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so i see the 5V power pin of BECs all 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 from 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? lol

i have 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 dont have 6 pins 5V input pins on the microcontroller as you would in a commercial flight controller (i think)

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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.

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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.

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