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Thread: Watts & Amps


  1. #1
    hookedonrc's Avatar
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    Talking Watts & Amps

    First let me say I am electrically challenged. I got to the point where I understood charging and cycling Nicads and Nimh's. Now I have more planes using lipo's than I used to have flying with only glow.
    So here is my question. How are Volts and Amps related in a power supply? I am using a Hitec X4 with 50watts per charging port. To power it I use an Electric Fly power supply with 12 amps. I am looking at another supply that out puts 24 amps. Does that mean I can charge my batteries in half the time? And that I will get up to 6 amps of out put on the 4 ports if I charge 4 batteries at the same time?
    I am sure I will get this one of these days.
    Will add something clever and witty if I ever come up with something.

  2. #2
    Rob2160's Avatar
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    If your charger is 50 Watts per port, this will be the limiting factor on charge amps..

    Amps x Volts = Watts.

    You have a limit of 50 Watts per port..

    If you are charging a 3S battery, it is approximately 12 volts - therefore the amps will equal 50 divded by 12 - IE just over 4 amps.

    It won't matter if you use a 12 amp or 24 amp power supply if you are just charging with 1 or 2 ports.

    If you want to charge all 4 ports simultaneously I would recommend the 24 amp supply.

    The easiest way to understand batteries is to imagine them as little fuel tanks..

    Imagine a 1 cell battery is a small tank with its own built in pump to deliver fuel.

    The Capacity - 2000 Mah or 5000 Mah simply tells you how much fuel it holds... like liters..

    If the C rating of the battery is high eg 30 C, this means the pump in the battery can emply the full tank in 2 minutes.

    If the C rating is only 1 C - it will take an hour to empty the fuel.

    Now batteries can be 1S, 2S, 3S etc..imagine a 3 S 2000mah battery is like 3 seperate 2000 liter tanks each with its own pump and they all pump into a single pipe..

    The more number of cells you have, the more individual pumps, therefore the fuel pressure will be higher (Voltage)

    So a 4 S 2000 Mah battery is going to have twice the energy of a 2 S 2000 Mah battery..etc

    I'm sure others can explain it much better than me but that's how I think of it.. ..
    ..... frakkin cylons...

  3. #3

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    Think of the classic water hose example.
    The water pressure is akin to the voltage.
    The hose diameter sort of matches with wire size, and the two set how much water can be delivered at a time or in a time period.
    Then think of amperes and ampere hours times the voltage as how much energy in watts and watt hours can be delivered.
    Watts = voltage X current
    or
    Watts = current squared X resistance (Useful for figuring overall effective resistance)

    ~ 746 Watts = 1 HP

    So, a 1650 Watt motor (E-Flite Power 52 for example) can deliver about 2.2 HP

    Test conditions Hanger 9 P51 40 (Don't believe the 7.5Lb listed in the specs when flap servos, retracts, and so forth are added)
    12x8x3 Master airscrew prop for maximum power or 12x6x3 for less power and longer flight time
    75-85A speed control (ESC)
    6 cell 5000mah 35C Lipo
    Measured current (Static, full power, fully charged Lipo) 75-78 A
    Wire gauge #10 battery to speed control using EC5 connectors
    11 Lb Model, ~150W per Lb or thrust/weight of 1.5 to 1 (Aerobatic power!) (12x8x3 prop)
    Last edited by chuckk2; 08-09-2013 at 04:35 AM.

  4. #4
    hookedonrc's Avatar
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    Ok...so if I am reading and understanding this correctly. I can charge 4 batteries at the same time on the Hitec X4 that are equal or less that 3 amps per battery without affecting the performance. If I have 4 batteries that have amp ratings that exceed 12 amps...i.e. say 18 amps for the 4. The "little fuel pumps" in the batteries are working at less than capacity to charge them. So if the total amps of the batteries that I am charging exceed the 12, it will take longer?

    So to work the charger at its optimum I should charge 1 or multiple batteries together up to 12 amps. For example - I can charge (2) 6 amp or (3) 4 amp rated batteries and the charge process is optimum. However, if I have (2) 8 amp rated batteries, the charge time will take longer since I have exceeded the amp rating of the power supply.

    Or...am I totally off track?
    Will add something clever and witty if I ever come up with something.

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    What cell count and capacity is your lipo's.
    Sorry officer i thought the speed limit only applied to people with driving licenses.

  6. #6

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    HiTec X4 50W per port with 200W total
    Let's use 4.0v per cell as an example
    Four cell Lipo battery @ 16v
    W=IE, so 50/16=~3.15A
    or for the same battery starting charge at 3.8v per cell (storage level)
    3.29A
    Remember that this is the maximum power available from the charger.
    As the voltage per cell increases, the current decreases to maintain no more than 50W
    Further, as the battery gets close to about 4.1v per cell, the current will drop considerably in order to
    maintain a safe maximum charging voltage per cell - - (~4.2v)
    And, to add to the confusion, a "balance charge" may decrease the current even more during the balance process.
    Obviously, a two cell Lipo might be able to be charged at double the current of a four cell, if everything else works out.

    The charge current rating of the battery is often higher than the charger's maximum current capability!
    Last edited by chuckk2; 08-09-2013 at 08:53 AM.

  7. #7
    hookedonrc's Avatar
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    trax de max. I have several different capacities, but all are the same cell count 3S.... I have an 1800mah; 1300mah; and 2200mah. I have been charging them with the balanced charger that came with the aircraft until I get more knowledgeable. Each of them have a Wh after the mah.... 1800mah (19.98 Wh); 1300mah (14.4 Wh) and 2200mah (24.4 Wh)..and then to top it off, they each have Continuous Discharge rates.

    Where have my old 1800 mah Nimhs gone???
    Will add something clever and witty if I ever come up with something.

  8. #8

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    ok charging a 3s battery the max charge rate is 3.8A. on the charger per port.
    But to charge a 3s at 3.8A requires 5A from the 12V supply.
    So to need to get the max 3.8A from each port for charging 3s, using a 12V supply would need a 20A
    These figures include the max losses that occur in charger only, Not including any losses from a supply.

    Your looking at 24A supply. As mentioned on an earlier post, this won't split into 6A each port because the 3s voltage is higher than the 12V supply. If you was charging 2s lipos that would split off into 6A per port.
    I hope it's starting to make a bit more sense. Electrics all about numbers, but it's making the input number match or exceed the output number using "Ohms law".
    Sorry officer i thought the speed limit only applied to people with driving licenses.

  9. #9
    hookedonrc's Avatar
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    Thanks it is beginning to get there. If I plan to charge multiple 3s batteries at the field using the Hitec it makes sense to use the power supply with the higher amp rating. As long as I charge 1 or two at a time, the 12amp will fit the bill. Looks like I may end up keeping my Electri Fly supply for times I am charging just a couple and getting the higher amp supply when I take several planes out fly. I appreciate all of the info.
    Will add something clever and witty if I ever come up with something.

  10. #10

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    Looks like the light is finally shining!
    As to the problem with Battery Chargers and AC/DC (usually built in, but also applies in general to an external supply).

    First, no power supply can be 100% efficient. A well designed Switching Power Supply might be about 80% on average.
    So if you have a 100W input AC/to DC power supply, 80W is about the maximum usable output.
    There are additional losses in the charger that reduce the actual charge capability further.
    As an off the wall number to play with, assume that the charger's circuitry is at best, also 80% efficient.
    So, our 100W from the AC power source is down to about 64W charge capability under almost ideal conditions.
    In practice, the actual charge capability might be something like 50W, Gee, that's familiar!.

    With a DC charging source, usually an automotive battery- - -
    A fully charged battery is usually about 13.8V, and the DC voltage from the automotive electrical system (engine running)
    using the same battery is usually no more than about 14.6V. This translates to a slightly higher voltage than many of the built in AC/DC
    supplies includes in the 50W rated chargers, with a slight increase in charge capability.

    One of my chargers lists about 40-45W using the internal AC/DC power supply, and up to ~50W when using an external DC source, such as a battery or
    vehicle electrical system with the engine running. Allowable DC voltage from an external source is listed as 11-18 VDC.

    Many of the 50W chargers can charge up to a 6 cell Lipo using the internal AC/DC power supply, or an external supply.
    Obviously, the internal charger circuitry has a switching power supply section that can roughly double the DC input voltage.
    Given the small size and fairly light weight, it's rather obvious that the switching circuitry in both the built in AC/DC power supply
    and the switching circuitry in the charger itself must run at fairly high frequencies. If the charger manufacturer "cheaped out",
    the filtering may be inadequate, possibly causing problems with a vehicle's electronic "stuff" if the vehicle's engine is running, etc.


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