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Field Charging Expectations

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  • Field Charging Expectations

    Most places I fly there is no AC power source.* I'm looking at purchasing a good car or marine battery for my 12v source to hook my charger to.(Not my car and end up stranded with dead battery)* Is there a formula to figure how many 4s batteries* I may be able to charge when in the field from that car/marine battery?? Should I look for a battery with the highest amp rating?

    Thanks, Lon
    Lon

    Youtube channel:* https://www.youtube.com/channel/UCKc...goSkIdw85eZfPA

    Hanger 9 Spitfire
    Freewing A-10 80mm, F-86 80mm, F-15 90mm, P-51D Iron Ass
    Fliteline P-38L Pacific Silver
    Flyzone DHC-2 Beaver Island Wings
    Freewing Pandora

  • #2
    RE: Field Charging Expectations

    Originally posted by Lon
    Most places I fly there is no AC power source.* I'm looking at purchasing a good car or marine battery for my 12v source to hook my charger to.(Not my car and end up stranded with dead battery)* Is there a formula to figure how many 4s batteries* I may be able to charge when in the field from that car/marine battery?? Should I look for a battery with the highest amp rating?

    Thanks, Lon
    Convert the charge rate to Watts: 4S is 16.8V at full charge, so, if you are charging a 5000mAh battery (5Ah) at 1C, Watts approaching full charge are 16.8 * 5, or 84W. That's the peak Wattage your charger would provide to that battery. The average voltage during charge is around 16V, so about 80W, average, for 1C. If you follow the 80% rule in battery discharge, that would be 80W for 0.8 hours, or about 64Watt-hours. If you allow 20% loss in the charger, which is a bit high for today's standards, but not much, that brings you back to 80Wh to charge one 4S 5000mAh battery. During discharge, a deep cycle battery will range from ~12.6 to ~11V, but let's call it 12V average during discharge into your battery. That means 80Wh / 12V = ~6.7Ah is taken from the battery per charge of the 4S. You should not draw more than 50% from a deep cycle battery if you want it to last, so you would need a 13.4Ah battery to charge one 4S 5Ah battery, 134Ah to charge that battery ten times. BTW, a car battery would be dead in no time doing this, since they are not made to be discharged; only one rated for deep cycle, as many marine batteries, would do. Given the cost and weight, I would go for a small inverter-generator, and bring my own, stable AC power with me.

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    • #3
      RE: Field Charging Expectations

      Hello Lon,
      I have a Turnigy 400Wx2 charger hooked to a Deka Extreme Eliminator Deep Cycle 12v batt rated @ 45Ah. When I charge a 4000mAh 30C 4S at 4A on the Turnigy charger the 12v battery providing the power is also hooked to a 12v batt charger(when at home) and the charger indicates that it is providing 4 amps to the Deep Cycle. By the numbers this would indicate that I could charge approx. 11 of the lipos (11x4 vs the 45 batt rating) during a session at the field. In actuality I can really only do about 8 before the Turnigy charger tells me my source is below acceptable voltage (no charger hooked up).
      So your question about high battery amp ratings becomes a matter of money as well as weight portability. The 45 Ah Deka that I am using weighs about 15 lbs and cost around $75. It is about the size of a garden tractor battery. To jump up to the 200 Ah and higher Deep Cycles(size of car batteries) the price becomes really stupid, $150 and up and they weigh in at like 40 lbs or more. It now is a matter of how much you want to spend and how strong you are !!!
      Hope this helps give some insight.
      Warbird Charlie
      HSD Skyraider; FlightLine: Sea Fury; FMS 1400: P-40B, P-51, F4U, F6F, T-28, P-40E, 1700 F4U and a Fox glider; Freewing: A-6, P-51; VQ: P-39; Dynam: ME-262, FW-190, Waco; ASM A-26; ESM F7F-3
      Incinerator Loss 16

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      • #4
        RE: Field Charging Expectations

        Originally posted by OV10
        Hello Lon,
        I have a Turnigy 400Wx2 charger hooked to a Deka Extreme Eliminator Deep Cycle 12v batt rated @ 45Ah. When I charge a 4000mAh 30C 4S at 4A on the Turnigy charger the 12v battery providing the power is also hooked to a 12v batt charger(when at home) and the charger indicates that it is providing 4 amps to the Deep Cycle. By the numbers this would indicate that I could charge approx. 11 of the lipos (11x4 vs the 45 batt rating) during a session at the field. In actuality I can really only do about 8 before the Turnigy charger tells me my source is below acceptable voltage (no charger hooked up).
        So your question about high battery amp ratings becomes a matter of money as well as weight portability. The 45 Ah Deka that I am using weighs about 15 lbs and cost around $75. It is about the size of a garden tractor battery. To jump up to the 200 Ah and higher Deep Cycles(size of car batteries) the price becomes really stupid, $150 and up and they weigh in at like 40 lbs or more. It now is a matter of how much you want to spend and how strong you are !!!
        Hope this helps give some insight.
        Clearly, the gauge on the charger is (typically) not very accurate. A charger may provide about 14V to that battery, so must be providing at least 5A while you are using your charger; it takes more than 4A at 13-14V to put out 4A at 16V. In any case, using 32Ah from a 45Ah battery isn't doing it any favors, and will shorten its life. I suspect you are actually drawing >40Ah from that battery to charge 8 batteries, when the charger stops because of low voltage.

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        • #5
          RE: Field Charging Expectations

          Thank you guys for your input. I've got several months to make a decision on how I'm going to charge my batteries on a "all day" outing. This summer, the small starter charger got me by, with only a few times I sat, waiting for a battery to complete it's charging cycle for me to fly again. I've purchased more batteries(going to run 5s in the IronAss) But before the Iowa weather permits me to go out for the whole day again, I'm going to purchase a new charger and a power supply, and probably a few more batteries.*I hated it the few times this summer when I had*spent all my batteries before noon, was charging as I flew, but there I sat, beautiful flying weather...waiting for the battery to be charged...sigh.

          I did get*4 flights in this afternoon!* 0-2mph winds, couldn't resist, even though the temp was a hand chilling 26.
          *
          Lon

          Youtube channel:* https://www.youtube.com/channel/UCKc...goSkIdw85eZfPA

          Hanger 9 Spitfire
          Freewing A-10 80mm, F-86 80mm, F-15 90mm, P-51D Iron Ass
          Fliteline P-38L Pacific Silver
          Flyzone DHC-2 Beaver Island Wings
          Freewing Pandora

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          • #6
            Sorry to revive - Somehow this old thread popped up when I was searching for field charging setups.

            Any thoughts on the AutoCraft Battery line as a field charger? Was thinking of a Platinum in the trunk inside a sealed battery box wired into the electrical system. Wouldn't drain the main battery and leave you stranded but would be capable of being charged by the alternator?

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            • #7
              It all depends on how much field charging capacity you need...

              Any automotive starting battery, you do not want to EVER pull below 11.0 volts (which will usually still start your car if its not cold weather) That's typically just a few charges of a 3S 2000 mah and you are done. Most of the 12V chargers have a low input warning and shutdown by 10.5 v. (your car may or may not start at that point if its in good condition)

              Pulling the automotive battery below 12.0V isn't good for it. They are not designed for deep discharge.

              "Marine" dual purpose trolling motor/starting batteries (typical of WalMart) are a compromise between deep cycle and starting batteries. They also really should not be repeatedly pulled below 12.0v.

              Golf cart batteries are really deep cycle and will tolerate repeated discharge to 20% of capacity remaining pretty well.

              The weight of automotive style batteries needed is about 70% more than golf cart batteries for the same useful field charging capacity without harming the batteries.

              There are some EXPENSIVE medical and special use batteries that are very near A123 (LiFePO4) operation cycle. Far lighter for the energy capacity than lead acid. I got hold of some that were being discarded (out of date) and have been using them for over 7 years (lost track) and combined with solar panels have never run out regardless of what I have charged. Calculated 320 amp hr 12v capacity plus 105 watts of solar cells, mounted on my model aircraft transporting trailer.

              Most people who need a lot of field charging capacity go with a portable generator.

              *******************

              Don't put any automotive battery in a sealed box in the trunk. There WILL be outgassing and it will get out and it will rot the carpet and rust the trunk floor and fenders,
              FF gliders and rubber power since 1966, CL 1970-1990, RC since 1975.

              current planes from 1/2 oz to 22 lbs

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