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  • in a 12V box for ham radio field day operations

    So I've been reading here for a couple of days in spare time. I'm sure I'm missing pieces here, but examples and getting them corrected is the best way I can learn.

    So I want to be able to operate for 48hr with a single radio (12VDC) at a 15% duty cycle (transmitting using 13A 15% of the time, and listening using 1A 85% of the time) using batteries and panels to charge them. I'll have about 14hr of daytime each day during the field day weekend. So I'm guesstimating 7hr of charge time each day. (I'm sure there is a better way to figure that)

    Panel needs: So I'm calculating 85%*1A+15%*13A over 24hr would be 806.4Whr. With a fudge factor of 30%, I'm planning for 1048.3Whr per day. To put that back when charging, I'd need 1048.3Whr/7hr=149.8W of solar panels.

    Battery capacity needs: absolute minimum would be to operate 24hr on battery only, assuming weather cooperates and I can get charging again during the second day. So for that 1048.3Whr/12V=87.36 Ahr, but this would completely drain the battery, so double it to stay above 50% and plan on 174.72Ah.

    So I'd be looking at at least 150W of solar into a 180Ah battery string. I haven't read about charge controller sizing, so I'm making assumptions here.... On average I'm looking at 2.8A of current outflow. To put that back the charge controller should be able to handle at least 3A. (I've got a 10A charge controller kicking around in my solar learning sandbox)



    my thoughts before I post and (hopefully) get all my mistakes corrected
    - I did the battery capacity on 24hr because it is safer, but if I'm getting 7hr of charge time, can I calculate for 17hr of battery only operation?
    - How different is real-life use from the basic calculations? Is that difference mostly encapsulated in the 30% fudge factor (no inverters involved, just DC)??

  • #2
    OK first mistake is you do not have 7 Sun Hours in a day. Going by you user name and state residence I assume the left coast island of Austin TX.

    Sun Hours depends on time of year, location, and panel orientation and tilt angles. In Austin are in December-January with Latitude tilt you get 4 Sun Hours. The peak occurs in June July with 0 degree tilt at 6 Sun Hours.

    Second problem I do not know if it occurred to you is are you calculating for using a PWM controller of MPPT. Makes a huge difference. If you use a MPPT controller you multiply the daily requirement by 1.5, and 2.0 if using PWM. So if you need a 1 Kwh per day using a MPPT controller the panels have to generate 1.5 Kwh, or 2 Kwh for PWM. So let's say you will operate in May and need 1 Kwh. Panel size = 1500 wh / 5.4 h = 277 watts. So you would be shopping for a panel of 270 to 300 watts. If you wanted year round then 1500 wh / 4 h = 375 watts.

    OK being a ham myself I know what field ops day is. Basically a pissing contest to rack up as many contact you can for a check mark. Basically you will use roughly 780 watt hours per day or 1570 wh in 48 hours. Don't want to bust your bubble here but you do not need solar, just an appropriate sized battery, and the right battery construction to get the job done. There are two manufactures that make excellent AGM batteries that can be discharged down to 80% DOD, and any can easily go to 50% DOD. Concorde At 80% DOD in 12 volts we are talking about a mere 170 AH. At 50% DOD only 260 AH. All you need is either a single 12 volt 170 AH, or a pair of 6 volt 260 AH AGM battery and forget solar. Just charge the batteries up at home on your radio power supply a day or two ahead of time. If you get in trouble in the field just connect a set of jumper cables to your vehicle battery and idle the engine for 30 minutes.

    But if you just have to look cool and want solar, use the same battery 170 to 260 AH with the panel and controller you already have. Won't be doing much, but you get the cool factor and can use the panel for some shade on your head.

    Here are some good battery choices for you.

    Concorde 12 volt GPL-30HT
    Concorde 12 volt GPL-8DA
    Concorde 12 volt PVX-2580L

    When you get home just use them in the shack connected to your power supply and they are ready to go as soon as the power goes out.
    MSEE, PE

    Comment


    • #3
      Originally posted by Sunking View Post
      OK first mistake is you do not have 7 Sun Hours in a day. Going by you user name and state residence I assume the left coast island of Austin TX.

      Sun Hours depends on time of year, location, and panel orientation and tilt angles. In Austin are in December-January with Latitude tilt you get 4 Sun Hours. The peak occurs in June July with 0 degree tilt at 6 Sun Hours.
      Ok, so I wasn't too far off (15% or so). I was basing it on expected 14hr between sunrise and sunset on field day weekend and halving that to compensate for the innefficiencies of not having direct sunlight unless I constantly adjust panel mount angle (though that's another project).


      Second problem I do not know if it occurred to you is are you calculating for using a PWM controller of MPPT. Makes a huge difference. If you use a MPPT controller you multiply the daily requirement by 1.5, and 2.0 if using PWM. So if you need a 1 Kwh per day using a MPPT controller the panels have to generate 1.5 Kwh, or 2 Kwh for PWM. So let's say you will operate in May and need 1 Kwh. Panel size = 1500 wh / 5.4 h = 277 watts. So you would be shopping for a panel of 270 to 300 watts. If you wanted year round then 1500 wh / 4 h = 375 watts.
      It's a PWM controller. I hadn't considered that factor. Another thing I didn't realize I didn't know.

      OK being a ham myself I know what field ops day is. Basically a pissing contest to rack up as many contact you can for a check mark. Basically you will use roughly 780 watt hours per day or 1570 wh in 48 hours. Don't want to bust your bubble here but you do not need solar, just an appropriate sized battery, and the right battery construction to get the job done.
      Hey... I'm here to have my bubble busted. Solar in this case would be a learning tool. That's all.

      There are two manufactures that make excellent AGM batteries that can be discharged down to 80% DOD, and any can easily go to 50% DOD. Concorde At 80% DOD in 12 volts we are talking about a mere 170 AH. At 50% DOD only 260 AH. All you need is either a single 12 volt 170 AH, or a pair of 6 volt 260 AH AGM battery and forget solar. Just charge the batteries up at home on your radio power supply a day or two ahead of time. If you get in trouble in the field just connect a set of jumper cables to your vehicle battery and idle the engine for 30 minutes.

      But if you just have to look cool and want solar, use the same battery 170 to 260 AH with the panel and controller you already have. Won't be doing much, but you get the cool factor and can use the panel for some shade on your head.
      Were my calculations off by a factor of two? I thought the 170Ah would only work for a day. Or is it because you're talking about batteries that I can discharge beyond 50% plus I'd get a bit of charge during the second day?
      Here are some good battery choices for you.

      Concorde 12 volt GPL-30HT
      Concorde 12 volt GPL-8DA
      Concorde 12 volt PVX-2580L

      When you get home just use them in the shack connected to your power supply and they are ready to go as soon as the power goes out.
      I'm making a list.... Thanks for the pointers.

      Comment


      • #4
        Originally posted by austinrob View Post
        It's a PWM controller. I hadn't considered that factor. Another thing I didn't realize I didn't know.
        Once you get to a panel wattage of 200 watts and higher, well really 150 today, it is less expensive to use a MPPT controller. With a MPPT controller you can use much less expensive grid tied panels. It takes a 300 watt PWM system to generate the same amount of power as a 200 watt MPPT system. For example:

        200 watt Grid Tied Panel = $200 to $250
        15 amps MPPT Controller = $200
        Total = $400 to $450

        PWM 2 x 150 Watt panels = $500 to $600
        15 Amp PWM Controller = $40
        Total = $540 to $640 dollars

        Both deliver 15 amps charge current to battery

        Originally posted by austinrob View Post
        Were my calculations off by a factor of two? I thought the 170Ah would only work for a day. Or is it because you're talking about batteries that I can discharge beyond 50% plus I'd get a bit of charge during the second day?
        OK I calculated 785 wh/day or 1570 wh/for the 2 day event. On a 12 volt battery that is 1570 wh / 12 volts = 131 AH. When using AGM batteries you can drain them down to 80% on occasional use like you intend without real significant capacity loss. Not something you want to do daily. So with that said 131 AH / .8 = 163 AH is all you need to CYA for the 2-day event with no solar needed. Or if you use a FLA battery you want to limit discharge to 50% so it would take a 260 AH battery and no solar. I am just trying to save you a few coins buying stuff you do not need to buy. In this case solar is not needed to do what you want.

        Now if you really want to do this with solar here is my recommendation

        Panel Wattage = 200 with MPPT, or 300 watts with PWM
        Charge Controller = 15 Amps either MPPT or PWM
        12 volt Battery Capacity = 120 AH

        The PWM system will cost you around $1000 using a Concorde GPL-30HT battery, and around $800 for a MPPT system. Or just get you a good 12 volt 160 to 170 AH AGM battery for around $350 to $450. Or heck if you do not mind FLA batteries go down to Sams or Walmart and buy you a pair of 6 volt golf cart batteries for around $300 or less. 6 volt golf cart batteries are 200 to 225 AH. Another quality battery is US Battery model US2000Xc a 6 volt 216 AH battery cost roughly $130/each or $260. No reason to go solar and save yourself some $500 to $700.
        MSEE, PE

        Comment


        • #5
          I'd say just get a couple pair of 6v batteries, and recharge them (in series) from a running car /truck . in 10 minutes, with a good alternator, you will get the same as days worth of power from 300w of panels.

          If you can't run a engine/generator, carry 2 days worth of batteries. Still cheaper and lighter than 300w of PV panels.

          And do a trial run in your back yard before you hit the field.....
          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


          • #6
            I do not know why I did not think of this earlier, had a dumb ass attack I guess. You may think you are going to have a 15 or 20% duty cycle but that is not correct. Not even close. Unless you are operating FM or AM th eduty cycle will be much less. Field OPS is SSB, not FM or AM thus TX power is only voice peaks and not carrier power. That means all you really need is a smaller 12 volt battery like a 80 to 100 AH AGM battery for $100 or there about.

            Charge the thing on your bench power supply Friday night, take it with you Saturday and operate, Saturday night take it home and recharge on bench supply, then Sunday go back out and finish up the contest.

            73's

            KF5LJW Out.
            MSEE, PE

            Comment


            • #7
              Count everything in watt hours. Your TX, RX, lights, fan, laptop. (plug in your real usage numbers)

              20hrs RX @ 1A @ 13V = 260wh
              4 hrs TX @ 13A @ 12V = 624wh 884wh total. x2 = 1768wh required harvest / 5 hours light = 353w of correctly aimed PV panels. And zero cloud cover.

              As long as your batteries can supply about 2KWh of power, you should be OK. They will be deeply cycled, but for what - 6 field days year ? 12 cycles to 95% discharge, should have 100 extra deep cycles in them.
              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

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