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How long to recharge batt bank?

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  • How long to recharge batt bank?

    Hi, folks! This is my first post here. I do hope I don't say anything stupid...which means I probably already have.

    I have built a small solar powered battery bank for use at a desert camp one week a year. I have two 230w 24v panels connected in parallel to an MPPT controller (SainSonic Tracer 3215RN 30amp 12v/24v). The controller is attached to a bank of six 12v 100-amp hour gel batteries.

    The unit is used in the desert north of Reno, which receives about 12.5 hours of sun during the week the system will be used. (Solar irradiance factor is about 6.8 at this location at this time of year.) My panels are hung at 55 degrees from vertical facing due south.

    The unit was built primarily to power refrigeration, and the fridge unit I'm getting (a large converted chest freezer) pulls 2.26 kwh in a day, or 188.3 amp hrs, according to my kill-a-watt. We are hoping to be able to power some sundry elements like LED lighting, a radio, etc, as well, but refrigeration is the primary goal.

    I have assembled all this with VERY little electrical knowledge, but guided by Boxwell's Solar Electricity Handbook. The system works and has been powering several smaller fridges in my garage, but not the biggie I'm about to get.

    I assume that if I pull 188.3 amp hours out of my bank, that leaves me with 411.7 amp hours. I know I don't want to drop below 300 amp hours, or 50% of my capacity. So let's pretend I pull 300 amp hours out of my bank in a day. What I still can't figure out long will it take me to recharge 300 amp hours back into my 12v bank with my 24v 460w array going through my MPPT controller?

    Any guidance/criticism/damnation will be welcomed.

  • #2
    1) are those batteries GEL or AGM ?? there is a BIG difference in the way they are charged. GEL is almost useless for off-grid usage, because they are not able to be quickly charged. There should be a spec sheet that explains all this in detail for your batteries. AGM cells can be recharged quickly and
    are better suited for deep cycle off grid use. Both types are pricey, much more than flooded deep cycle batteries.

    2) 12V, 600AH - 6 batteries in parallel. Asking for problems. 2 or 3 batteries in parallel, possibly OK.
    With 6 batteries, it becomes difficult to wire according to the gold standard: Wiring on the diagonal

    3) Minimum charge rate. Useable Insolation at BRC in summer is actually only about 6 hours max, so you have to manage your loads, to stay within your harvest capability of 6 hours, at 50% of panel rating. When you add the battery recharge inefficiencies, lost power from "hot in the sun" PV panels, you derate your system by 50%. What is your inverter ? It's standby losses ?
    600ah of batteries generally, needs to be recharged with 60A (10% of nameplate)
    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



    • #3
      You have a few problems.

      1. The big one is the battery type and configuration. If they are Gel batteries, they are completely unacceptable for use in solar. Gel batteries need to be charged up very slowly. There are a few that can handle high charge rates but there are few of them on the market. A 600 AH Gel battery should not be charged with more than 30 amps. You will be at 38 amps which you can probable get away with. If they are AGM no problem as Mike indicated.

      6 batteries in parallel is a nightmare and you will not be able to charge/discharge them equally. Ideally you only want to have only 1 string of batteries. If you need 600 AH, then buy 600 AH batteries like 3-4 volt 600 AH batteries

      2. You indicated you will be using 2300 watt hours per day, and now the Sun Hours are 6.8 Sun Hours. That is all fine and well, but may not have much to do with your application if it is not being used right now. To generate 2300 wh/day the panels must generate 3450 wh/day. To do that with 6.8 Sun Hours requires a minimum panel wattage of 3450 wh / 6.8 h = 507 watts, or a 500 wat panel. You only have 430 watts. But like I said that only applies right now. If you do this in say in October your Sun Hours fall to 5.6 Sun Hours which means you would need 3450 wh / 5.6 = 616 watts or about 600 watts.

      But here maybe your saving grace. You said this is just a one week job, and that you are using a fixed 55 degree tilt angle which may or may not be a good angle for year round. For off-grid in your area year round latitude angle is best and for Reno area is approx 39 degrees. But you are not using this year round, just 1 week out of the year so point them directly into the sun at solar noon. For example in October with a fixed array at Latitude angle is 5.6 Sun Hours to 7.2 Sun Hours. That would be more than enough with a 460 watt panel array.
      MSEE, PE


      • #4
        Sorry for my ignorance...I was responsible for the solar part of the build. Someone else was responsible for the batts/inverter part so I wasn't as familiar with that side.

        The batts are AGM. Here is a link to their specs:

        They were bought 1-year used for $30 each from a govt surplus solar project that requires the batts be replaced every year. We had to buy 6, it was the minimum. Should we NOT use some of them? I'm well aware of the dangers of buying used equipment, but we use this system 1 week a year and our budget for the whole project was $1k, including the fridge conversion, so I didn't have that much to work with.

        The bank is currently wired using the second method in the link specified, with the positive going into one end of the bank, and the neg coming in from the other. I realize that wiring up the final "gold standard" way will not work with 6 batts, but I could certainly wire it according to example 3. (Though the document specifies isn't not much different from example 2.)

        My inverter is a Cobra CPI 2575 2500w/5000w peak which claims 5% inefficiency, but I don't believe that for a second.

        After our group crunched numbers last night, we've decided to go with a smaller fridge. (The one in my garage only eats 480watt hrs a day, so even 2 of those would still eat less than half the electricity of the mammoth one.)

        Still, it would be interesting to know how much time it will take the panels to recharge the bank if it's pulled down to 50%. I've been correction on insolation, should be a factor of 6. 460w panel at 24v, through the MPPT into the 12v bank, each batt is 100 amp hours and we have 6 to play with. Any estimates/suggestions? Again...this system only need to perform for 1 week out of the year, spanning Labor Day.


        • #5
          Originally posted by TheBenStarr View Post
          They were bought 1-year used for $30 each from a govt surplus solar project that requires the batts be replaced every year.
          That is why they need changed out every year. They are not made for cycle service. You can use them but do not count on them having their rated capacity. It will be a crap shoot.

          Originally posted by TheBenStarr View Post
          After our group crunched numbers last night, we've decided to go with a smaller fridge. (The one in my garage only eats 480watt hrs a day, so even 2 of those would still eat less than half the electricity of the mammoth one.)
          Well there is a reason we use 5 day reserve capacity, clouds. A 5 day capacity in reality only gives you 2.5 days to 50%. If you discharge to 50% and have a cloudy day, you shut down until you have had a complete full sunny days to recharge. So you are out 2 days minimum with a 50% capacity design and no generator.

          With 460 watts of panels and the batteries I think it will work out with a smaller demand
          MSEE, PE


          • #6
            We are well aware the batteries will not have full capacity. Which is why we spent $30 on them, as opposed to $230. We only need them to work for 1 week.

            Cloudy days are EXCEPTIONALLY rare in this part of Nevada at this time of year. Exceptionally. So much so that when a cloud DOES cover the sun, everyone in the city whoops for joy.

            And we have a generator to use as a backup, it's not a crisis if we run out of battery. We're not talking life or death situations, even if there's NO battery OR generator.

            My primary question is I don't understand how long it will take to recharge the battery bank using our solar panels, if we drain the bank to 50% capacity...does anyone have any insight on that, or how to calculate it? I have 2 books on solar power, and they are chock full of formulas for calculating all sorts of figures on my solar system...but neither explains how to tell how long it will take to refill the batteries after draining them. (Which means it's either really easy, meaning I'm really stupid...or it's not something that can really be calculated because there are too many variables.)

            We're in the process of draining the bank to do a practical test, but the system is currently in North Texas, which is a long way from Northern Nevada. And it's taking a LOOOOOOOOOOOONG time to drain the bank...2 fridges running on it for 48 hours only drained the bank about 5% according to our meter.


            • #7
              Ben I am going to let you answer your own question as it will help you understand what is going on

              A battery capacity in Watt Hours = Volts x Amp Hours

              To recharge a battery you must replace the amount of Watt Hours used x 1.5 if using a MPPT controller, or x 2 if using PWM

              So let's say you use 1000 watt hours in a day. That means you have to generate 1500 watt hours with the panels. So how many panel watts do you have and how many sun hours?

              Watt Hours = Watts x Hours

              So if you have a 500 watt panel and MPPT controller; how many sun hours does it take to make 1500 wh?

              For the rest of the story try reading this Sticky. It will answer all your questions.
              MSEE, PE


              • #8
                One little thing about the used batteries - even for your casual use.

                Since you are putting these batteries into a bank, I'd definitely try to balance them by giving each a decent full charge individually before using them as a bank. Being used, and not knowing what each battery's true health is, makes this even more important than normal. Knowing this, if the system doesn't perform, don't blame the batteries per se, because you have no real history of their use/abuse, other than they are just cycled out of commision each year. It could be quite possible to have one bad apple to spoil the whole bank.

                A decent charger will help you spot these problems. Re: draining the bank - I hope you aren't trying to do a full-on load test to full discharge (10.75v or so) as a bank without knowing the health of the batteries individually. That test could kill off weak, but usable batteries. If they are each fully charged to start with prior to putting them into bank service / testing, then you stand a better chance of not killing weak ones in an unbalanced bank.


                • #9
                  Thanks SO much folks, and my apologies for saying stupid things. Sunking...that was exactly what I was looking for, and it should have been that simple in my mind, but after reading SO many books and SO many threads on so many forums, I had convinced myself that it wasn't. PNJunction, we did individually charge each battery before assembling them into the bank. (They were already full when we bought them, it didn't take long to get them up to max charge.) Each one tested out just fine, we're keeping our fingers crossed.