Understanding the BEC and LVC features of your Electronic Speed Control

[AEAJR]

The Electronic Speed Control, ESC, is something we think of as being the throttle for our electric planes. But it usually incorporates two other functions that are often overlooked or misunderstood by new and experienced pilots alike.

I have written two short articles, posted below, that may help you understand their function and value in your plane. If you don't understand what they are and how they work you could damage your batteries or end up in a crash and have no idea why.

I hope you find these helpful.

Ed Anderson
aeajr on the forums

[AEAJR]

The Low Voltage Cutoff Feature of your ESC
By Ed Anderson
aeajr on the forums

Many electronic speed controls include a feature called the low voltage cutoff
circuit, the LVC. The LVC watches the voltage that is being delivered by the
battery. When it gets below a certain level, it will cut power to the motor to
preserve power for the radio system. This will allow you to keep control of the
plane and land it in a glide.

Power draw by your receiver and servos is a tiny fraction of what the typical
electric motor draws. As the battery drains it will exhibit a voltage drop. You
may feel this in the way the plane flies. The plane may become sluggish or it
may not be able to climb under full power. This is a clear indication that the
pack is getting low.

A battery that can't sustain voltage when the motor is on, can still provide
plenty of power for the flight electronics and may be able to do so for quite a
while, but don't test it. If your motor cuts, enjoy the glide, but set up to
land as soon as possible. I always teach new pilots how to glide their planes
so, if the LVC cuts the motor, they don't panic.

If you practice flying your plane with the motor off, then an LVC cut will be no
big deal. You might even find you enjoy gliding, which can extend your flying
time. I often glide and thermal my electric planes just for fun.

LITHIUM BATTERIES CHANGE THE ROLE OF THE LVC

If you drain NiCd or NiMh packs too low, usually there is little damage. Just
bring them back to charge a little slower than normal. If you drain a lithium
cell below 2.5V resting voltage, typically the cell will be damaged. So, in
this case the LVC is protecting your plane and your battery packs.

Most lithium friendly ESC will cut the motor off if the pack voltage drops below
2.7 to 3.0V per cell under load. At this level there is very little useful
charge left in the pack and the voltage will continue to drop fast.

Note that when you cut the load of the motor the voltage will likely pop back to
3.1, 3.2 or even 3.3 V per cell. If you check your batteries after you land,
you may think that LVC has malfunctioned, but it has not. The battery may be
3.3 V/cell resting but it can't sustain it with the motor running.

One thing you might want to be aware of is that the voltage sag will be less at
lower throttle settings. If the LVC cuts the power at a particularly bad time,
you may be able to get a short burst of motor operation at a reduced throttle
setting. A short run at half or quarter throttle may be all you need to get you
over that fence, past that tree or properly aligned with runway. But don't push
it by trying to extend your flight with lots of short bursts. However if it
will help you avoid a crash, or two short runs, to save the plane, are worth the
risk to the battery pack.

CONCLUSION

The LVC was put there to protect the radio, but if you are using Lithium
batteries the LVC can protect them too. It is best to be sure your ESC/LVC is
lithium friendly. That means either that it can be set manually, or that it
senses how many lithium cells you have and sets automatically. Even if it is
not designed for Lithium cells, if you can set the cut-off at something above
2.75V per lithium cell, then you should be OK.

Understanding how the LVC works will make it your friend.

OTHER RESOURCES

Sizing Electric Power Systems
http://www.rccyberflyer.com/forum/showthread.php?t=272

[AEAJR]

THE ROLE OF THE BEC IN YOUR ELECTRIC PLANE
by Ed Anderson
aeajr on the forums

In the world of electric motors the electronic speed control, ESC, takes the
place of the throttle 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.

In most of our radio systems, the receiver is designed to operate between 4 and
6 volts. To match this, the typical BEC supplies power to the receiver at about
5 volts by stepping down the motor battery voltage. However the higher the
voltage of the motor battery, the harder the BEC has to work to get the voltage
down to 5 volts. In doing this work the BEC generates heat. The greater the
voltage reduction, the more heat the BEC generates. As a result most BECs have
to be disabled if the motor battery pack is over a certain voltage. ESCs that
are designed specifically for high voltage use often do not have an integrated
BEC.

BECs are also rated by how many amps they can deliver to the receiver. The
greater the number of servos installed the greater the amperage the BEC must
deliver and the more heat it generates in the process. However, with most
integrated BECs, the higher the voltage of the motor battery pack the lower the
amperage the BEC can deliver. This is often where problems occur. It is this
heat load that leads to the need for a compromise as to how many servos an
integrated BEC can support.

For example, if the motor pack is 8.4V, then a given BEC might be able to
support 4 servos. If the motor battery voltage is higher, say 11 volts, then
the same BEC may need to be derated to handle only 3 servos. Since more heat
will be generated by the larger step down from 11 volts to 5 volts, the amp load
has to be reduced or the BEC will overheat.

Note that the voltage rating for the ESC may be different than the voltage
rating for the BEC. Your ESC may be rated for 14.8 volts 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 four-cell lipo, your ESC may be fine
but your BEC may be heading for a failure, resulting in a crash.

According to Dimension Engineering, a maker of BECs, "Many people don't realize
that their ESC's BEC rating is misleading. With the linear BEC built into most
speed controls, the current rating decreases as pack voltage increases. For
example, several popular 25A ESCs with "3A" BECs are only capable of supplying
0.5A when running from a 3s pack".

If you are flying an RTF or "receiver ready" model, there may not be ESC/BEC
documentation included. 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
get too hot causing a thermal 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 could be the BEC.

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/showthread.php?t=604621&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. The receiver could overheat or you could cook your servos.

How you fly your plane can also cause heat build-up. For example, an Easy
Glider that is flown for 1 minute to get to altitude might have enough airflow
to eliminate the built up heat. But if you fly it 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.

Note that there are different kinds of circuits that are used to create the BEC
function. There are linear BECs, which seem to be primarily what is found
integrated in with the ESC. These seem to be low cost but are more affected by
the voltage of the motor pack. Then there are switch mode BECs that seem to
tolerate these higher pack voltages better than the linear BECs. It appears
many of the after market BECs are of this type.

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.

The Ultimate BEC
http://www.hobby-lobby.com/ubec.htm

Novak 3 amp BEC
http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXSGC1&P=7

For very large servo counts - 6 amps
http://www.horizonhobby.com/Products/Default.aspx?ProdID=SQBDBEC008V

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.

[Grasshopper]

Very informative Ed. Great post as always.

Thanks.

[marchino61]

I have an E-Flite 20A brushed ESC which I have fitted to my Super Cub along with a 4 channel Rx, using the stock motor (the ESC says it can be used with motors only up to 400, but presumably it is OK with the Cub as the motor is geared down).

The leaflet that comes with it says you can use 5-cell to 10-cell NiMH batteries, and that the ESC has a low-voltage cut-off. There is no indication that this is programmable.

My question: what is the LVC of this ESC? How does it adapt to batteries of differing voltage?

[sherwi@sbcglobal.net]

Very good and informative post Ed. Thanks for sharing with me and others that need this kind of information.

Sherrill

[AEAJR]

Originally Posted by marchino61

I have an E-Flite 20A brushed ESC which I have fitted to my Super Cub along with a 4 channel Rx, using the stock motor (the ESC says it can be used with motors only up to 400, but presumably it is OK with the Cub as the motor is geared down).

The leaflet that comes with it says you can use 5-cell to 10-cell NiMH batteries, and that the ESC has a low-voltage cut-off. There is no indication that this is programmable.

My question: what is the LVC of this ESC? How does it adapt to batteries of differing voltage?

Is this your ESC?
http://www.horizonhobby.com/Products...ProdID=EFLA105

If so, this is the manual:
http://www.horizonhobby.com/ProdInfo...105-manual.pdf

This is a good example of an ESC that comes with very limited instructions.

1) It says the BEC supplies 3.6V at 1 amp. That is scary low voltage to the receiver. I would be worried about this BEC. A small voltage drop could result in a loss of radio contact. It also provides no guidance about how many servos that can be supported. I think I would limit this one to 2 servos and nothing bigger than a speed 400 just because the specs concern me.

I presume it can maintian BEC operation all the way up to 12 volts.

2) The LVC operation is not explained at all. I have no idea how it decides on cut-off point. It is CLEARLY not set-up for Lipos. Using this with Lipos is likely to lead to damage of the battery pack.

I would suggest you contact the maker. Your manual has the contact for warranty service as Horizon Hobby.

[oimmuk]

This has been a concern of mine in that the sharing of power from the batteries between the RX and Motor, when the LVC kicks in and shuts the motor down how much flight time would you have with a LIPO pack before damage occours. Will the BEC channel the current you need to the RX without damaging the LIPO's? If so how much glide time do you have? Will it be enough to land safely? I read about people "Puffing" a pack and that is a not too unsubstantial amount of cash to play around with.

I have yet to dive in and buy equipment , not till I understand mre about it. Truth be told I am leaning toward seperate packs for control and motor, I know this would increase weight and cost in power vs weight concerns, but I would rther let the ESC shut down the motor safe with the knowledge that I might have yet another hour or two on the RX/Servo pack. Cal me a worry wart but I would rather be safe than sorry. Keep the info comeing folks I am learning from all of you.

[AEAJR]

Let's suppose you have a electric plane flying a 2 cell lipo, 7.4V and 1300 mah capacity. BTW the fully charge resting voltage level is 8.4V on a 2 cell Lipo. Under load the voltage sags. 7.4V is an approximation of the sag voltage. Naturally that will vary with much of a load is put on the pack.

You run the motor at full power. Let's assume the motor pulls between 10 and 15 amps. The LVC kicks in at 3V per cell or 6V. This is the voltage under load which will be lower than the resting voltage.

When the LVC cuts the power to the motor, the pressure is off the pack, and the voltage starts to rise. It might make it all the way back to 6.4 or higher with only the receiver and servos to support.

Now, how much does your receiver and servos draw? If you have 3 micro servos, you might be pulling 300-600 miliamps. That is about 1/25 the load that the motor applies.

How much time do you have to land? Well I would not recomend you find out, but you might have 10 minutes of glide time. Or it might be only 5 minutes, but it is not 15 seconds.

At a 600 ma load, 5 minutes is only going to draw about 50 MAh from the pack. That is 1/26 its capacity. When the LVC kicks in you probably have 20% of the pack's capacity left.

The point is not that you should glide for 5 minutes, you should land ASAP, but don't panic, you have time to do it right. That pack can stay well above any danger level for quite a while, but land right away anyway. Don't push it.

BTW, the story would be similar for NiXX packs. The only difference is that NiXX will not be severely damaged if you run them down really low. Your receiver might cut out, but the battery will not be seriously damaged even if you were to run it flat. After the BEC cuts the motor, you will have plenty of time to glide in for a landing.

Hope that helps.

[oimmuk]

It does, and Thanks.

[AEAJR]

Let's bring this one back to the top one more time.