by Jim Kitt
As with most electrical systems in all applications, the motorized RC vehicle requires a device that regulates current in order to provide a safe flow of electrons from the source, to the load. The Electronic Speed Controller is used to provide the correct current to the motor and to the receiver. The ESC’s we typically buy comes with a built-in Battery Elimination Circuit (BEC). This internal device provides voltage to the receiver, and therefore the servos, from the batteries that are powering the motor. This means that a separate receiver battery is not required if they are combined in the ESC.
The BEC will typically have programming logic monitoring the battery pack voltage, and as a precaution, it will channel power to the receiver and control surfaces and cut off the motor power when voltage is too low to power both. This is known as the LVC, or low voltage cutoff.
In most manufactured speed controllers, there is a limit to the amount of voltage where an internal BEC can be applied, and this is typically 6 cells, or 22.2 volts. If you need more than 6 cells to power your aircraft, you have two options: you will either use a high voltage ESC in tandem with a separate, external BEC, or use a separate battery to power your receiver.
ESC’s that require an external BEC will contain an OPTO switching mechanism to prevent high voltages, or rapidly changing voltages from the battery from damaging the motor.
If using a separate battery, the battery is connected to the battery port of your receiver and the ESC is connected to the throttle port. If a separate external BEC is used, the BEC is connected between the battery and the speed controller and connects to the battery port of your receiver. The ESC is then connected to the throttle port.
It may also be your preference to include a separate BEC on smaller planes for a few reasons. I have found that many ESC’s do not provide 6 volts to the receiver, which means my servos are operating on only 4.8 volts and not reaching their true potential for speed and torque. Because of this, I use an external BEC on all planes, even those being powered by only three cells. I will typically buy OPTO ESC’s and add a $16 BEC to the setup.
I can also alter the wiring of my ESC that already contains an internal BEC in order to accommodate an external BEC that will provide the 6 volts. By removing the power wire from the connector that attaches to my receiver – which is usually the middle wire of the three going to the connector – I will disable the internal BEC, which is very important in order to keep the receiver from getting power from two separate sources. This is easy to do, but obviously necessary. The connector from the ESC is then plugged back into the receiver’s throttle port, and the external BEC is connected to the battery port.
With the external BEC attached, the circuit should be set up in this configuration: the battery is connected to the BEC, the BEC is connected to the ESC with the BEC connector running to the battery port of the receiver, and the ESC is connected to motor with the ESC connector running to the throttle port of the receiver.
There are really good BEC’s out there for under $20.
For bigger planes that can manage the additional weight, I truly recommend that you use a separate receiver battery. The newer A123 Systems, 6.6 volt, 2 cell, 1100mAh nanophosphate batteries weight only 3.3 ounces, and they extended the time between charges considerably compared to NiMH and NiCd batteries. They use safer battery chemistry than a LiPo, they eliminate the need for a regulator, they don’t self-discharge, and they provide a battery that you can safely recharge in about 5 to 7 minutes. Most of them now come without balancing plugs since most users are becoming comfortable with the fact that these batteries really don’t need to be balanced until they are old enough that you may as well discard and replace them after 1,000 cycles
Internal Resistance and Ohm's Law:
In the circuit defined above, the voltage coming from the battery will decrease linearly as current increases. The voltage reduction is, for the most part, related to the internal resistance of the wiring and cell construction and configuration. So when the volts go through a circuit, some of the energy is converted to heat because of the resistance, and our system will get warm or hot depending on how well, or how efficiently, we combined our motor, ESC, number of battery cells, and prop size, and how low or high the discharge rate is of our battery.
The bottom line here is that if we have a decent battery with a decent discharge rate, and we matched the Amp rating of our ESC to the motor and number of battery cells, we should get less of a voltage drop when we go to full throttle then if we did not configure the setup properly.
Here is a real-world example of something that happened to me, why it happened, and how you can make sure it is not happening to you. It is simple to check if you have a simple Watts-Up meter or the Turnigy Watt Meter and Power Analyzer.
My new motor setup on a new plane was cutting out about two minutes into the flight. When I checked for heat, I found the ESC was on the hot side, and so was the back plate on the motor. I used my power analyzer and found that the voltage on a fully charged 4 cell battery under load was only 12.8 volts. My Amps reading was 53A with 640 Watts. Since the fully charged battery under no load was reading 16.6 volts, I figured that something was wrong. The first thing I did was check the circuit to make sure everything was tightly connected and secure, but everything was fine. After getting the same results with another battery, the next step in my analysis process was to do some math.
My motor was 966 Kv from a top-of-the-line brand name. My battery was a 4 cell, 3000mAh, and the same size motor from the same manufacturer was turning a 13x6.5e APC prop on 3 cells in another plane. For this new plane, which was slightly heavier, I added another cell to increase the volts, and decreased the prop size to decrease the resistance. My initial calculations suggested that the 53 peak Amps in the 3 cell setup may actually diminish because of the increase in voltage and decrease in prop size. Because of this, I thought the 60A ESC would be sufficient. Considering my Amp reading during my testing was peaking at 53A at wide-open-throttle (WOT), I was not really sure what was causing the issue and began thinking that one of the components was faulty. But since everything was working, something must be wrong with the setup configuration.
The next thing I did to try to find the problem was to consult the manufacturer. As the old saying goes, “if all else fails, read the instructions.” I went to their Web site and one of the first things I read was that they recommended the 80A ESC for this configuration. Hmmmm... If I increased the voltage and reduced the prop size, I should not be significantly increasing the Amps. How can I possibly be wrong on this?
My original setup on 3 cells generated 533 Watts and 48 Amps in the smaller plane. This means that since Watts = Amps x volts, my voltage was 11.10 volts total, or 3.70 volts per cell, under WOT load.
My 4 cells setup was generating 680 Watts and 53 Amps. This means that my voltage was 12.8 volts total, or 3.20 volts per cell, under WOT load.
680 Watts divided by 53 Amps = 12.8 volts
12.8 volts divided by 4 cells = 3.2 volts per cell
Okay, red flag. The difference between the 3.70 volts and the 3.20 volts is significant enough to raise a question.
If I was getting 3.70 volts per cell on the first setup that ran fine, the drop in voltage could be the evidence of a problem. Any loss of current will mean that the voltage must be converted to heat since energy can’t just simply disappear. To test this, I used Ohm’s Law.
From my original setup’s readings, I can calculate that 11.1v divided by 48 Amps equals 0.23125 Ohms. This is the representative value of internal resistance.
3.7 volts per cell minus 3.2 volts per cell equals 0.5 volts per cell, which means that my 4 cell setup can be losing 2 volts to heat somehow.
If voltage = Amps x Ohms, then Amps equals volts divided by Ohms:
Amp = 2.0 volts / . 0.23125 Ohms
This means that 8.65 Amps is potentially being converted to heat instead of Watts from my motor. To get a feel for just how much heat this is, multiply the 8.65 Amps times the 12.8 volts and you will get 110.7 Watts. Now imagine how much heat is given off by a 110 Watt light bulb.
This tells me that my power system setup is severely inefficient, and by finding the problem, I should increase the amount of volts going to the system, and therefore the Watts, or power, being generated. If I was getting 680 Watts already, I’m now very curious how much power I am missing from this 4 cell setup.
A few days later, and I had a new 80 Amp ESC, and it was applied to the system immediately, along with my power analyzer. The results were eye-popping.
63 Amps
14.9 volts
940 Watts
The missing element to my calculations was the fact that I had not considered the increase in Amps when adding another cell. I took for granted that the increase in volts would keep the Amps manageable, but I was simply wrong.
The result in increasing the Amp capacity of my ESC was the conservation of my volts per cell, the increase in Amps, and therefore a huge increase in Watts.
Original 3 cell setup:
3.7 volts times 3 cell = 11.1 volts
11.1 volts times ~48 Amps = ~533 Watts
60 Amp ESC on the second setup:
3.2 volts times 4 cell = 12.8 volts
12.8 volts times ~53 Amps = ~678 Watts
80 Amp ESC on the second setup:
3.725 volts times 4 cell = 14.9 volts
14.9 volts times ~63 Amps = ~940 Watts
This also means that I was losing almost 28% of my true power to heat by not using the correct ESC with the number of cells, and the prop size. The [1] motor, [2] speed controller, [3] battery cells, [4] prop diameter, and [5] prop pitch, must all be correct in order to minimize the loss of power to heat and provide a highly efficient power system.
This small episode in my electric conversion experience has taught me more about the interrelationship between these five elements in my power system setup than any single event to-date.
I hope this experience can help you too, so keep it in the back of your mind in case the day comes when you feel power is being lost to heat, instead of going to Watts from your motor. Stay cool, and stay powerful.