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FAQ The BEC condundrum: A guide to BEC's and Rx packs

rcbirk21

70cc twin V2
So I have seen this question more times than I can count: "Should I run a bec or an rx pack on my electric plane?" Well, this question is both easy and hard to answer, so I figured I would write up something people can reference based on facts and a little bit of personal experience. Lets start with some basic definitions:

BEC - BEC stands for battery eliminator circuit, and its intention is to do exactly that, eliminate an additional battery. A BEC would be wired in parallel with the power inputs to the esc. The receiver output plugs are the plugged into the receiver, just like a battery pack. They have three main stats and measurables to watch out for; input voltage, output voltage, and output amperage. All three of these stats play a very important role in which BEC, if any, will work for your application.

1) input voltage - the voltage that the bec can take from the main packs. This is a huge factor, as you need to have the bec able to take the voltage from the packs you are using, or nothing will work properly and you will be frying components left and right. If you are running a 3s setup, make sure the bec can handle that. If you are running a 12s setup, make sure you find the correct bec that can handle that voltage. Also be careful of pro-rated bec's. I will go into detail with this is the output amperage section.

2) output voltage - this is another very important spec to understand and use to your advantage. This is a HUGE advantage for bec's, being able to select your output voltage. This is simply the voltage that your receiver, and therefor your servos, will see. Some bec's, like the castle becs (both bec and bec pro) as well as the western robotics and many many other becs have selectable voltage. Some are more selectable than others. For instance, the SportBEC from Dimension Engineering has a 5v/6v selectable voltage output. Honestly, that really does nothing for us 3Ders as we like to run the highest voltage our servos can take, and nowadays there are not many servos that wont take at least 6v (unless you are a heli guy and have some tail servos that only take 4.8v, in which case this bec wouldnt be the best choice anyways). Other bec's, such as the castle series and western robotics give you much more control. The castle has 0.1v increments up to 8v, and 0.5v increments above 8v up to about 11v (that high number may be wrong, its been a while since I have had to go about 8.5v on a castle bec :D). Western Robotics bec's have a couple different options depending on which bec you are using, but they typically go in jumps that equate to different voltage levels such as 6v, 7.4v, 8.5v, 9v (or something similar). This all depends on what servos you choose. If you servos can only take 6v, make sure you pick a bec that is capable of supplying that. If you have HV servos, make sure your bec is capable of outputting a higher voltage, as high as 8.4v (a fully charged 2s lipo). It is also important to know HOW to change output voltage for a bec. With the castle you must use a castle link, while others such as the ZTW, dimension sportbec, and the western robotics can be changed on the device without having to use a computer to program.

3) Output Amperage - This is a very touchy subject, and when misunderstood, causes a lot of people to discount BEC's and favor an rx pack (not that there is anything wrong with that, its just good to be informed and make the best possible decision for your application). This is simply what the bec is capable of supplying amperage wise to the receiver and servos. This can be either very straight forward or very tricky, depending on how the company rates their bec's. A simply example is the ZTW UBEC's. They are named by their output amperage. For example, the ZTW 6A UBEC is capable of supplying 6a at all voltage outputs (as listed by the specs on the webpage for the product). Castle has a more tricky label, especially when it comes to the bec pro. If you look closely at the label for the cc bec pro, it says 20a maximum output. Although it says 12s/20a capability, that is NOT 20a while running 12s. They actually pro-rate the current output on different voltages. This is how it actually breaks down:

bec pro output amps.JPG
(source)

That chart means that if you are running 12s input voltage, you actually only have about 8a available, not the listed 20a. This is why it makes much more sense to hook up the cc bec pro to 6s input voltage if you are running a 12s setup made of 2x 6s packs in series. This also brings up the topic of servo amp draw, but we will discuss that later. The last product worth discussing in this section is the Western Robotics bec. Although they are very expensive, they have vastly superior stats to almost any other bec on the market. While they do not list a chart like castle does, they do not list any limitations, so as long as adequate cooling is supplied, 15-20a peaks on high voltage setups (up to 14s!) should work! I have sent an email to them asking for clarification. I will update when I receive an response.


There are also two major types: linear, and switching. To be honest, that does not matter much anymore as you really should always get a switching bec. I could go into the specifics of the electrical differences between the two, but it honestly does not matter that much and the prevailing thought is that switching BEC's are more reliable and more robust in order to handle the power output of todays digital servos. If you are interested in reading up on the differences between the two, check out this site.

All of these facts apply to both internal and external bec's. A lot of people prefer running external bec's as they know what they are running. A lot of internal bec's specs are not as easy to find, and very often are cheaper and not as robust. Obviously this is not the case with all internal bec's, such as castle's new talon series of esc's with a very robust bec.

examples:

Western Robotics Hercules Super BEC

Western Robotics Hercules Super Mini BEC G2

Castle Creations CC BEC and CC BEC Pro

ZTW 6A External BEC (UBEC)

ZTW 12A External BEC (UBEC)

Dimension Engineering SportBEC

RX pack - Again, this term is exactly what it sounds like, a battery pack that is dedicated to powering the 'rx', or the receiver, as well as the servos and anything that is drawing power through the receiver. Again, there are things to worry about. C rating does matter, as well as capacity and voltage.

Advantages

BEC - There are a number of advantages of bec's. First of all is the convenience factor. Not having to plug in an additional battery before each flight and/or worrying about what state of charge the pack is in is very convenient and makes it much quicker to get a ton of flights in, or is one less thing to add to your pre-flight check. Another great advantage of bec's (but not all bec's) is selectable output voltage. Being able to experiment and change your mind without changing your equipment is great.

RX Pack There are plenty of advantages to the rx pack as well. The first is that a seperate battery pack allows an imperfect system to function. With a bec, if you exceed the amperage limit, you can burn something up and lose control. With an rx pack, if you pull too much current, you will simply have voltage sag. You will lose performance, but will not lose control. That in and of itself is enough reason for a lot of people to use an rx pack. You can also add redundancy with 2x rx packs hooked up independently to the receiver. This works great on larger planes, but on smaller planes it is simply too much weight and complexity to be worth it. Also if something happens to your main pack (such as losing a cell, battery ejection, plug comes unplugged) you still have servo power.

Disadvantages

BEC - With a bec, servo setup becomes a very important aspect. Making sure that servos are not binding and/or drawing excess current is extremely important as it can cause a bec to be overloaded and fail. It is also important to know what your servos are drawing if running a bec. I highly recommend a product such as this that can measure current draw and can be used after setup so you know what your specific servos and setup are drawing. Half the battle is knowing, and once you know, you can make much more informed decisions on your setup. Also if you lose your main pack for any reason (such as losing a cell, battery ejection, plug comes unplugged), you lose power to your servos also.

RX Pack - One of the biggest disadvantages of an rx pack is having another component to monitor that degrades. Its a fact that batteries degrade over time and need to be monitiored. Load testing as well as monitor voltage after flights becomes very important. How many flights can you get out of one pack? How big of a pack do you want to run? How do you secure it make sure the connection does not come out? Those are all important questions that need to be answered. Another battery is another component to keep an eye on, keep charged up, and make sure it is not sagging voltage. The more discharged the battery is, the lower the output voltage. That means you will lose servo power the more drained the pack is, which is not the case with bec's.

Points of Failure

I find this argument to not be an argument at all. Both ways have points of failures, and while there may not be the same number of points of failure, I feel that the likely hood of either one failing is pretty similar. For example, a bec can fail due to component failure such as a build up of heat, too high of a current draw from the servos, or just being a faulty component. An rx pack can fail if it is going bad and doesnt hold enough voltage, if it isnt charged, if the connect on it comes unplugged, if a solder joint goes bad in the pack, a cell goes bad, or any other number of problems. Those problems also become amplified if you buy a cheap chinese pack that are known to have bad solder joints, tabs come off cells, etc.

Conclusion

This comparison really comes down to comfort level and convenience. Whatever makes you most comfortable in the air is what you should run. If you are comfortable with a bec and love not having another battery pack to worry about, bec's are for you. If you are constantly nervous while flying that your bec is going to fail on you and it is negatively effecting your flying, then by all means rip out that bec and put in a receiver pack! This page is not meant to support one side of the argument. It is meant to inform everyone so they can make the decision that most positively impacts their hobby experience, because after all that is what we are trying to accomplish!
 

econpatric

70cc twin V2
Thank you for the write up. Very informative. I especially like how you compare and contrast. It might also be fun to see how you compare various types of RX packs, like lipo, life etc..
 

rcbirk21

70cc twin V2
you know i thought about that, but the problem is there are so many different thoughts on what voltages are safe for what servos, that i really did not want to touch it. some servos are rated for 6v. does that mean 6v regulated or 5s nimh pack? same with 7.4v rating. How high is safe? is a couple tenths of a volt going to cause damage or not? then the whole concept of regulators with rx packs is another dragon to tackle, and then you deal with setting voltage outputs on the regulator which again brings it back to what is safe.

sorry for the rambling, and thanks for the kind words :D
 

Steve_B

70cc twin V2
For those wanting to add the extra assurance of dual redundant power supply to the ESC and servos it can be done without using a separate battery and without adding any significant weight.
If you have an ESC with a built in BEC, plus a remote uBEC, the standard approach is to isolate the live wire of the ESCs BEC in order to prevent interference. However with a small mod you can utilise the outputs of both to provide redundancy. You need to put a couple of shottky diodes in the wires from the BECs to stop them interfering with each other. The diodes do cause a small voltage drop so it's best to use BEC's with adjustable voltage to compensate for the drop.
The idea is that if either BEC fails, or cant meet demand, the other takes over or shares the load.

It's a very simple idea, the diodes are tiny:
attachment.php
 

Neil_SC

Banned
Steve_B,
I've been looking for a good diagram like yours showing where the diodes go and part numbers. In my case, I have the 20A CC BEC Pro as the secondary/backup BEC. Which Shottky diode should I get?
 

Neil_SC

Banned
I forgot that the CC BEC Pro has two outputs to the RX, which are obviously in parallel. So I can run two of those 9A Schottky diodes on each output and the total output would be 18A, which is pretty close to the CC BEC Pro's 20A rating.
 
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