THE ROLE OF THE BEC IN YOUR ELECTRIC PLANE
by Ed Anderson
aeajr on the forums
Updated 1/24/15

In the world of electric motors the electronic speed control, ESC, takes the
place of the throttle servo used on fuel powered planes. It regulates the speed of the
motor by pulsing the power to the motor to achieve the desired motor speed.
However most ESCs also have two other functions, the LVC and the BEC.

The LVC, low-voltage-cutoff circuit, will cut power to the motor and preserve
power to the radio system so you can land your plane safely when the motor
battery is getting too low. In the case of lithium batteries, the LVC, can also
save your battery packs by preventing them from getting too low. If you started
with NiXX packs and have switched to lithium packs, be sure your LVC is set
properly or you could damage your lithium packs.

The BEC, the battery elimination circuit supplies power to the receiver and the
servos. It is the BEC that will be the main focus of this discussion.

The name, battery elimination circuit, comes from the fact that, in the "old
days" of electric planes, you had a battery pack to power the motor and another
one to power the receiver. In order to save weight, the BEC was introduced to
eliminate the need for that receiver battery pack.

BEC, battery elimination circuit, is a generic term that applies to all circuits or
devices, whether in an ESC or as a separate device, that step the
voltage to the desired level. You could also call them voltage regulators.
They take the power from a battery pack and reduce the voltage to the level
desired. For example, an 11.1 V 3S lipo pack gets stepped down to 5V to run
your receiver and servos. Most are fixed but some can be set for the desired
output voltage.

There are two types of BEC in common use, linear and switching.
Whether you do it with a switching or a linear BEC the effect is about the
same. I am not aware of any reason to believe that one is more reliable than
the other or that an external BEC is in any way better than one integrated
into your ESC. The critical issue is the sizing of the BEC to meet the
amperage and voltage needs of your equipment.

It is worth noting that linear BECs are more commonly used with lower
voltage battery packs. That is because the linear BEC uses a resistance
process to drop voltage from, say 11V to 5 V and this generates heat.
Nothing to be concerned about but that is how it works. So once you get past
a 4S lipo or a 12S NiXX pack the step down becomes enough that most
manufacturers go to the switching BEC design. This uses the same type of
switch on/off process that your ESC uses to regulate the speed of your
motor.

There is nothing inherently more or less reliable in either design from a
practical point of view. It is just a matter of the most appropriate device for
the use case. I would have no hesitation to use a linear BEC
on any pack for which it is rated. Nor do I have a big preference for
external vs. internal BECs.

One thing to note is that linear BECs are rated for output based on input
voltage. So, a linear BEC might be rated for 3 amps output when used on a
2S pack but only 2 amps output when used on a 3S pack. Again, that is related
to the resistance method. The higher voltage drop generates more heat so
they derate the device for safety. But if you only need 1 amp, who cares?

The biggest issue we face when talking about BECs is that we really don't
know what we need in amperage. Do you know how many amps any given servo
draws? Did you know that the number goes up when the servo is under load?
And, of course, the total amp load goes up if you are moving more than one
servo. And a stuck servo's amp draw can go very high.
Typically we evaluate the size of be BEC based on the number of servos,
what has worked in other planes or what the MFG recommends.

As an example, the Radian Pro Bnf has 6 micro servos and a Spektrum receiver.
It was originally shipped with a BEC rated at 750 mah. Now, most people would tell you that
that is not a large enough BEC for 6 micro servos. But there were a LOT of
Radian Pros shipped with them and most flew just fine. Later they shifted
over to a larger BEC, 1.5 amps I think, to provide a greater margin for safety.

Note that the voltage rating for the ESC may be different than the voltage
rating for the BEC. Your ESC may be rated for 6S/22.2V but the BEC may have
to be disabled over 12 volts and you will have to power the receiver separately.
If you don't take note of this and pop in a 6S lipo, your ESC may be fine
but your BEC may be heading for a failure, resulting in a crash.

If you are flying an RTF or "receiver ready" model, you can be confident that the
BEC chosen is appropriate when used with the recommended battery pack. As
an example, the manufacturer of the plane may
designate that the plane takes an 8.4V pack. At that voltage the included BEC
may be fine. However, if you decide to pop in a three cell lipo, a problem may
only be a launch away. The BEC may do fine for a couple of flights, or maybe 5
minutes or may fail 100 feet out, and down you go.

We also have the variable of which servos are being used. Different servos draw
different amounts of current. If the current draw gets too high, the BEC will
overload causing a shutdown of the BEC. This protects the BEC and
prevents a fire, but cuts the voltage to the receiver. The net effect is that
you lose all power to the radio system and you lose control of the plane.

In the case of an overheated BEC, if there is enough cooling air going through
the plane, the BEC may come back quickly as it cools. This could look like a
radio glitch, but it could be the BEC operating on the edge of total failure.
If your ESC is very hot when you land, the cause could be the BEC operating at
the edge of its capacity. When we see these glitches, we often think the
problem is the radio system, but in fact the cause is power to the receiver.

When we were switching from 72 MHz radios to 2.4 GHz radios a lot of people thought
their 2.4 GHz receivers were failing but what was actually happening was that the
2.4 GHz receivers pulled more power, more amps, which overloaded the BEC in
the plane. If the BEC was just adequate for the 72 MHz receiver, which may have
only needed 20 mA and you put in a receiver that needed 100 mA then a BEC that
was just adequate for the 72 MHz receiver could cut out with the 2.4 GHz receiver.
We are more aware of this now and this has become less and less of a problem.

A CASE STUDY

This pilot was flying a new Spektrum 2.4 GHz system. All was fine till the plane
suddenly went dead and crashed. All sorts of speculation were offered about what
the cause could be and much of it was focused on the Spektrum 2.4 GHz system.
After the plane was recovered, everything seemed to work OK so it must have been
a radio hit, right? However, due to the diligent work of the pilot, it was
determined that the BEC had failed due to overload. You can read the actual
account at this link in posts 2986 to 3006.
http://www.rcgroups.com/forums/showt...04621&page=200

This is not the only account of this type that has been reported, but this was
one that was worked out over a short time with a very clear outcome. Note also
that the pilot had to run his test for several minutes before the failure
appeared. Thus, everything seemed fine at first; it seemed that the BEC was
handling the load. But over several minutes' heat built up in the BEC. Combine
this with the heat from the motor and the battery and, perhaps not enough
cooling airflow and the BEC shut down.

BE COOL FOOL!

With good airflow a BEC overload may be avoided. Regardless of what radio
system you are using, make sure you have enough cooling air going through your
electric plane. This is especially true of foam planes as the foam acts as an
insulator. You may have a cooling air vent in the front somewhere, but the heat
can't get out unless there is an exit air hole large enough to allow good
airflow. If you are pushing the limit on any part of your power or radio system,
not enough cooling air can cause damage or failure to your motor, ESC, BEC or
battery packs.

How you fly your plane can also cause heat build-up. For example, an Easy
Glider that has the motor run 1 minute to get to altitude then glide might have enough airflow
to eliminate the built up heat. But if you run the motor constantly for 10 minutes,
the heat build up could be enough to cook your BEC, your battery pack, or some
other part of the plane.

Be cool fool, and make sure you have enough airflow in your plane. If your
battery is very hot, or if your ESC is very hot, you may need more cooling.


OTHER CAUSES OF BEC PROBLEMS

You could be configured properly. Your BEC may be rated to handle your servo
count and you could have plenty of cooling air but still have problems. If you
have a servo push rod that is dragging or is otherwise placing a high load on
the servo, this can increase the amp draw of that servo. If that servo gets
stuck, the amp draw will go way up!

Servo loads are expected to be variable. A servo will move, put a load on the
BEC then come back to neutral and the current draw will drop. In between loads,
the BEC has a chance to cool. However a jammed servo will draw a lot of power
and that draw will be constant. You can see why it is very important that your
servos move freely, without binding. Check those control rods for kinks,
obstructions or things that could get in the way.


ENTER THE COMPUTER RADIO

In the past it was common to have 2 ailerons run off of one servo, so three
servos were typical of a 4-channel electric plane. With more and more people
using computer radios, there is a tendency to put 2 servos on the ailerons
meaning more load on the BEC.

Also, with a computer radio it is easy to add a little aileron to rudder mixing,
moving 3 servos at once. Now add a little up elevator in the turns and all four
servos are pulling power. Go to a full house electric sailplane, with flaps
following ailerons, rudder mixed in and a little up elevator in the turn and you
now have 6 servos, all moving at once. We begin to see how the BEC can become
challenged to keep up.


WHAT IF YOU NEED MORE?

If you need more power than the integrated BEC in your ESC can supply, or if
your motor battery voltage is higher than the BEC can handle, you will need to
disable the integrated BEC and put in a separate receiver pack or a separate
BEC. Many companies make after market BECs that can handle these higher
voltages or higher servo loads.

Remember there are two different types of BECs. Both work but they work
differently. Most, but not all aftermarket BECs seem to be switching BECs,
but be sure to read the instructions. If the amp output is different based on
battery voltage then it is a linear BEC. Nothing wrong with that, just be aware
of the rating for the voltage battery pack you plan to use.

Regardless of what type you have, follow the instructions carefully or risk
losing your plane. And be sure to provide plenty of cooling air.

Listed below are some examples of after market BECs.


Dimension Engineering has several BECs
http://www.dimensionengineering.com

The SMART BEC - Combines BEC and LVC that is Lithium aware
http://www.dimensionengineering.com/SmartBEC.htm

SUMMARY

The ESC is the heart of your electric power system. The BEC is the part of the
ESC that powers your radio system. Keep it cool and make sure you read the
instructions so you don't overload it. Forget these tips and you may be
picking up pieces of your plane, wondering what caused that crash.