Are the power connections for BECs of ESCs made in parallel or series with each other?

Question

I see the 5V power pin of BECs all are connected to the flight controller, but within the flight controller, is this registered as a 4 5V batteries in series or parallel? In series would not make sense since that would be 20V (assuming quadcopter) going into the flight controller.

In parallel makes more sense since that would be 5V and like 10+ amps going into the flight controller to power low voltage devices including camera equipment.

But I was told by someone that the 5V BEC pins are not connected parallel to each other since that would damage the BECs. But like, it has to be series or parallel right, one or the other?

I have a microcontroller I am using for flight controller and I was thinking of just hooking up all the 5V pins to a 5V power rail and the ground pins accordingly. I don't have 6 pins 5V input pins on the microcontroller as you would in a commercial flight controller (I think).

Answer 1

The exact wiring architecture will differ between flight controllers, but (if the BEC is used at all for board power) it's likely that only one BEC is actively used to power the FC. The others would just be open circuits, i.e. disconnected and unused.

As a general rule in circuit design, it's not a wise idea to combine different power sources/rails unless you use decently complex rail combination circuitry, or if the rails were specifically designed to be combined. This is because any slight voltage differential between the rails will cause current to try to flow between the rails, an undesirable situation because it begins to violate assumed design constraints for both the device and the individual power sources.

It's not that efficient power switching circuitry to change between rails is nontrivial, but it usually just adds often unnecessary complexity (and weight/space) to a board.

Answer 2

Most multicopters use ESCs that do not have a BEC, which saves space and weight (and the red wire).

Instead they use a single BEC, usually built into a power distribution board (PDB) that can be stacked under the flight controller.

It's also increasingly common to integrate 4 ESCs and the BEC on one board, replacing the PDB.

If you have ESCs with built-in BECs, the usual practice is to only connect one of them.

You're correct that connecting them in series would provide far too much voltage, but it's not a good idea to connect them in parallel either (and connecting them all to the + and - pads of the flight controller would be a parallel connection).

You can either cut the red wire in the other servo plugs, or pull the pin out of the plug after lifting the retaining tab with the tip of a knife. In either case you should ensure the exposed wire can't touch the frame or any other electronics and cause a short circuit. If you're soldering the servo wires, you could remove the red wire entirely.

The reason for this is that most modern BECs are active 'switching' devices that adjust their output based on demand, and they aren't designed to be connected in parallel. Older BECs used linear regulators which could be paralleled. Linear BECs are usually only rated for 1amp, and comes with a warning that you should limit yourself to two servos when running from a 3s LiPo. If your ESC has a 3amp or larger BEC, it's a switching BEC.

Anecdotally, I've tried paralleling BECs, and it seems to work but just one BEC will provide plenty of power and you risk a voltage fluctuation that causes the receiver or flight controller to reboot in flight, which could easily cause a crash.

Answer 3

Contrary to the previous answers, on a traditional flight controller (taking the KK2 board as an example), all 4 BECs can be connected together if the user blindly wires it that way. Whether this is recommended practice is below.

Traditional ESCs (before blheli was a thing) use a 7805 linear voltage regulator as a BEC. A lot of ESCs (the fire red series, and the myriad of ESCs based directly off of that) use multiple 7805 regulators in parallel. So by hooking 4 esc's together, you're really hooking 8 78[m]05 regulators in parallel.

In theory, this arguably works acceptably for increasing the possible output current (not that a flight controller needs more than ~200ma on the 5v rail), and in practice, it still "works" albeit less so.

Assume 8 regulators, all linearly regulating to 5v +/- 0.1v. Let's say regulator 1 outputs 5.1v, and all others output <5.1v. Reg1 will output the brunt of the current, and only when it is so overloaded that it starts to drop out, will the others actually start to source current. At this point, Reg1 is approaching overheat and since Reg2 on the same ESC shares a thermal plane, it's not far behind. The cooler regulators will still pick up the slack, but at the expense of extreme heat and wear on the parts.

The running recommendation is to allow one ESC's BEC to power the system, and to disconnect (cut the red wire) of all the other BECs. This doesn't solve the ESC's design issue of paralleling two 7805's, but it drops the number from 8.

When switching regulators are involved (SBEC), the short answer is do not wire them in parallel. They behave quite differently and could damage each other.