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Chagring a P3A in the car with inverter calculation

Discussion in 'General Discussion' started by Nighthawk, Apr 24, 2016.

  1. Nighthawk

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    Hi i am going out bush for a few weeks and would like to use my P3A. i am planing on using a 300w sca inverter.(dont think 150 will be enough)

    My question is: i have a deep cycle battery (second car battery hooked up) and trying to work out how many charges i get out of it before running my car. I don t have the exact details of the battery because i cant get to it right now but was looking for a rough estimate using standard numbers (will it last only a bit or will i get a fer few charges out of it). I am trying to work it out but i am not really good at this so i hope we mite have a sparky here that mite be able to help me.

    Thanx for any help
     
  2. Mutterranch

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    I would like to know this too!
     
  3. cja844

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    With a rough calculation on a regular deep cycle I would say 6-8 before battery is dead! I would not run it on actual car battery without car started just in case. And that is on a deep cycle and not just a normal car battery (would be considerably less on regular car battery). And p3a has a 65 watt charger so 150 inverter should be fine while charging one battery at a time!
     
  4. cja844

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    Ps it may be even less than that due to inverter safety shut off, I know mine cuts off when battery gets close to 12 volts
     
  5. Sagebrush

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    I'm not an electrical engineer. But here's what you are trying to do:

    1. You have a battery (that's probably a marine/RV and not a true deep cycle) that produces about 13.6 volts DC.
    2. You hook up an inverter to this so you can plug in your 120 volt AC charger.
    3. The charger has a converter that changes this AC voltage back to DC at 17 volts. It's the black box that's inline on the cord.

    This inverting from DC to AC to DC is horribly inefficient–that's where the heat is coming from in your inverter and why it probably has a fan and fins. The efficient way of doing this is with a converter (changes your 13.6 volts DC to 17 volts DC) . Amazon sells them for $20 or so.

    A DJI Phantom 3 battery is rated at 4480 milliamps or 4.48 amps. Your battery (I'm betting it's a Group 24 marine/RV battery) has 40 useable amps. So... you should get roughly eight charges before you start damaging your battery–realistically seven. (Don't take it below 12 volts.)

    I charged my batteries recently from a 12 volt source while camping. I took them down to about 50% and it would take about an hour and twenty to bring them back up all the way with a DC charger.

    SB
     
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  6. Steve vaughan

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    Get at least 400 watt my 200 watt won't charge my 400 wat works great 800 works best
     
  7. Nighthawk

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    Thanx for all the help. I ended up getting a 300 watt Pure sine power inverter and going to test it tomorrow before leaving town. If i have trouble i will try the 600 but sounds like it will be no problem there.

    Thanx for the heads up of the deep cycle battery. I always though they are designed to nearly completely drain and charge over and over but i am just gonna make sure now that i will run my car every couple of charges to keep the battery topped up.
     
  8. Nighthawk

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    The caliber 300 watt Pure sine inverter seems to work fine on the P3A battery. I will post back when i come back from the trip about average charging times for anyone interested.
     
    SirAchie likes this.
  9. PEEFA

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    Why not just get a car charger? What is the point in trying to convert from battery voltage to mains Ac voltage and then back down to 17.4 v. It makes no sense and not very efficient.


    Sent from my iPhone using PhantomPilots mobile app
     
  10. With The Birds

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    The simplest way to look at this is to consider the available Ah from the battery, the conversion losses in the inverter and charger, efficiency of the charging process and battery capacity of the P3 lipo when connected.

    A typical deep cycle battery or 100 Ah will give you 80 Ah (80% DOD). Terminal voltage at full charge will be around 12.8v dropping to 12v at 20% charge so 12.4v is a reasonable average.

    Inverter and DJI charger are both switch mode so 90+% efficiemcy can be expected (from each).

    The P3 battery is 4.480 Ah so if you fly 70% out of it you will need to put 3.130 Ah back in to fully charge.

    We also need to allow for the fact that the P3 battery is 15.2v so we need more Ah from the charging battery than what is put into the battery being charged (15.2/12.4 = 1.23).

    From this our 80Ah abailable from your charging battery gives us max 65 Ah for charging phantom batteries (80 /1.23).

    Taking losses into account and phantom batteries at 30% we have (65 × .9 × .9) / 3.130 = 16.8 charges.

    Add in the consideration that the charging process itself isnt 100% efficient (battery chemistry) and that your car alternator is unlikely to fully charge the deep cycle i would say 13-14 charges is more likley.

    Will be interestwd to see what you get.