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  • Small home solar system question.

    I bought one of the Harbor Freight solar systems a couple of months ago. I want to test out a small system to see if I want to invest further in solar. I have the kit and a 1,000/2,000 watt inverter. My question: What are the best batteries I can use and how many?

  • #2
    Welcome. Sorry for your loss, the HF panel kits are worth nearly nothing, but they do work great as a science project !

    a) 1, 45w kit is undersized for even 1, cheap-o deep cycle battery. So the battery is guaranteed to not be properly charged, and will fail in a couple years. But, good enough for a science project.

    b) you could run a 300W inverter for an hour, maybe two, before you get into deficit charging, depending on the size of the load on the inverter.
    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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    • #3
      To run a 1000/watt battery inverter effectively without tripping off line from under voltage requires a 12 volt 500 AH battery. A 12 volt 500 AH battery requires a minimum 500 watt panel.
      MSEE, PE

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      • #4
        So are you saying I need a 45 amp hour battery? Like I said I want to experiment to see if I want to make fuerther investments in solar power. I have seen the youtube videos showing the same 45 watt panels charging two deep cycle batteries. But, they do not tell how long they last or how long the batteries hold up. I am giving thought to buying 2 deep cycle acid batteries from CostCo to further the experiment.

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        • #5
          I do not know if the HF panels can even keep up a 50 AH battery. A 50 AH battery can only support about a 100 watt inverter.
          MSEE, PE

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          • #6
            A 1000 watt inverter at full load will drain 100 amps from a 12v battery bank..

            Most inverters are 80-90% effiecent and there is a NO LOAD draw usually on most inverters that tell you how much they will use with no load being on.

            Hence a 300 watt inverter will last about 3 hours.. the issue is if voltage drops below say 12.06 volts (about 50% of DOD).. the battery will not last long..

            Now if your just running a few 13w CFL bulbs and such for testing you should be fine..

            Get a Killawatt meter from HD or Lowes or Amazon for $28 and test for wattage of the items you intend to run.

            This will give you an idea of what you can run safely without ruining the battery prematurely.

            Also running items with the SUN out and on the panels will reduce the draw coming from the batteries..

            Figure a 45 w kit will get you at best case about 20-25 watts getting to the battery..

            Hope this info helps some..
            1160 watts, Midnite 150 , Xantrex SW2000

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            • #7
              With no real application in mind, and since it is just for experimenting, think SMALL at first.

              Nearly all vids I see with the HF freight panels are running in a "deficit-charge" scenario - too much battery / too big inverter etc.

              I think maybe the first application might be to just size the battery so it isn't too big. I think a 35ah battery or less would be reasonable. And maybe a 120 watt inverter like a pure-sine-wave Samlex PST-12S 12V might be nice. Try to run things off DC if you can, and use the inverter only when necessary. Charge up your phones, and other small items. Charge up AA batteries. Run some simple led lighting. Maybe grab a digital multimeter. Perhaps even get a " P3 kill-a-watt" meter to start measuring the AC power stuff so you can plan for any bigger upgrades down the road.

              The reason for the smaller battery at 35ah or so, is that you don't want to use more than 50 percent of it's capacity at any one time. So that leaves you with 17ah to use and recharge in a reasonable amount of time with those panels. It all depends on your location and solar insolation hours, but I'm going with a generic 10am to 2pm common solar insolation hours availability. It might take more than one day to fully recharge. And of course, how long the battery lasts all depends on how much power your devices draw.

              If you develop a taste for solar, the first thing I'd upgrade is the charge controller. I think the Morningstar Sunguard 4.5a unit is nice enough for a very first upgrade for that kit as a start. The charge controller that comes with the kit is a simple on/off type, which isn't real efficient, and not nice to batteries in the long run.

              If you are like me, the HF stuff turned out to be more of a "proof of concept" and imagination starter, and then the upgrades began after reading the great threads on the board here. I'm still trying to get it right, but that's the fun part of it.
              Last edited by Mike90250; 07-06-2012, 08:41 AM. Reason: typos, 10pm typo

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              • #8
                Originally posted by Mike90250 View Post
                Welcome. Sorry for your loss, the HF panel kits are worth nearly nothing, but they do work great as a science project !

                a) 1, 45w kit is undersized for even 1, cheap-o deep cycle battery. So the battery is guaranteed to not be properly charged, and will fail in a couple years. But, good enough for a science project.

                b) you could run a 300W inverter for an hour, maybe two, before you get into deficit charging, depending on the size of the load on the inverter.
                Mike nice post thanks man.

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                • #9
                  Originally posted by PNjunction View Post
                  With no real application in mind, and since it is just for experimenting, think SMALL at first.

                  Nearly all vids I see with the HF freight panels are running in a "deficit-charge" scenario - too much battery / too big inverter etc.

                  I think maybe the first application might be to just size the battery so it isn't too big. I think a 35ah battery or less would be reasonable. And maybe a 120 watt inverter like a pure-sine-wave Samlex PST-12S 12V might be nice. Try to run things off DC if you can, and use the inverter only when necessary. Charge up your phones, and other small items. Charge up AA batteries. Run some simple led lighting. Maybe grab a digital multimeter. Perhaps even get a " P3 kill-a-watt" meter to start measuring the AC power stuff so you can plan for any bigger upgrades down the road.

                  The reason for the smaller battery at 35ah or so, is that you don't want to use more than 50 percent of it's capacity at any one time. So that leaves you with 17ah to use and recharge in a reasonable amount of time with those panels. It all depends on your location and solar insolation hours, but I'm going with a generic 10pm to 2pm common solar insolation hours availability. It might take more than one day to fully recharge. And of course, how long the battery lasts all depends on how much power your devices draw.

                  If you develop a taste for solar, the first thing I'd upgrade is the charge controller. I think the Morningstar Sunguard 4.5a unit is nice enough for a very first upgrade for that kit as a start. The charge controller that comes with the kit is a simple on/off type, which isn't real efficient, and not nice to batteries in the long run.

                  If you are like me, the HF stuff turned out to be more of a "proof of concept" and imagination starter, and then the upgrades began after reading the great threads on the board here. I'm still trying to get it right, but that's the fun part of it.
                  I think the HF kits are an excellent starter kit. For a little over 200 dollars(kit and cheap abttery) you can start collecting and using solar energy. What these kit will teach and have taught me most of all is lesson's in production,useage and capacity.

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                  • #10
                    Originally posted by PNjunction View Post
                    If you are like me, the HF stuff turned out to be more of a "proof of concept" and imagination starter, and then the upgrades began after reading the great threads on the board here. I'm still trying to get it right, but that's the fun part of it.
                    Exactly. Thanks for the response.

                    Comment


                    • #11
                      Maybe a better start would be would be to determine the smallest battery I could recharge with it without frying and move upwards from there.

                      I'm not good at it, but here's my train of thought...

                      The fastest charge rate would be for an AGM sealed lead acid which is typically C/4. The imp of that 45watt panel system is somewhere near 2.5 amps measured across the terminals with an ammeter under the best of conditions. No load, just the meter. The fastest you should charge a FLOODED battery via solar would be C/8, but you may find that a sealed AGM more convenient - preferably a real deep-cycle type, not a gel or a ups battery. At this stage, I wouldn't get something too expensive. Consider these first experimental batteries as sacrificial.

                      So 2.5 * 4 = 10 amps. That would be the smallest size battery I'd use off the top of my head. BUT, when you actually put it across a battery that is well discharged, you de-rate the power by about .75 or so. Takes into account the voltage drop of the battery connection, charge controller efficiency, panel heating etc. So now....

                      2.5 amps * .75 = 1.875 amps measured when actually charging the battery with no load. Or you can think of it as your 45 watt panel is actually about a 34 watt panel when doing anything really useful)

                      So, 1.875 * 4 = 7.5 amps. Looks like the absolute smallest AGM sla battery I'd use on that would be a 7.5 ah. AND, unless you want to kill the battery in a few short cycles, only use 50% of the capacity. So, 7.5 / 2 = 3.75 amps usable.

                      The battery would recharge pretty easily in a perfect 4-hour "solar insolation" day, ie 10am to 2pm, especially because you are not going beyond removing half of it's capacity. So you only need to replenish 3.75amps in that case. Each location is different obviously. I'd find the WORST hours, usually wintertime hours to base my solar insolation hours on.

                      If you were willing to charge for many days without a load, I'd probably make the battery no larger than 35ah - again only 50% usable, 17.5ah. Better yet, only use 20 percent of that 35ah batt each day to make the it last longer. Try to use only 7 amps of current total each day with the 35ah batt. So this 35ah battery would give you some breathing room, and the ability to recharge properly in a day or 2 or 3 depending on your usage.

                      You can calculate the loads if they are steady current, or you can measure the SOC by letting the battery rest for 4 hours minimum, and taking a voltage reading. For AGM's, it would read

                      AGM:
                      12.8v or more: 100% charged. (I typically see 13v or a little more on fresh AGM's)
                      12.6v: 75% charged.
                      12.2v: 50% charged.

                      So run your stuff, but remember that these values above will be more accurate if the battery has rested for a few hours doing nothing - no load and no charge. Going beyond 35ah batt means that the battery will sulfate faster than you can recharge it - ie a deficit-charge scenario which is something to avoid. This is typically C/13.

                      There's my feeble attempt at it so far. The guys here are far more accurate at this than I am, so I welcome all criticism of this post!
                      Last edited by PNjunction; 07-06-2012, 02:56 PM. Reason: typos

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                      • #12
                        Max charge rate for an AGM is C/4, and FLA is C/8. So you go that right.

                        So you have cheap Harbor Fright panels of 45 watts which on the best days of their life can generate 3 amps.

                        So for AGM 12 AH, or FLA 24 AH. So that is the smallest AH battery set you can run.
                        MSEE, PE

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                        • #13
                          Whew - thanks. I've got to redo my calcs and re-read the threads and stickies. Which strangely enough is fun for me.

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                          • #14
                            Originally posted by PNjunction View Post
                            Whew - thanks. I've got to redo my calcs and re-read the threads and stickies. Which strangely enough is fun for me.
                            Well when you do the calculations run your daily maximum watt hours the batteries can support.

                            Battery Watt Hour Capacity = Volts X Amp Hours = Watt Hours

                            You can use about 20 to 30% of the capacity each day if your panels can generate that much to replace the energy. Last calc to do is compare that to what it cost to buy the power from the electric company which is around 10 to 15 cents per 1000 watt hours. If you do that it might enlighten you a bit.
                            MSEE, PE

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                            • #15
                              Another question: I have a 1000/2000 watt inverter and I know my 45 watt panels and 1 or 2 batteries cannot use even close to that. What will happen when I hook up my inverter? Will it work? Will I lose some measure of performance with this setup. The Harbor Freight 45 watt kit, the 1000/2000 inverter and 1 or 2 deep cycle batteries fromCostCo.

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