. It is all about Amp Hours and how batteries charge. Think of it like an hourly job, whatever that is. Amp Hours = Amps x Hours. If you make $10 hour, and need to make $100 per day to live, you need to work at least 10 hours or you are SOL. Second challenge is your boss does not pay overtime, instead regressive pay. After 5 hours your hours, you hourly wage goes down with each passing hour and after 10 hours you are done making $5 per hour. That is how a battery charges aka Absorb cycle works.

            When you set your charger to say 60 volts, when morning comes and if you are observant, you have never seen 60 volts on the battery until later in the day. Bulk is nothing more than the constant Current Phase of the battery charge cycle. A charger is both a limited Current Source and a Regulated Voltage Source. You may set the voltage at 60 volts, but that does not mean the Charger will put out 60 amps, because current is limited by the Controller and even more restricted by panel power. If the battery was at say 49 volts in the morning, sun pops up and controller is providing max charge current, the voltage is only going to go up .1 to .5 volts if even that much. The Controller in Bulk mode will provide the batteries with as much current as the panels can supply aka Constant Current. It will pump Constant Current until the battery voltage rises to whatever set point like 60 volts. Once the battery Open Circuit Voltage charges up to around 59.5 to 59.8 volts the Charger Voltage and Battery OCV are approaching equilibrium and now the battery Internal Resistance determines how much current is flowing. At this point the Charger can provide enough current to hold a CONSTANT VOLTAGE with a funny name called ABSORB. When you reach this point the battery is 80 to 90% charged up. However Charge Current rapidly tapers falling off to ZERO Amps when the batteries reach saturation, current stops, and charging stops. If the voltage was high enough the battery is fully charged and fully saturated. It is not a timed event and it takes several hours that are undefined.

            OK reminder Amp Hours = Amps x Hours. We cannot control time or hours, but we can control Amps. Instead of setting the voltage to 60 volts and having the Controller go into Absorb at say 3 in the afternoon thus cutting current back and slowing things down, turn the voltage up and force the controller to stay in Constant Current mode charging as fast as we can and get as many Amp Hours in the batteries as physically possible with a limited amount of Sun Hours or Time we cannot control. So instead of slowing down, tapping the brakes, coasting to Stop at the Finish Line, we instead put the pedal to the metal, floor it, and hope we make it to the Finish Line before it gets dark and we run out of Time.

            MSEE, PE

            Comment


            • #7
              Originally posted by Riley5781 View Post

              Thanks again for the advice and mentoring. I am getting to 100% SOC (according to my temp compensated hydrometer) and even with backing off the absorb time (now 180 min), it's still getting to 100%. That's my concern and purpose behind my questions. for summer time, my panels are able to fully charge and more....
              OK my bad, I thought you were under charging. Reduce Absorb Time until you just notice the the Specific Gravity start to Fall Off. Keep the manufactures voltage settings and work on time a rare commodity with solar. Going back and rereading this is a Vacation Home or Part Time Use? If so relax a bit because if you are only talking being there say half a week giving the batteries a few days to completely recover and recharge. That is when you can pretty much use manufactures settings. Just keep in mind manufactures settings are Ball Park Settings to start with. They maybe high or low depending on your use and environmental conditions. You still use your hydrometer to set the Voltages and Absorb time.

              MSEE, PE

              Comment


              • #8
                Perfect... you have confirmed my approach (at least for the summer months). For next few weeks of August the place will be used activly but then reduce down into Sept.
                At that time I will need to think about how to balance the reduced consumption against the reduced sunlight hours.

                Once we get into the winter months, I basically charge up to max and then disconnect all loads and panels... just let the batteries sit... that worked well last winter for extended periods.

                Comment


                • #9
                  Originally posted by Riley5781 View Post
                  Perfect... you have confirmed my approach (at least for the summer months). For next few weeks of August the place will be used activly but then reduce down into Sept.
                  At that time I will need to think about how to balance the reduced consumption against the reduced sunlight hours.

                  Once we get into the winter months, I basically charge up to max and then disconnect all loads and panels... just let the batteries sit... that worked well last winter for extended periods.
                  I will give you the best tip you will ever get. When you know you are going to be gone a while long term, or even a few days, take a few steps to preserve your batteries.

                  Set all voltages Bulk = Absob = Float = 54 volts. What you are doing is making your solar system a Float Charger like the pros use to maximize cycle life and remain fully charged ready to go when you come back. Batteries in Float Mode can last a long time and is the kindest gentlest charging method there is. No gassing because you stay below gassing and corrosion voltage. High enough voltage to keep iron sulfate on the grids and plates. Keep them cool and can last a few seasons with no use. So when you leave turn everything off load wise, set voltages to float on the controlerrs, check water level, and keep cool, and your batteries will have had a nice time in the Battery Spa having good food and sex while your are gone. When you return they are all rested up and ready to go.
                  MSEE, PE

                  Comment


                  • #10
                    Thanks Sunking. I am going to do this over the weekend as I am packing up for the season and moving out for a few weeks..... I will be disconnecting all loads (turn off inverter/charger) so it will be just the charge controller, control panel and combox active (very small loads). It's hard to disconnect the Combox because of termination point is on that device.

                    Over winter I am thinking it still might be better to totally disconnect the batteries because the panels will get covered with snow and there's no power at all coming from them. I worry that the small drain of the electronics (charge controller, SCP control panel and Combox) will drain the batteries. I would like to totally disconnect the batteries for the snow months....
                    Does this strategy make sense?
                    Last edited by Riley5781; 09-14-2018, 06:29 PM.

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                    • #11
                      Sunking,

                      How often do you go through the measure & adjust process? Monthly? Quarterly????

                      Thanks.

                      S.

                      Comment


                      • #12
                        The SCP & combox both draw their power from the Xanbus, and over winter, you should disconnect them both, as they WILL load the system and deplete batteries if you have no charging
                        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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                        • #13
                          Originally posted by PNW_Steve View Post
                          Sunking, How often do you go through the measure & adjust process? Monthly? Quarterly????
                          It is Dynamic and an ongoing process. Although not always obvious to us mere mortals, depending on your location, time of year, you gain or loose 2 to 15 minutes per day of Sun. So if I had to put a time on it I would say; at least once a week when you run your genny to top off batteries as part of your regular maintenance routine.
                          MSEE, PE

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                          • #14
                            Originally posted by Mike90250 View Post
                            The SCP & combox both draw their power from the Xanbus, and over winter, you should disconnect them both, as they WILL load the system and deplete batteries if you have no charging
                            Yes, agreed but I assume it's not that simple to just disconnect the SP and Combox due to cabling the required terminators. Int eh dead of winter with snow on the panels it's just easier to get the batteries fully charged and then disconnect the batteries with the breaker. I think the solution works well until there's snow....

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