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BEC Question

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Old 05-26-2010, 11:56 AM
  #1  
rascal329
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Default BEC Question

I am new to electric and have built my first electric plane. I am using a 5S battery and have read in a few places that it is not good to use the built in BEC when using more than 3S batteries. Can someone explain the reasoning behind this, is it a valid concern, and what is the risk (i.e. what happens if you do use the BEC with more than 3S)?

I have been researching this and can't seem to find a definitive answer. Being a noob I would like to understand this better.

Thanks for any info or guidance someone could provide.

KB
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Old 05-26-2010, 04:45 PM
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patnchris
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Default RE: BEC Question

The built in voltge regulator in most ESC's is designed to provide the proper voltage to the reciever to power it and the servo's. The internal circuitry is designed to function at a maximum of 14 volts input. Voltage over that can, in most cases burn out the circuits. There are some ESC's that have higher rated voltage reg's, but I haven't seen any that will function over 6S. I have substituted voltage reg. for BEC (battery eliminator circuit) for clarity. The BEC is so one can eliminate the need for a separate RX pack. Unless you purchase a special ESC that is designed for high voltage input, and has a built in switch mode BEC you will need to run a separate reciever battery or purchase a voltage regulator that can be wired to the battery output and regulate the voltage to 6.0 volts or less. External BEC's (voltage regulators) come in two varieties. The linear ones are usually good for 4-6S input. The switch mode versions can normally be used for higher input voltages.

The easiest way to get this together is to purchase an ESC (like the E-Flight 60 amp Pro) that has a built in switch mode BEC and is rated for 6S. The BEC will handle up tp 3 amps to power the RX and servo's. This is usually good for 4 standard servo's. The other easy option would be to purchase an ESC that can handle the higher voltage with no BEC. In this case you will run an RX pack, just like in the old days. The down side is that you need to make sure the RX pack is charged before flying.

JMHO

Pat
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Old 05-27-2010, 06:02 PM
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Default RE: BEC Question

You tell em Pat. Something else to know about BEC's. the more voltage you input into them, the LESS amps they will output. This goes for built in and stand alone units.

This could lead to a problem if say you don't read the fine print on the ESC/BEC instructions, or forget about it. Your system could be in need of say 3 amps, but because of you running a high cell count pack, the BEC is only capable of say 2 or less.

For example I copied the below from Castle's site on their 10 amp stand alone BEC.


Peak: 10 amps
Continuous:
<12 volts input = 7 amps*
<24 volts input = 5 amps*
*Ratings are determined with a 5mph airflow on the BEC. Servo connectors are not rated for current in excess of 5 amps. Users are encouraged to replace the connectors if more than 5 amp currents are anticipated.
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Old 05-27-2010, 06:51 PM
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rascal329
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Default RE: BEC Question

What about the difference between linear and switched BEC's? What I think I'm reading is that what you are saying is true in regards to linear BEC's but not an issue with switched BEC's. Please correct me if I'm not understanding this correctly.

Thanks,
KB
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Old 05-27-2010, 11:00 PM
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patnchris
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Default RE: BEC Question

Switch mode BEC's can usually handle a higher input voltage as they generate less heat. However, they are still limited, in the voltage they can handle, by the internal components.

Before you make the decision to jump into a switch mode BEC, there is a drawback. Linear BEC's generate almost no RF interference. Switch mode BEC's (specially cheap ones) can generate RF interference to the point that it can effect radio range and performance.

These problems can be limited by installing the BEC away from the reciever antenna and by not crossing any servo leads around it. Some are much better than others, so if you were to decide to go this route, be sure and reasearch you purchase. Get recommendations from other flyers that use them......If you cannot find any reviews, online, or cannot find anyone that has experience with a product, it's probably best to avoid it.
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Old 05-28-2010, 12:50 PM
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Default RE: BEC Question

I will not be using speed controls internal BEC's any more. Even though I pushed the limit and used 6 mini analog servos (which the manual said it would handle and many people told me don't do it) it overheated in flight and I lost all control and got to watch it glide into a fence at a 45 degree dive from 250-300 feet. there is no question in my mind it was the BEC in my E-flite pro 60 amp speed control that did it.
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Old 05-28-2010, 04:37 PM
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rascal329
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Default RE: BEC Question

Sorry to hear of your loss but it has convinced me to not use the onboard BEC. I bought an external today and will put it in my plane before I fly again. I now have a Phoenix ICE 75 and a CC BEC. Hopefully things will work as advertised.
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Old 07-03-2010, 01:42 PM
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Default RE: BEC Question

If you still have questions you might take a look at this e-book on electric flight:
http://www.rcuniverse.com/forum/m_7100376/tm.htm
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