solar math gurus please read

topic posted Thu, August 13, 2009 - 12:07 AM by  RICK
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5v pocket projector x 2a = 10 watts an hour

5v hdd palyer x 1a = 5 watts an hour

10w + 5w = 15 watts an hour

5 hour a day proposed use time for both so 15 watts x 5 hours = 75 watts

75 watts x 1.5 (system losses) = 112.5 watts a day

112.5 watts / 5 hours sun a day (winter solar insulation) = 22.5 watt solar panel

22.5 watts x 5 hours sun a day = 112.5 / 12 volts = 9.375 Amp Hour battery at 12v

9.375 amp hours x 2 (charging losses) = 18.75ah

18.75ah x 5 (allowing 20% discharge of batteries) = 93.75 amp hour battery bank @ 12v


is my math ok???
posted by:
RICK
Los Angeles
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  • Re: solar math gurus please read

    Thu, August 13, 2009 - 3:06 AM
    Well, it's not watts an hour, it's just watts. watts an hour is a rate of change in power consumption. Your 75 watts, then, would be 75 watt-hours (Wh), and you 112.5 would also be in watt-hours.

    No math errors, though, only a semantic one.
    • Re: solar math gurus please read

      Fri, August 14, 2009 - 5:44 PM
      i agree with glen, you should be good here.
      you have taken a very conservative approach with your inefficiency variables which means you will probably do fine. however if you hedge it a bit more you might be able to carry around a lighter battery pack that is less certain to perform. choice is yours.

      how do you plan to go from 12v battery pack to your 5v devices?

      let us know how things worked out after bm if you can.

      n
      • Re: solar math gurus please read

        Fri, August 14, 2009 - 5:48 PM
        wait,

        the last multiplying times 5 would be for if you want to have 5 days of storage in your battery pack. you would in that case not have toe power in solar to regenerate that entire pack, but if you only use your load like you said, you should be able to restore that much every day.
        • Re: solar math gurus please read

          Sat, August 15, 2009 - 3:52 AM
          You sure about that? If these are lead-acid batteries, I wouldn't discharge them below the 80% mark (okay, maybe 75%) on any kind of regular basis.

          On going from 12V to 5V: I do NOT recommend using a 7805 for this -- it just creates a resistance and effectively "burns off" the difference, which will also introduce heat dissipation issues. You can, however, use a 78HC05 -- they cost more initially, but are, at worst, 85% efficient.
          • B
            B
            offline 120

            Re: solar math gurus please read

            Sat, August 15, 2009 - 5:54 AM
            IF you can afford to ignore cost the voltage translations are always more efficient using dc -dc converters. Many dc-dc converters are 95% efficient or better. Regulators as Glenn says are not power efficient.
            • Re: solar math gurus please read

              Sun, August 16, 2009 - 7:54 AM
              Further to B's point, the 78HC05 is a DC-DC converter with the same pinout as the 7805. That's why it earns my recommendation. As I say, 85% is its *absolute worst possible* performance; it only goes up from there.
          • Re: solar math gurus please read

            Sat, August 15, 2009 - 5:54 PM
            i was trying to maintain his variables cuas they were the ones he likes,
            but he also multiplied by 2 right before that so the battery wont be discharging even to 50 percent.
            • Re: solar math gurus please read

              Sun, August 16, 2009 - 7:55 AM
              True. There is a very large margin of error in there, as there should be.
              • Re: solar math gurus please read

                Sun, August 16, 2009 - 12:07 PM
                thank you all for your insight and help, to clarify where i am going with this i thought i would show you some of the components i have bought so far:

                i got 2 of these:

                cgi.ebay.com/ws/eBayISAPI.dll

                specs:

                High efficiency module---17.25% Mono-crystalline Silicon
                Low iron tempered glass
                Suitable for prolonged outdoor use, water proof
                EVA laminate and TPT backsheet
                Anodize aluminum frame
                Bypass diode minimizes power drop by shade
                10 years limited warranty of 90% power output and 25 year limited warranty of 80% power output.
                Maximum Power: 10.3W watt
                Maximum-power Voltage: 17.8 volt
                Maximum-power current: 0.58 Ampere
                Open-circuit voltage: 22.1 volt
                Short-circuit current: 0.69 Ampere
                Size: 350 x 285 x 18 mm ( 13.8" x 11.2" x 0.7" )
                Weight: 1.25 KG (2.8 pounds )
                Surface of the cell: 125 x 125 mm
                Temperature: -40 ° C / +80 ° C
                Temperature coefficient of Pm (%): -0.47 / ° C
                Temperature coefficient of Im (%): +0.1 / ° C
                Temperature coefficient of Vm (%): -0.38 / ° C
                Tolerance: + / -5%



                one of these but will upgrade to mppt version soon:

                cgi.ebay.com/ws/eBayISAPI.dll

                specs:

                12V, 5A Solar Charge Controller/Regulater
                Handles up to 5 amps of array (solar panel) current
                Three LED indicator: Over Discharge, Over Charge, Charging.
                Cut-in Voltage: 11V. Cut-out Voltage: 14.2V
                Prevents over-charging the 12 volt storage battery being charged by your solar panel. It also prevents the discharging of your battery during hours of darkness.
                Provides three sets of connection: Input, battery, output
                Max input power: 60W
                Dimension: 3'' X 3'' X 2''
                Condition: Brand New. Bulk package


                and this lot of four 12ah agm batteries:

                cgi.ebay.com/ws/eBayISAPI.dll

                specs:

                FOUR (4) 12V 12Ah SLA Sealed Lead Acid Battery

                ZEUS PC12-12F2 *No wires or cables are included*



                PC12-12F2 Battery Specifications

                Weight (lbs) 9.69

                Volts 12

                Ah 12

                Length (in.) 5.94

                Width (in.) 3.86

                Height (in.) 3.74

                Terminal F2 (1/4”)

                SLA batteries may be used in a wide range of applications, such as UPS Systems, Emergency Lighting, Underwater Fish Finders/Cameras, Fire & Security Alarm, Game Feeder’s, Game Decoy’s, Radio Control, Robotic, Invisible Pet Fences, Communications, Telemeters, Camera’s & Photographic Equipment, Hobby Equipment, Small Toys, Street Lighting, Pumps, Solar Power
                Not sure if our batteries will work in your application? Feel free to contact us for varification.
                Power-Cell Battery is a leader in competitively priced sealed lead acid batteries. Our ZEUS line is comparable to many well known manufacturers, including Universal, Power-Sonic and Yuasa batteries. While are batteries ship out with 80-90% charge in them, it is strongly recommended to top charge prior to first use to prolong the life of your battery.
                REASONS TO CHOOSE ZEUS:



                · Maintenance Free

                o Valve regulated

                o Safe to discharge in any position

                o Electrolyte is absorbed into the positive and negative plates and separator

                o No need to add electrolyte



                · Ease of Shipping

                o Requires no special handling

                o Completely sealed construction



                · Life Expecancy

                o Will last up to 5 years in stand-by applications

                o 200 – 1000 cycles in cyclic applications depending on depth of discharge



                · Ease of Design

                o Wide variety of sizes and amp ratings

                o Maximum flexibility in the design

                o Pack assembly, stand alone, standy-by and cyclic applications



                · Shelf Life & Storage

                o Stored in fully charged conditions

                o Top charge every six (6) months in ambient temperatures to prevent permanent loss of capacity

                o Store at temeratures less than 50 degrees F & 10 degrees C or lower to extend shelf life





                hows it sound?
                • Re: solar math gurus please read

                  Sun, August 16, 2009 - 12:10 PM
                  obtw dc to dc converter:

                  cgi.ebay.com/ws/eBayISAPI.dll

                  specs:

                  This is a professional Step Down DC / DC Converter circuit.

                  Dimension: 44 mm*22mm*7.2mm (L*W*H)

                  Input Voltage: 4 - 25 VDC;

                  Output Voltage: 1 - 10 VDC ( Default ) or 1.22 - 6.7 V ( more high-precision )

                  ( Adjustable ) (To work normally, the input voltage must be at least 1.55V higher than the output voltage)

                  Maximum Current: 3 A

                  The efficiency can reach 90% -93% with a quite low heat power!

                  If the output side is short-circuited, the circuit will be automatically protected!

                  The quiescent current of the circuit is 5 mA.

                  The input side is connected with a red LED, while the output side a green LED

                  The total consumption of the circuit is about 10 mA.

                  The value of adjustable resistance on the circuit board is 50K






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