ElectroFlightRC - Basics of Electric Flight

Battery Eliminator Circuits (BEC's)

Table of Contents

Powering Your Receiver and Servos

SBEC (or switching bec)

LBEC (or linear BEC, sometimes just called a BEC)

External BEC, UBEC

Internal BEC

Powering Your Receiver and Servos

Traditional Method:

No I’m not talking about making sparkling wine in the traditional method of Champagne. With electric planes, you have the option of powering your receiver and servos the same way as for traditional glow and gas planes. I will not go into detail on this subject as there is lots of information out there and lots of RC hobbyists who know this stuff. Lets focus more on the options available to us with electric planes.

BEC:

This acronym stands for Battery Eliminator Circuit. This is essentially the same thing as a regulator, but you can run it off your main battery pack - because it can handle higher voltages than traditional regulators. You can still run the BEC off a separate battery pack if you like. That can give you extra safety in case something goes wrong with your main battery pack - failure in either battery pack is just as likely and a main battery pack should be just fine unless you push them too hard or do not maintain them properly. See the section on batteries for more info on this subject.

Typically, a BEC is used for weight reduction. With a BEC, there is no need for a separate battery (and potentially a regulator) to power your receiver and servos. Another useful benefit is that you don’t have to remember to charge your receiver packs (and probably lose power to your receiver/servos if the receiver pack gets drained too far down and you forget to charge it after a few flights).

Some ESC’s have built in BEC’s. You have the option of using the built-in BEC or you can still use a separate BEC. Personally, I prefer to use an external BEC even if my ESC has one built-in. I just like the extra level of safety in case my ESC gets fried - in that case I would still have power to my receiver and servos.

Just one thing to keep in mind if you are going to use an external BEC with an ESC that has a built-in BEC. You will either have to disable the built-in BEC on the ESC - if the ESC has that programming option - or you can remove the red wire (positive) from the cable that connects the ESC to the receiver. That red wire is the positive wire which provides power from the ESC to the receiver if you use the built-in BEC. All I do in the latter case is carefully pull up the retaining plastic pin on the little black connector to remove the red wire - leaving only the black wire (ground) and the white wire (signal) in the connector. I then fold the red wire back and tape it to the cable using electric tape for insulation. This is better than cutting the red wire because some ESC’s require all three wires in the connector in order to connect the ESC to the programming card or USB cable connector. So if you want to program the ESC, all you have to do is re-insert the red wire in the connector and then remove it again once you are done with the programming.

BEC Ratings:

Input voltage range:

This represents the voltage range the BEC can accept on the input side. This is important to consider if you are going to connect the BEC to your main battery pack. Sometimes, this is represented as a cell count (i.e. 3s to 6s) instead of a voltage range. So that would be 11.1V (3s x 3.7V) to 22.2V (6s x 3.7V) in the example.

Output voltage range:

This represents the voltage range the BEC is able to output. Almost all BEC’s will be able to output 5V (or close to that like 4.8v) and some will have the ability to also go higher (typically up to 6V - or 5.9V). How the output voltage is set varies depending on the ESC. Some of them have a simple jumper that can be used to set the output voltage to a number of preset output voltages (i.e. 5V or 6V) and others can be programmed using a programming card or USB cable (with PC software) and the voltage can be changed in 0.2V increments (within a min and max voltage). Read the instructions that come with your BEC to find out if it supports different output voltage settings and if so, how to change it.

Output Amp range:

This represents the Amps the BEC is able to output to the receiver and servos. The manufacturer will normally specify a constant Amp draw rating and a separate peek (or surge) Amp draw rating. For example: 5-7.5A means 5A maximum constant Amp draw and up to 7.5A surge Amp draw (usually for a short period like 5 to 15 seconds max). Beyond that, the BEC could burn up and you could lose power to your receiver and servos (brown out). Be sure to read the instructions that came with your BEC to make sure you understand it’s capabilities.

One thing to be aware of, about certain BEC’s that are built-in to ESC’s. In some cases, the output Amp rating can change depending on the number of cells in the battery pack connected to the ESC/BEC. In some cases, the BEC may not be available for higher numbers of cells even though it is available for less cells. For example, the Castle Creations Phoenix ICE ESC’s have the following BEC Chart:

Again, be sure to read all the information in the instruction manual and make sure you understand what you are doing. If you don’t, ask questions at your RC Club or on online forums.

Types of BEC’s

SBEC (or switching bec):

These work by switching the input voltage off and on very quickly to produce their output voltage. They are more efficient and hold a constant output until the main pack is below a certain threshold. This is the better BEC type to purchase.

LBEC (or linear BEC, sometimes just called a BEC):

These work by using a type of bleeder circuit to produce the output. More heat is produced and the output voltage goes down as the main pack voltage goes down.

External BEC, UBEC (UBEC is a brand name bec):

This is simply a BEC of any type that is self contained and not part of an ESC with built-in BEC. All external BEC's will have greater amp ratings vs. their built-in cousins (or at least they should).

Internal BEC:

This is a BEC that is either switching or linear and built into an ESC, these usually have a limited amp rating.