Internal/external BEC threory

[Espeefan]

I've got two senarios regarding BECs. Both internal and external. I'm looking for some input on the subject.

Senario 1 -

I'm running a dual speed controller, dual motor crawler. Both speed controllers have internal BEC. Both speed controllers are identical, and the BEC rating is 6 volts at 3 amps each. I know everyone suggests removing one of the speed controller's BEC wire so that the receiver is powered by only one of the speed controller's BEC. A search confirms this over and over again, but I beg to differ. In theory, two power supplies that are identical would not harm the receiver or electronics. They would be no different then plugging in two receiver battery packs in parallel with one another. The 6 volts does not add, but the combined current each speed controllers supplies would be double of what one BEC would do on it's own. In other words, the total current draw of servos, or any other load, would simply be split up between the two speed controllers' internal BEC. In theory, one could assume that you now have 6 amps (give or take) of current to power any devices.

Now my question is, why does everyone recommend removing one speed controller's BEC wire? You aren't feeding to much voltage to the receiver, that's for sure, but current? Say you have 6 servos on one receiver. Each one draws 1 amp, which could actually be conservative. Granted not all 6 will be drawing 1 amp all the time, continuously, as even in aircraft applications, the chances of working all the servos continuously would be impossible! If a receiver can regularly run 6 servos, clearly the receiver has the ability to handle up to 6 amps, or more, even if not continously. Why is it then that everyone suggests doing away with one of the speed controller's BEC wire? The current drawn may not even be a concern, as the combined draw will probably be below what the speed controllers can provide. Just because you have 6 amps of potential current capacity, a couple servos will be lucky to draw half that much in normal use. You may very well not even come close to maxing out the current draw on them, hence not putting your receiver in any danger.

Senario 2 -

Along comes an external BEC. The recommended way to wire one of these things up is to again, disconnect the BEC wire from the speed controller, and then let the external BEC unit run the entire sha-bang. Just like this -



Makes sense to me. I understand in this senario, the reason to disconnect the speed controller's internal BEC is to allow full potential use of the speed controller on a higher cell count, without overloading it. By taking away the BEC duties, we ease the load on the speed controller, preventing thermal shut downs, glitching, or just poor performance. Now this is what I find interesting. The external BEC unit may very well have greater current capacity then a single speed controller by itself. It might very well have great capacity then two speed controllers doubling up on BEC duties, as I described above. Take Castle's 10 amp BEC. Yes, I realize it may not be capable of a true 10 amps in all applications, but it will still be more capable then two speed controllers with rather low 3 amp BEC ratings, correct?

Now, here's what doesn't make sense to me. If people say damage can result from using dual speed controllers, and using each one's BEC together, then why doesn't a 10 amp external BEC destroy things or cause damage just as easy? Two speed controllers together - maybe 6 amps. One external BEC - perhaps as much as 10 amps. Do you see what I am getting at?

So tell me what the reason is for the suggestions of disabling one of the speed controller's BEC functions, in a dual ESC set-up? Is this just a suggestion that someone threw out, and then everyone ran with it? It makes no sense to me. I see no reason why you would have to do this, despite countless search results agreeing that you need to disable the BEC function on one speed controller.

Any thoughts guys? Has any actually tried this and had some real damage done? Somehow I don't think so. Based on my theories (right or wrong) a guy with dual ESCs shouldn't even need an external BEC to run multiple servos, unless he/she is running the ESCs at the limit of their max voltage input rating, correct? If they share the BEC duties, that's double what one could do by itself.

[constantmotion]

Just follow directions.....They are there for a reason

[One-eyed Jack]

In theory what you say about parallel connection is correct. The problem is that each one may try to produce a slightly different voltage and they then work against each other and they will produce excess heat. Been there done that. Taking the red wire out of the esc bec keeps from doing that. The cc bec is NOT rated at 10 amps in a crawler application. There is no air flow. That 10 amp per cc is with a 5+ mph wind blowing across bec. Based on all the post using an external bec and the posted failure rate I don't use one. The bec on a rooster crawler esc is 1.5 amp. I use a 9100T JR servo and have good service with it. An underpowered servo will draw more current with the same load that one with power to spare will. I realize that CC says you can run two BEC in parallel and work fine. If they put out the same voltage,which is unlikely it should work the same as two batteries. There are a few post on here that say they have done this and other methods of wiring the bec with sucess. I have seen more where they are burning up the bec. If I have heard I wired it just like it shows and it won't work once I have heard it a thousand times over the past 40 years. When "I" hook it up just like it shows I bet you can not guess what happens. Yes you can run the esc bec parallel and I have done it. I also have discovered excess heat in the power leads so I no longer do so. If you want to run 3 or more cells with a esc that is only rated for two I would use a receiver battery. Yes,extra weight but 10 times more reliable. I run a cc esc rated for 3 cells on 4 cells on a Revo for more than a year now. 40mph on 3 and 50+ on 4

[Espeefan]

Originally Posted by One-eyed Jack

In theory what you say about parallel connection is correct. The problem is that each one may try to produce a slightly different voltage and they then work against each other and they will produce excess heat. Been there done that.

Interesting. I have run a dual brushless ESC, dual motor crawler, and I have not taken any precautions as far as diconnecting one speed controller's BEC, or the other. I have not noticed any heat problems at all. I ran a battery pack through and, at best, the speed controllers were luke warm.

Originally Posted by One-eyed Jack

The cc bec is NOT rated at 10 amps in a crawler application. There is no air flow. That 10 amp per cc is with a 5+ mph wind blowing across bec.

That's strange. You're telling me that the Castle BEC needs a 5mph breeze blowing across it to put out it's rated 10 amps? This BEC is designed for aircraft use, this much I know, but seriously, guys install these things in fuselages, which are closed up tight. I don't see how they'd get any breeze in this application either. Nobody runs these things with the BEC hanging in the breeze. The only exception is park flyers that have a stick fuselage and foam wings. These aren't the norm for RC aircraft.

Originally Posted by One-eyed Jack

Based on all the post using an external bec and the posted failure rate I don't use one. The bec on a rooster crawler esc is 1.5 amp. I use a 9100T JR servo and have good service with it. An underpowered servo will draw more current with the same load that one with power to spare will. I realize that CC says you can run two BEC in parallel and work fine. If they put out the same voltage,which is unlikely it should work the same as two batteries. There are a few post on here that say they have done this and other methods of wiring the bec with sucess. I have seen more where they are burning up the bec. If I have heard I wired it just like it shows and it won't work once I have heard it a thousand times over the past 40 years. When "I" hook it up just like it shows I bet you can not guess what happens. Yes you can run the esc bec parallel and I have done it. I also have discovered excess heat in the power leads so I no longer do so. If you want to run 3 or more cells with a esc that is only rated for two I would use a receiver battery. Yes,extra weight but 10 times more reliable. I run a cc esc rated for 3 cells on 4 cells on a Revo for more than a year now. 40mph on 3 and 50+ on 4

I wasn't really talking about any external BECs specifically. High failure rate is something I don't know much about yet. It could be you are more likey to hear about failures then you will the sucesses. People tend to ask for help when something doesn't work right. When it's all good, nobody mentions it. In my original post, I was simply talking about the theory of it all. If I had an external BEC, I would not be using, nor needing the ESC's internal BEC. I was simply saying I saw no reason I couldn't use the internal BECs from two speed controllers together, without the need for an external BEC, but point taken. I guess I am still free to experiement and find my own conclusions. Thanks for the input.

[Espeefan]

Originally Posted by constantmotion

Just follow directions.....They are there for a reason

I wasn't asking for advice on the installtion or wiring, nor was I asking about running an external BEC, in conjunction with a speed controller's internal BEC. Why, if I spent the money to buy an external BEC, would I need the ESC's weaker, internal BEC?

More to the point, I was simply suggesting that in theory, if one runs a dual ESC set-up in a crawler, he or she could get away without needing any external BECs because the combined current capacity of both ESCs working together should be adequate to run the typical servos used on crawlers.

[oscaroc]

Here's a retarded question, What is a BEC?

[SMFtwstdHICK]

I asked the same thing a while back. But, still don't know the advantage of one. Nor what they accomplish. But,what can ya do. Guess it's one of those "learn for yourself" things.

[Espeefan]

BEC = battery eliminator circuitry. With electric RC models, the internal BEC of the speed controller is what powers your on-board electronics, normally. Mainly the servos and the receiver. The BEC takes the voltage from your crawler's battery pack, which powers the motor(s), and drops it down to 5 or 6 volts, which is what your radio gear needs to function. In this way you do not have to run a separate 4.8 volt to 6 volt receiver battery pack, sometimes called a hump pack. The speed controller's internal BEC takes the place of the receiver pack, saving you some weight, and reducing some un-necessary equipment.

Now external BECs do the same thing. Why run an external BEC? Because many times the internal BEC of a speed controller is not good enough to run both the motor(s) of the crawler AND supply sufficient power to high torque servos at the same time, especially as you start to max out the ESC with the higher input voltages. The more cells you run, the less effective the internal BEC becomes. One servo may be fine, but if you run two or more servos (like 4 wheel steering, and or a DIG transmission) then the internal BEC of the speed controller can be over-tasked and the ESC shuts down, glitches, overheats, or just plain runs out of power to spin the motor(s).

An external BEC usually has a higher current rating, and since it's a switching type regulator, it is capable of providing a cleaner, more reliable voltage and higher current supply to run on-board electronics. When you start to tax a speed controller by running more cells, the internal BEC performance suffers. Not so with an external BEC. If you need more power to run servos, an external BEC is a good upgrade, and sometimes the only way to make things work.

[SMFtwstdHICK]

I got it, okay,makes sense now. Just takes me a second. lol. i run an 8.4 7 cell and no extra battery pack for my rig. I can't recall my ECS, I believe it is a Microrunner? Seems to power everything okay right now. But, if this BEC setup helps in maintaining the rig and helping out my servo and winch. THen me thinks me needs one.

[SuperRoo]

oscaroc there is know such thing as stupid questions.

I thought the BEC is a function of a receiver.

Isnt it the receiver that allows the ESC or any other device to supply the voltage to itself through the servo input channels or through the battery input circuit? By allowing the voltage to enter the receiver through the servo inputs is what I know as a BEC.

Everything else is a function of the ESC by way of a current limited voltage regulator or a external current limited voltage regulator like those sold by Castle Creations and known as a BEC (for some reason).

-----------------------------

Back on topic, Espeefan are you running two batteries with your setup or one?

[Espeefan]

Originally Posted by SMFtwstdHICK

I got it, okay,makes sense now. Just takes me a second. lol. i run an 8.4 7 cell and no extra battery pack for my rig. I can't recall my ECS, I believe it is a Microrunner? Seems to power everything okay right now. But, if this BEC setup helps in maintaining the rig and helping out my servo and winch. THen me thinks me needs one.

Most speed controllers have no problem providing enough BEC current to operate at least one digital high torque servo, or two standard servos. The problem comes in when you add another servo or function. Typical BEC current ratings are usually around 3 amps. Some as high as 5 amps. The 5 amp rated BECs will easily handle two digital, high torque servos no sweat, but the 3 amp BECs may be questionable. It never hurts to try it. The worst that happens is you realize the internal BEC current of your ESC is not up to snuff and you then get to decide what external BEC to purchase.

One other thing to note is that if you run your ESC at it's maximum rated input voltage level, as well as run extra servos on the internal BEC, you begin to ask more of it, and it may very well thermal on you, glitch, or cut-out. There lies the problem, and reason for running an external BEC.

[One-eyed Jack]

"I" did not make up the 5 mph requirement, Castle Creations did. It makes no difference where they install them or how many run them in closed enviroments and get away with it. CC says the 10 amp rating is with 5mph wind. In order to get max performance ie 10 amps the wind to cool is required. There are few if any mfg. products that will deliver the Max except in ideal conditions and for short duration. Try to get a 100% continuously and you will get a failure. I have yet to see a mfg. co that would not blow its own horn to the MAX.

[Espeefan]

Originally Posted by SuperRoo

oscaroc there is know such thing as stupid questions.

I thought the BEC is a function of a receiver.

Isnt it the receiver that allows the ESC or any other device to supply the voltage to itself through the servo input channels or through the battery input circuit? By allowing the voltage to enter the receiver through the servo inputs is what I know as a BEC.

Everything else is a function of the ESC by way of a current limited voltage regulator or a external current limited voltage regulator like those sold by Castle Creations and known as a BEC (for some reason).

-----------------------------

Back on topic, Espeefan are you running two batteries with your setup or one?

All radio receivers, that I know of, can be powered using an external BEC, an internal BEC - coming from the speed controller, or a dedicated receiver battery pack. Your discription of how BEC power gets into the receiver is correct. It will enter through the positive and negitive (or common) pins of whatever channel you happen to plug the speed controller, or external BEC into. Even a separate receiver battery pack can be plugged into a spare channel, and it to will power the receiver. All of the positive and negitive pins are tied together. Only the third pin, which is for the signal, is separate. The positive and negitives are always 'live', and so it does not matter if the voltage enters the receiver from a specific channel, or from the battery slot. The are electrically identical. This is why you can plug a battery monitor into a spare slot and it will show you the voltage condition of the receiver battery pack.

But, getting back to the point, BEC is not a function of the receiver. It's a function of the speed controller or an external BEC. It could be said that the circuitry (the way it's built) of the receiver allows the BEC to power it from any channel, but the actual stepping down of the voltage, and regulation happens inside the ESC or external BEC unit. Make sense?

On my crawler, I run one battery pack, which powers both ESCs. I could run a complete dual everything set-up, but one battery pack has no problem running two separate speed controllers, each with their own motor.

[Espeefan]

Originally Posted by One-eyed Jack

"I" did not make up the 5 mph requirement, Castle Creations did. It makes no difference where they install them or how many run them in closed enviroments and get away with it. CC says the 10 amp rating is with 5mph wind. In order to get max performance ie 10 amps the wind to cool is required. There are few if any mfg. products that will deliver the Max except in ideal conditions and for short duration. Try to get a 100% continuously and you will get a failure. I have yet to see a mfg. co that would not blow its own horn to the MAX.

Jack, point taken. If it comes directly from Castle's mouth, then I believe it. Also, I seriously doubt in a crawler application that anyone comes close to 'maxing out' the 10 amp rating. Even if it is only a 5 amp continous rating.

[One-eyed Jack]

True

[JohnRobHolmes]

Originally Posted by Espeefan

I seriously doubt in a crawler application that anyone comes close to 'maxing out' the 10 amp rating.

Two big digi servos could to that easily. Many super crawlers run two BECs, and on separate circuits. Two CC BECs in parallel do not double the power because of small differences in output voltage. One BEC will take the brunt of the amp load, just like internal BECs will do.

The reason to run the RX via the ESC's internal BEC and the servos via a Castle BEC is for added interference rejection and system redundancy.

An RX pack is more reliable in simple terms, but adds weight and may not have a place to fit on 2.2 rigs.

External BECs are needed because many people run 3s lipo, which renders the ESC's internal BEC useless for powering a strong servo.