Do I need a BEC?

J

Jim Schultz

Member
So I just bought 6 Afro 30A onto ESCs for my new hexacopter. I did not realize that these didn't have BECs when I bought them so I was wondering if I need external BECs for my ESCs. I am using 6 800v SunnySky motors and a 4s 6200mAh lipo.
 

Motopreserve

Motopreserve

Drone Enthusiast
The BEC (battery elimination circuit) inside an esc allows the esc to power the Rx and/or FC, so depending on your flight controller, you may or may not need a BEC.

If your FC is powered directly from the battery, you should not need a BEC.
 


Motopreserve

Motopreserve

Drone Enthusiast
Is that right? I know many people do actually run BECs for Pixhawk. Or at least the older APM 2.6. Maybe that's just for redundancy?

You might want to do some research on that.
 

J

Jim Schultz

Member
Seems like most people use an ESC with the Pixhawk. If I were to use an ESC leading to the Pixhawk would I then need BECs for my motors?
 

Motopreserve

Motopreserve

Drone Enthusiast
No. ESCs, and then the motors (thru the ESCs) are powered by the battery (positive and negative leads on one side esc). The internal BEC on an esc sends power the other way back to the Rx/FC through the servo lead. It is to eliminate the need for a separate battery specifically to power the Rx. This is a hold over from RC Planes.

The pixhawk may be sensitive to power requirements - and since the power from an internal BEC is only about 5v, it may be best to get a dedicated BEC to step the power down from the battery.
 



Motopreserve

Motopreserve

Drone Enthusiast
These are two very separate issues.

ESCs and motors are powered by the same leads that come directly from the battery - typically by soldering the positive and negative leads (should be something like 14 or 16 gauge wire) directly to a PDB or squid. That PDB or squid receives power directly from the battery (therefore the voltage that your battery is rated for.

The pixhawk needs a lesser voltage than the ESCs/motors. So either an internal BEC (on the esc) or another source of this lesser voltage is needed.

I'd search a bit and see how to best and most securely power the pixhawk. If people are confident with the built in BEC on the power module - you are all set. If people feel the need for a redundant power source (I thought I recalled reading this - but I'm not very familiar with the pixhawk), then you may want a separate BEC to get power to the pixhawk. That external BEC would derive power from the PDB/Squid as well.
 


crayfellow

crayfellow

Member
many people use a dedicated BEC with Pixhawk because they are running larger voltage and/or current than the standard power module can withstand. This requires replacing the standard 4S max voltage / 90A max current Pixhawk power module with an attopilot voltage/current meter.

@Jim Schultz since you are using 4S you can use the standard Pixhawk power module. Obviously with whatever motor/ESC setup you settle on, check your max current readings on the bench (or check specs for max draw for your total system) to ensure you are within the 90A limit.

You can provide redundant power, if you want, by using a 5V BEC such as this connected to the "servo rail" on the Pixhawk. But its redundancy will of course depend on where it's drawing its power vs. the supply to the power module. At the very least it would keep power to the FC in the event of a failure of the built-in BEC on the power module.

Also if you want to go up to 6S voltage-wise but will still be under the 90A limit, the rctimer power module is a nice option.
 


dazzab

dazzab

Member
The Pixhawk can be powered from three sources. With a power module, on the output rails, or with USB. The most common way to power it is with a power module and if you are ok with one power source that's all you need. If you want some redundancy you can put +5v on the output rails as well. This can be from a BEC on a ESC but a lot of people don't like this as ESCs are known to have issues and if they do it is possible for a voltage spike to end up on the Pixhawk output rail. What I've been doing is to power the Pixhawk with a power module and also with a dedicated quality +5V BEC connected directly to my main power and the Pixhawk output rail. The Pixhawk is programmed to monitor and use the most stable power source available. So if for some crazy reason the power module fails, it will automatically get it's power from the +5V on the output rail provided by the BEC. But for the most part it will use the power module input. Also, if you are running any servos or relays off the Pixhawk power rail that require power then you must supply power to the output rail anyway.
 

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