Announcement

Collapse
No announcement yet.

Could you guys help me with a 24V 100Ah solar battery system?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Could you guys help me with a 24V 100Ah solar battery system?

    Hi all,

    I'm in Puerto Rico and wanted a back-up solar power system for this coming hurricane season and just power cuts in general. I'm not really interested in a whole home power system as I can't afford it. I was able to get a deal on x2 12V 100ah AGM batteries (new) for the battery bank. I've already used a kill-a-watt meter to measure how much power my appliances use, so it'll be a case of powering what I can, for as long as I can - aiming to discharge the battery no more than 30-40% to extend its lifespan.
    So working backwards;

    1. Am I correct that the max PV array for 24V 100ah AGM battery is going to be between 400-700W? Or could I go higher? If it's 700W, I would aim for x2 350W panels.
    2. Charge controller for 700W array would need to cope with (700/24) 29.1Amps but a 40A mppt would be the choice?
    3. On a 24V 100ah battery what's the upper limit for an inverter? Should it match the PV array which if the max is 700W, my inverter should be 700W?

    Thanks for any input you have!

  • #2
    You are building it backwards.
    1) calculate loads
    2) calculate solar / sun at your site & panels
    3) chose battery to meet your load requirements

    For me, a 700w inverter would only run lights. Generally, it's not large enough to start a fridge.

    If you have 29A of expected charge rate, a 30A controller usually is fine, arrays seldom put out nameplate power.
    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


    • #3
      Thanks Mike,

      So I used a kill-a-watt meter to measure the power draw of the appliances in our office which is what I want to be able to keep up and running during the day.
      [B]DayTime:[/B]
      PC and monitor (164Wh), 8hrs = 1312W
      Laptop (70Wh 1hr charge),
      Fan (60W), 6hrs = 360W
      Office laser printer (say less than 5mins per day) (not critical) "Maximum: Approx. 1320 W". - this would require a more powerful inverter, is this too dangerous for a 24V 100Ah AGM battery?

      Total daytime: 1672 Watts

      [B]Nighttime use:[/B]

      One fan: 60W, 10hrs, 600W.
      Lights 7pm-11pm (60W) total 240W.

      Total nighttime: 840W

      Now I'd love to have the PV array sized to the maximum the 24V 100ah AGM can safely cope with. If I have more power to play with during the day that would be great.
      If the 700Wh array is the max that would give 700 x 5.53 (sun hours in PR) = 3,871 watts, but factoring in system inefficiency (x0.75) = [B]2,903 watts[/B].

      If the 24 100ah battery is full at the end of the day (2400 watts) then the nighttime use of 840 watts depletes the battery to 1560 watts (65% capacity) which is a 35% discharge.

      So I'll work around cloud cover by decreasing loads accordingly and never try to deplete the battery below 35% - I'll use a battery protect that would cut the system at this point. I guess my main concerns is the max size of inverter and max size of PV array that I could use with the battery and a 30 (or 40amp) solar charge controller.

      Thanks!

      Comment


      • #4
        First, batch all your printing, so the laser printer only has to "warm up the fuser" only once or twice a day. Leave it [B]off [/B]the rest of the time.

        AGM can handle some pretty hefty charge rates, the 20A from a decent MPPT controller won't hurt it.

        You don't fully comprehend the solar power curve throughout the 5 hour solar day, you only get your peak 600w for about 20 minutes when panels are aimed precisely at the sun, then the sun moves away. You won't get 700w for 5 hours. Additionally, as the batteries charge up, they accept less amps and start charging slower, so at the end of 5 hours, you won't have really filled the batteries, you need more panels (with their own controller) aimed more easterly to get started charging earlier. Also, the recharge efficiency for AGM may start at 95% efficient, but will degrade as the battery ages, so you need to plan for 3 year old battery charging at 85% eff.

        There is so much "seat of the pants" learning that's really hard to easily document.

        There are not really any good meters that can tell you what's left in the battery, while the battery is in use. After a battery sits idle for 4 hours, the voltage can be used as a indicator. otherwise you need a meter that can count amps in vs amps out and calculates an efficiency coefficient/battery age, to arrive at a good guess.
        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


        • #5
          Ah true - totally slipped my mind when I was doing the calculation, thanks for bringing that up. Haha, yeah I see what you mean about "seat of your pants" learning. It'll be a fun learning experience for me as long as I keep safety at the forefront of my mind.

          OK so far:
          [B]1. [/B]Gotta keep in mind battery efficiency drop of 85% at 3 years and work off that to begin with to avoid problems later on. Meaning I should consider my 2400 watt capacity battery to actually be 2,040 watts at full charge.
          [B]2.[/B] Not easy to tell what's left in the battery while in use - which makes it tougher working with smaller margins of error on a smaller battery bank. Better to play it safe with a larger margin of error (less loads or bigger battery) which in my case means less loads.
          [B]3.[/B] 700W PV output at 5.52 sun hours is not simply a case of multiplying due to solar power curve throughout the day.
          [B]4.[/B] If using the printer then batch print and turn off.
          [B]5.[/B] At end of the day the battery may not be fully charged due to slower charging the closer battery is to full.

          Ok, so doing the calculations... PV array of 700W with system inefficiency (25%) = 2,903 watts but factoring in solar curve throughout the day means less than this. Starting the day with a battery 35% depleted (840 watts needed to charge or thereabouts) plus a daytime need of 1672 watts = 2512 watts input from the PV array.
          A 700W PV array is not sufficient.

          You are right, I'd need more panels on a different charge controller. Or would it be possible to increase the PV array and move up to a 40A mppt charge controller? If I added another 350W panel, it would give a 1050 watt PV array. I just don't know if this is safe for the battery.

          Comment


          • #6
            Adding a 2nd array, on a different axis, will help spread your solar harvest more evenly. intutievly, if the larger array is east biased it will put more power into a low battery, and the smaller array with a west bias, will continue at the reduced rate of absorb later in the day..

            The following sketch shows the charge rates changing over the course of a day, sunrise, power increases as sun rises higher, end of absorb battery draws less amps, nothing different happens internally in the controller. ( note opinions differ, with the EQ stage happening at the end of absorb in most cases, Sealed batteries never should be EQ'd )

            SolarChargeProfile.png
            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


            • #7
              Ok thanks Mike! That's useful to me.
              I could have my 700W array facing east bias and then I could add another 350W panel with slight west bias (on a different charge controller). That would give a total input into the battery bank of 1050W. This would be a theoretical max charge rate of 43.75% of the battery capacity. In reality I'm guessing it would be around 35% of battery capacity. Would that be safe enough for an AGM battery?

              Comment


              • #8
                safe enough, because not all panels will be producing at full power all the time, it's called Virtual Tracking. Costs is a 2nd array and controller, but it levels the power curve thru the day.
                Link:
                https://www.solarpaneltalk.com/forum...778#post356778

                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


                • #9
                  Great, thanks Mike, I think that's given me the path forward for the system that I want to develop. Appreciate the teaching and advice! I'll probably be back for more once it comes time to set it up just to make sure I've all the fuses, wire sizes and general safety stuff correct!

                  Comment

                  Working...
                  X