Important! Rx Batteries, BEC's & Servo Current!

May be a daft question
On a multi motor model could you use more than one BEC/UBEC to power the Rx having one as a kind of back up. So if one of the ESCs fails or UBEC fails the other would take over?

Only daft answers Dean, if it's a switching Bec, then NO WAY! If they're Linear then OK but if the output section goes down on one the it'll pull the other one down as well. I suggest you look at Power boxes.

As Klippy says, they will fight each other, and YOU will lose!

Crikey those powerboxes aren't cheap!!
I am allergic to electricity :lol: so don't really understand circuits Etc. Give me a Hydraulic or pneumatic circuit and I'm happy!
I suppose I was thinking of using two batteries like using two hydraulic pumps and using a non return valve on each pump incase one pump failed, would a diode do the same thing? Or is it not quite that simple? Would they still fight each other if they where both working?

Yes you can, hydraulics and electrics are very similar in nature. You will have to suffer a 0.6volts drop across the diodes in the forward direction. I would use what we call a 'stud' diode, you can mount them on a piece of alloy plate to take care of any heat.

I have been trolling around looking for the answer to ; how many servos can you safely run from a 3amp SBEC & came across this topic from eight years ago. Things have moved on since then but I do wonder how relevant it still is ?
Take my Tomcat for example. In its normal configuration (wheels) it has four nine gram digital servos, on floats that becomes five, the extra being for the water rudders. If you allow for some diversity & the draw from the RX I am starting to think that 3amps is not enough ! Thoughts ?

You might get some drop if they're all moving together, the worst case scenario is when one is stalled for some reason. Here's a thought, water is pretty solid stuff, is a 9gm servo sufficient power wise?
A finger tip test immediately after landing might be an idea.

But surely you don't use the water rudder at landing or take off speed. Disaster would surely follow if you did.
When manoeuvring the water rudder will be used in conjunction with only the elevator and fin rudder, both of which are also at a low air speed.
My own guess is that when manoeuvring on water the total electrical servo load will be significantly less than in normal flight.

As implied, servos under no load (not moving) draw next to nothing, amps-wise.
Let's break the loads down.
As you fly electric, the speed controller load on the bec never varies from a few milliamps, so you can discount that.
The same applies to the receiver.
So now we are down to three servos.
When taxiing on water, the aileron servo does nothing, and the elevator servo is probably held fully up, so once it has transited, that is doing nothing as well, so that just leaves the rudder servo.
In flight, the ailerons and elevator are used most, with occasional rudder inputs for stall-turns or 'pretty' skidless banking turns. Unless you are flying 3D or extreme deflection manouevres, the load on the servos is small and brief, as they don't have to move very far from neutral.
So it is unlikely that two servos can draw enough from the bec to cause a voltage brown-out, providing neither of them is overloaded to the point of stalling, whereupon the current drawn will shoot up, as per ANY electric motor. (it's all to do with back emf, and other stuff. You can look it up if you really want to be baffled by science!)
Digital servos do draw a little bit more power than a conventional analogue one, because they are constantly micro-correcting for position errors.
Stiff control linkages can make things worse, so if you have one - fix it!

If you were around in the late 60's/early 70's, you will recall that we used to fly quite happily using ONE four-cell 500mah nicad battery, that (hopefully) lasted all day, using four servos, if you were rich enough to afford four!

Thanks lads, interesting stuff. I managed to find some more info on the ESC i'm using & the SBEC is rated at 3.5 amps continuous, so that has put my mind at ease.
I would probably go for a 5 amp if I had flaps though.

Waterplanes.
The air and water rudder are often sharing one Servo, and are used as required. On take off they are lightly used to keep the correct heading.
High wing models on floats (any model on high floats). Taking off. Ailerons are also lightly used to keep the wings level as both wave and turbulence seem to conspire to prevent flight.
So to calculate for worst case scenario is good practice. Most smaller "normal" size models seldom require more than the standard esc current rate