Can a RX battery be used in combination with a BEC? - RC Groups
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Mar 21, 2012, 12:38 AM
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twinster2's Avatar

Can a RX battery be used in combination with a BEC?

OK, here is the deal. I've just about finished with a Royal B-17. A schematic of the wiring setup is attached. Originally, I was going to disable the BEC on all four ESC and power the RX and servos using a separate 4.8 volt RX battery. Although this will work fine, I'm starting to think about a redundant power source like activating the BEC on one of the BEC as well as using a separate battery. Has anyone ever tried this before and can it cause problems. Since the battery would be a 4.8 volt NiCd pack one concern is that the ESC could possibly overcharge the battery pack. Perhaps if I put a diode in the battery pack wiring so that it could not be charged. Your ideas and comments are welcome.
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Mar 21, 2012, 01:45 AM
222 km/hr Parkjet flyer
solentlife's Avatar
i have a nifty little box that takes two Rx power sources. It has a main and a back-up ... so if your main power fails - it instantly switches to the back-up and you continue on. t has LED's to tell you which is working. It weighs and about size of a small servo.

It's called a "Battery backer" and sold by MFA in UK .... but I'm sure that someone else does similar.

I'm told it will also work with two BEC's ... but never tried. I had it for large models back in the 1980's when we were recc'd to have back-up power in them as safety. I also used two Rx's in those models .... same reason.
Mar 21, 2012, 09:50 AM
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JurassicJet's Avatar
I sorta have the same issue. Completing a large B-25 and I am trying to figure out how I want to power my receiver.

I was thinking about using a Life A123 for the receiver, maybe two with a "Y" connector.
You make a good point about the BEC overcharging your other pack. I would certainly think that it would if it is connected on the same line. Now, if you plug it into a different channel port will it still do the same thing? How is the channel port bus in the receiver configured....that would be the question to answer.
I diode would work too.
Mar 21, 2012, 10:47 AM
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The + and - buses are just that. Buses. So they are all common in the RX. The only one that isn't would be the third pin, or signal. Obviously.

I have been wondering this same thing lately though too. I was thinking a 5 cell battery with a diode would probably work? Simple anyway.

I mention 5 cells because then your battery would still be doing something after going through the diode's voltage drop.

So with one diode on the battery side you would stop the BEC from overcgarging the battery, but would there be any benefit to a diode on the BEC line? I think not, and you wouldn't want the voltage drop there anyway.

You would want a standard switch and/or charge jack for the battery of course.

I didn't do this yet, but I almost did on a .40 size conversion I just finished. I could have used a little payload up front for balancing and making sure there is always DC going to the receiver and servos can't be a bad thing, right?

Are we missing any obvious issues here? Maybe I'll try the above soon.
Last edited by Oregon Craig; Mar 21, 2012 at 10:49 AM. Reason: Got an F in spelling?
Mar 21, 2012, 11:15 AM
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Doug Bartley's Avatar
IMO on both planes discussed above, I'd use a single separate Bec of adequate amperage to deliver 6V to the RX.
I don't think these redundant systems are really required anymore. The old stuff (nihm packs etc was unreliable, it was a necessity. Modern batteries, lipo, A 123 eliminate the good battery/ failed battery syndrome Rx packs where so famous for in the day.
If your using 2.4 TX, 6v assures that under all circumstances, the voltage stays high enough to prevent Rx brownouts. I use a small 1500/1800/ 15C 2S lipo with the Turnigy Megabec 8 (8A) (6-8 servos) or the Turnigy 8-15 (15a max) (6-10 servos) . Both of these have been fooproof for 100 flights + in various planes including a 50cc Aeroworks. Typical consumption on my planes is 100-120mah per flight, so 8 flights in a day, far more than I ever fly!!
If redundancy is really a necessity in your mind, use a Smartfly, it allows 2 batteries with auto switching, a separate power supply to RX, even from the servo power grid, all regulated. FWIW Doug B
Mar 22, 2012, 08:35 AM
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JurassicJet's Avatar
Interesting about the Smartfly. Which model of Smartfly do you use.
Mar 22, 2012, 11:00 AM
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Disarray's Avatar
I have a 3 motor foamy that I am using 3 different brand ESCs on. I am currently using all 3 BECs at the same time. :O
Mar 22, 2012, 10:39 PM
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Doug Bartley's Avatar
Originally Posted by JurassicJet
Interesting about the Smartfly. Which model of Smartfly do you use.
i don't use them at all, but others flying gas planes in our club do. There are several, but the basic 2 for electric planes are a 7.5a and 15a unit. Depending on servo count 7.5a should handle 6 and 15a for more at 6v. the top of the line units have adj voltages etc, some require more voltage to servos. they seem to be ery well made, no one i know has ever had an issue with a Smartfly. doug b
Mar 23, 2012, 05:52 PM
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vintage1's Avatar
when using multiple ESCs it is perfectly possible to split the servo load amongst them.

You cant breakup all the regulated +5v outputs of the ESCs except one which feeds the RX and use them to feed different servos.

The easy way to do this is to make some sort of power panel out of PCB and servo pins

That way you can have redundant servos - typically use one for each aileron, on different servos and power supplies and one for each side of the elevator.

If you have retracts give them their own regulator/ESC and if you use two servos for them, each one with its own ESC/BEC.

That way any regulator shutdown wont kill the model completely, unless its the RX one.

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