Portable (off grid) RC charging system

The field I fly at is not electrified, and I'd like to setup a charging system for a trailer.

I'll need 24V's worth of Pb batteries, and be able to draw 40 amps peak current, charging worst case 4kWHr of batteries over 2 days, or 2kWHr/day, at say 85% charger efficieny. So I'll need to supply say 5kWhr over 2 days, very bursty.

This will be used to power my LiPo charging system.

I will charge the system back up during the week, using the grid at home. Later, I'll look into setting up solar panels.

So, for now, how many Pb batteries will I need? Two in series, or two sets of two in parrallel, in series? (I'm concerned about the sustained 40 amp draw).

Should I go with true deep cycle (can they handle 40 amps), marine, or automotive?

Cost, weight, and longevity are all factors. I'd like to keep it under 200 lbS, mabe even 150.



  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    Re: Portable (off grid) RC charging system

    Welcome to the forum LW.

    A few more details will probably help as there are significant losses at each point in the system/for each energy conversion step.

    But starting with what we know:
    • 4,000 WH * 1/24 volts = 167 AH @ 24 volt worth of DC storage

    Normally, we would suggest a maximum discharge of 50% for longer battery life--But you could discharge to 20% state of charge if you use something like a forklift/traction motor battery... But for the moment, lets assume AGM type battery. These are more efficient and higher output current for their size (one reason they are used for computer UPS systems--Some of them can be drained in 15-30 minutes--albeit, not with very good cycle life. So I will use 50% discharge for the time.
    • 167 AH * 1/0.50 maximum discharge = 334 AH @ 24 volt

    Using Concorde's AGM batteries, 4x 6 volt @ 405 AH batteries would work:

    PVX-4050HT 4x 120 lbs @ $610 each

    So, the first issue. Batteries are not cheap, and lead acid are not light.

    Next charging... We normally suggest ~5% to 13% rate of charging. And you can go as high as 25% rate of charge with thermal management... For a 405 AH battery bank, the minimum AC charger would be:
    • 405 AH * 0.05 rate of charge = 20.5 amps minimum (at 24 volt charger)

    If you where going to charge with with solar--There are two numbers to worry about. One is the rate of charge, and the second is how much energy per day (and the amount of sun where you fly).

    Using our 5% to 13% rate of charge rule of thumb:
    • 28.8 volt charging * 405 AH * 1/0.77 panel+controller losses * 0.05 rate of charge = 757 Watt minimum array
    • 28.8 volt charging * 405 AH * 1/0.77 panel+controller losses * 0.10 rate of charge = 1,515 Watt array nominal
    • 28.8 volt charging * 405 AH * 1/0.77 panel+controller losses * 0.13 rate of charge = 1,969 Watt array "cost effective maximum"

    And then based on how much sun you get... Start with 4 hours per day (of noon time equivalent sun) minimum 9 months of the year (a reasonable guess unless you are in a coastal area with heavy marine layer or up in the north east.
    • 2,000 WH per day * 1/0.5 * 0.69 panel+controller+AGM eff * 1/4 hours of sun per day = 724 Watt array minimum

    So, two days of no-sun (sizing of battery bank) and a recommended off grid solar array used every day would be ~757 to 1,969 watt array.

    If you assumed you had full sun every day of flying (don't fly in poor weather) and that this is week end flying, you could cut the battery bank by 1/2 -- to ~170 AH and cut the minimum array to 1/2 also--or:
    • 212 AH AGM battery * 28.8 volts * 1/0.77 losses * 0.05 rate of charge = 396 Watt array minimum

    If you wanted to save some money, you could use 4x 6 volt @ ~220 AH flooded cell golf cart batteries (probably 1/3 or less the price per battery vs AGM). But your peak current of 40 amps is pretty high for a flooded cell battery bank... Nominally, the maximum recommended load would be C/8, and maximum surge load would be C/2.5 -- However, if your maximum current is not for a "long period" (less than tens of minutes?) you could look at C/5 discharge rate:
    • 220 AH * 1/8 = 27.5 amp long term discharge (recommended)
    • 220 AH * 1/5 = 55 amps short term discharge (less efficient, less AH available from battery bank)
    • 220 AH * 1/2.5 = 88 amps "surge current"

    And this then gets us into the whole solar/battery/generator question... A 40 amp @ 24 volt battery charger (assuming power factor corrected modern LiPo battery charger) would need:
    • 40 amps * 28.8 volts charging (?) * 1/0.95 PF * 1/0.85 charger eff = 1,427 VA circuit (11.9 amps @ 120 VAC)

    A Honda eu2000i (or similar Yamaha, etc.) inverter generator would supply such power for ~$1,000 at 50 lbs. And 1-2 gallons of gasoline for a weekend of use. Put it on a 100' extension cord, you probably would not even hear it. Make up an extended run gas tank, and you could run it for several days between fill ups (Honda eu2000i uses a fuel pump and can draw a light vacuum in its fuel tank--put a sealed gas cap+hose on it and drop the hose end into a 5 gallon tank for extended run). You will get ~2,000+ hours out of an eu2000i (or Yamaha) with normal maintenance (these engines are not really rebuild-able).

    Much cheaper (short and long term) vs AGM or other lead acid battery bank--Let alone the additional of solar panels (take home or store on site?). (yes, this is a solar forum, but we are a cheap and practical bunch here too).

    Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LoneWolfRCLoneWolfRC Registered Users Posts: 3
    Re: Portable (off grid) RC charging system

    Thanks Bill for the detailed answer. I was running some of the same ideas through my head after poking around the forum, though your specific answers are even better.

    Even using the grid to charge the Pb batteries, I think I'd be abusing cheap Pb batteries to power my LiPo charger. I already have a couple of 6V UPS batteries and it takes no time to drain those, unless I'm charging small Lipos. And the cost of good Pb batteries I was looking at, based on recommendations in other applications on this forum, to keep me running all weekend would be approaching the cost of a generator it seems, if I wanted the Pb batteries to last.

    So, along those lines, do you know of any generators that put out straight DC, preferably 24 volts? At 12 volts, my LiPo charger would still be limited to 40 amps input, which would limit charging 6S LiPo batteries to about 20 amps. I do have modified computer power supplies to produce ~1175 watts at 24 volts.

    This would get rid of the inefficiencies of using computer power supplies to create DC from a generator's AC output.

    I guess computer power supplies are 80 to 85% efficient, and the LiPo charger is 85% to 90% efficient.
  • BB.BB. Super Moderators, Administrators Posts: 30,520 admin
    Re: Portable (off grid) RC charging system

    We had a discussion on one brand of DC generators here:


    I would suggest the AC generator(s) may be handier if you can use AC power for other applications.

    The 45 amp at 24 volt unit is ~$1,458 in gasoline... I think they will do what you want pretty efficiently.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • LoneWolfRCLoneWolfRC Registered Users Posts: 3
    Re: Portable (off grid) RC charging system

    Thanks. I'll look at that. But as you point out, AC has more uses--I'll be able to sell the inverting generator decision to my wife a lot easier. (Keep the fridge running in the event of a long disruption in electricity).
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