Battery Elimination Circuit Load Rules (Tutorial)

Discussion in 'Electronics' started by LukeWarm, Dec 4, 2011.

  1. LukeWarm

    LukeWarm Top Gun

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    We have a lot of questions in the forum about BECs and their rating.
    Most of the ESCs (electronic speed controls) have BECs (battery elimination circuits) built into them that allows you to use the motor battery to power the receiver. The BEC supplies power to the receiver through the ESC's control connection. Calculating the amp draw of your plane’s electronics is not that difficult, this is necessary so that you know you are using a receiver battery or a BEC that is rated at a high enough amperage to sufficiently power the receiver and the electronics it controls.

    There are 3 ways to supply power to a receiver;
    1) An ESC with a built in BEC
    2) A stand alone BEC
    3) A separate receiver battery, this is the cleanest power.

    The ESC's connection to the receiver:
    There are three wires from the ESC that are connected to the receiver. Two of them, the Control wire (white), and the ground wire (black), are used to control the throttle. The small red wire is used to deliver the power from the ESC's BEC, to the receiver and everything connected to it. The ESC gets its power from the motor battery; so if the ESC does not have a BEC, the red wire serves no purpose.

    If you use a different receiver power source other than your ESCs built-in BECs, Unplug the red wire to the receiver from your ESC to disable its BEC. Never use two power sources to the receiver at once (see NOTE). The receiver’s power connection can be plugged into any open channel on the receiver.

    What if I use an under powered receiver power source?
    If you use a power source with insufficient current (amperage) to supply power to your aircraft's flight control system, your receiver will lock up and become nonresponsive. Afterward, if you're lucky, it may recover and become responsive in time for you to regain control of the plane before it hits the ground.

    An over powered power source will run cooler, be more dependable, and last longer; buy one that is better than you need.

    The Servos:
    The servos are by far the primary user of the power that is used by the flight control system. It is for this reason that their consumption is what determines the power requirements of the systems power supply. Normally the amp draw on the servos we use is less than .5 amps per servo. The harder a servo must work to do the job you are asking it to do, the more power it will use. In extreme conditions, a servo can consume over an amp, but those times few and fast. There are things that you can do and choices you can make to make the servos job easier or harder, so there is no good solid number I can give you. But for most conditions “.5 amps per servo” will serve you well.

    The amp draw on servos varies with load;
    1. If the servo arm has more than one hole, if the control rod is in the hole closest to the servo; it will have a shorter throw, have more torque, and draw less amps.
    2. If the control arm on the control surface has more than one hole, if the control rod is in the hole farthest from the control surface ; it will have a shorter throw, have more torque, and draw less amps.
    3. If you have large or heavy control surfaces, the servos will have a higher amp draw.
    4. The faster the plane goes, the higher the servos amp draw will be.
    5. The more the plane or its control surfaces weigh, the higher the servos amp draw will be.
    6. The more radically you fly the plane, the higher the servo's amp draw will be. This is because most of the servos are being heavily worked all at one time.
    7. If the control surface's hinges and the connection to their servos are very free flowing, the servos will have a lower amp draw.
    In flight, not all servos will exert the maximum load at the same time, so you can sometimes get away with a slightly under powered BEC.
    But, If you have a large, fast plane with stiff hinges on many large control surfaces that are configured to where they have a lot of travel, And you fly radically so that many big servos are used heavily all at one time; You are the worst case scenario for a BEC’s load burden.

    The ESC’s BEC:
    The better ESCs have a BEC that will handle 3 amps or more, but most ESCs we use have a 1.5 to 2 amp BEC. A higher voltage main battery will give Most BECs less available amperage; If a BEC with a 7.4 volt battery has an out-put of 3 amps, the same BEC with a 11.1 volt battery has an out-put of 2 amps. Increasing the Battery voltage by 1/3rd, decreases your BECs out-put amperage by 1/3rd. This isn't always true, some BECs out-puts are unaffected by their in-put voltage. Check the specs on your BEC.

    Normally, Overloading the BEC will take out the BEC only and leave the motor part of the ESC working. You can safely get more performance out of properly ventilated and cooled electronics, because it can now dissipate more heat faster. It is safest to buy a BEC that has a high enough amp rating so that it can easily do what you need it to do. Better quality equipment gives better performance.

    The Receiver:
    On all Futaba receivers the amp draw can be up to 8 amps continuous or 10 amps peak. Others are similar. The average receiver will handle anything the average foamy pilot will throw at it. The transmiter is power hungery, but the receiver its self uses very little power. Most of the power that goes to the receiver is used by the electronics it controls.

    The BEC supplies power to everything except the motor:
    The power that goes to the receiver supplies power to the receiver and everything that is connected to it; gyros, lights, the control part of the ESC, measurement recording devices, and of course the servos. The max load is reached only if everything is heavily worked at once.

    The Conclusion:
    The size of BEC you need depends on your equipment and the way you fly. You need to use a receiver battery or a BEC that is rated at a high enough amperage, that the plane’s electronics has sufficient current to function correctly. There are two ways to find the amperage needed from the receiver’s power source, by using an amp meter OR by calculating the total maximum amperage all your components will draw based on your equipment's specifications. If you use an amp meter to find the amp draw, it's better to use a separate receiver battery during the test, not a BEC. An in-flight test with an onboard amp meter is the only way to get accurate numbers, fly the plane like you normally do.

    For most of us that use the recommended servos;
    1) if you have a 1.5 amp BEC, use 3 servos max
    1) if you have a 2 amp BEC, use 4 servos max
    2) if you have a 3 amp BEC, use 6 servos max
    3) For 6 to 10 servos, use a 5 amp receiver battery or a BEC
    Max has little to no headroom, a number less than max is safer.

    These recommendations are assuming that you are building the plane to fly. If you are building it to crash, use a more conservative set up. Servo amp load is very high during a crash and can take a BEC out.

    The voltage:
    Above we have discussed current but said nothing of voltage.
    On the AR600 6-Channel Sport DSMX Receiver
    The operating Voltage Range is 3.5 to 9.6 Volt DC.
    Most BECs and RX batteries are between 4.8 and 6.6 volts.
    6.6 volts would be very hard on our servos; but at that higher voltage, they will be fast and strong.
    With 6.6 Volts DC, our servos may work, but they will have a very short life.
    In my opinion, 6.6 Volts DC is too high for the servos we use.
    Our servos are rated for 4.8 to 6.0 Volts DC.

    Your Receiver will work fine at up to 9.6 Volt DC, but check the specs of operational voltages for everything you connect to the Receiver. Some of the bigger servos will work OK at that higher voltage.

    NOTE: Unless you really know what you are doing, dont do this. To use two power sources at once to supply power to the receiver, the voltages have to be exactly equal, otherwise when you connect them, you might puff a battery or burn up the BEC. You must also have a separate better connection between the batteries so that they do not equalize through the receiver; this could fry the receiver.
    Dual-motors-Dual-BECs
    Servo load info Source: http://homepages.paradise.net.nz/bhabbott/Servo.html
     
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  2. LukeWarm

    LukeWarm Top Gun

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    I redid post one to make it more user friendly. Also here is some additional info.

    LBEC:
    Most BECs are this type. Linear BEC reduces the voltage from that of the motor battery down to 5v or 6v basically by putting a resistance in the line. It gets hotter as the battery voltage goes up and as the current goes up. So they're generally limited to powering 4 servos off a 3S battery, or 3 servos off a 4S battery.

    SBEC:
    Switching BECs reduce the average voltage down to 5v or 6v by switching the supply on and off several thousand times per second so, with no resistance in the line, they don't heat up like linear ones do. They generally handle higher input voltages and more servos.

    UBEC:
    It stands for Ultimate BEC, and is the trade name of a particular manufacturer, but it's used by many others these days to mean a stand-along switching BEC.

    OPTO:
    The "OPTO" ESC require that a separate battery pack be installed to power the receiver. Because the motor control circuitry is optically isolated from the line to the Rx, glitches due to motor noise are greatly reduced.
     
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  3. nice Luke, makes it more understandable. lamen terms are always helpful
     
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  4. squishy

    squishy Top Gun

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    great post luke!!
     
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  5. YoungSC

    YoungSC Rookie

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    ..and my naive understanding captured for all to see :)
     
  6. LukeWarm

    LukeWarm Top Gun

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    For us, an extra battery to power the receiver is normally unnecessary weight. Most of the ESCs (electronic speed controls) have BECs (battery elimination circuits) built into them that allows you to use the motor battery to power the receiver.

    If you use a different receiver power source other than your ESCs built-in BECs, Remove the red wire out of the plug that connects to the receiver from your ESC; this will disable your ESC's on-board BEC. Do not connect two power sources to the receiver at once (see NOTE). The receiver’s power connection can be plugged into any open channel on the receiver. In a receiver's circuit board, all positive connections are connected directly to each other, and all negative connections in a receiver are connected directly to each other. The control wires each have there own connections on the receiver's circuit board because they are the only wires with a signal that changes and controls.

    NOTE: Unless you really know what you are doing, dont do this. To use two power sources at once to supply power to the receiver, the voltages have to be exactly equal, otherwise when you connect them, you might puff a battery or burn up the BEC. You must also have a separate better connection between the batteries so that they do not equalize through the receiver; this could fry the receiver.
     
  7. Grey

    Grey Ace Pilot

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    Just to put 2 and 2 together.

    You need motor amps + BEC amps from your battery. If you are using 3 servos you don't really need to account for it for normal flight.

    But if you fly at WOT a lot with 6 servos, retracts and lights? Add the amps ;).

    An addition;

    BEC amps include;

    Receiver and everything powered from the receiver, not just the servos.
    • Servos
    • Servoless retracts
    • Lights
    Sorry, I just know more than one person who has made the wrong assumption on that one and smoked an internal BEC.
     
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  8. phoenix_md

    phoenix_md Airman

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    Thanks Luke Warm. I loved this article. It really made sense out of a very confusing area of the hobby.
     
  9. LukeWarm

    LukeWarm Top Gun

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  10. LukeWarm

    LukeWarm Top Gun

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    There are 3 ways to supply power to a receiver:
    1) An ESC with a built in BEC
    2) A stand alone BEC
    3) A separate receiver battery, this is the cleanest power.

    [​IMG]
     
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  11. HiFlite

    HiFlite Ace Pilot

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    Hi Luke,
    I have a Spektrum AR8000 rx and there isn't a BAT port. How can I put an external BEC in? Thanks.
     
  12. squishy

    squishy Top Gun

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    Plug it into any channel. In most RX's all the + and - pins are connected together, this is called a bus and that's how the ESC gets power to the RX through the throttle port. Just plug the BEC into any unused channel, or use a V-cable on any used channel if you are out of ports...
     
  13. HiFlite

    HiFlite Ace Pilot

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    Thanks Squishy.
     
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  14. squishy

    squishy Top Gun

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    Here, I made you a picture even. I hope this explains it well...
    inside the RX.jpg
     
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  15. HiFlite

    HiFlite Ace Pilot

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    I had the gear channel open so I just plugged the BEC into that. Thanks.
     
  16. LukeWarm

    LukeWarm Top Gun

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    Great job Squishy, you know your stuff!!

    I did a major rework of post 1.
    Enjoy
     
  17. LukeWarm

    LukeWarm Top Gun

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    I added this today to post one.

    The voltage: Above we have discussed current but said nothing of voltage.
    On the AR600 6-Channel Sport DSMX Receiver
    The operating Voltage Range is 3.5 to 9.6 Volt DC.
    Most BECs and RX batteries are between 4.8 and 6.6 volts.
    6.6 volts would be very hard on our servos; but at that higher voltage, they will be fast and strong.
    With 6.6 Volts DC, our servos may work, but they will have a very short life.
    In my opinion, 6.6 Volts DC is too high for the servos we use.
    Our servos are rated for 4.8 to 6.0 Volts DC.

    Your Receiver will work fine at up to 9.6 Volt DC, but check the specs of operational voltages for everything you connect to the Receiver. Some of the bigger servos will work OK at that higher voltage.
     

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