Hardcore RC car enthusiasts are always looking to push the envelope. A big part of doing this is understanding what is happening within the power system. Without this level of understanding it is very easy to get yourself in trouble and that’s when your power system starts smoking. Something to avoid at all costs!
The largest challenge in RC is how can we understand or better predict the amount of current that our cars need? Therefore, we are going to discuss why it is difficult to predict how much current an RC car motor will draw. Also, why is this more difficult than predicting current draw for RC airplanes.
The three main components in an RC Car
To demystify the mechanics behind the current pulled, let’s introduce power system components and their respective specifications. This will serve as an example for our RC car.
LiPo Battery
First off, the LiPo battery (lithium-polymer battery) has its specifications typically marked on the front of the battery. In our example consider the specification of 77A (2.2Ah x 35C continuous rated). This 77A figure identifies the amount of current that the battery can generate.
Electronic Speed Control
The electronic speed control (ESC) has its rating also marked right on the front of it. In our example, let’s consider a rating of 60 amps. This is the maximum continuous current that it can support.
Brushless Motor
Lastly, the brushless motor has it’s specification accessible from the manufacture’s spec sheet. Consider our example to be able to deliver approximately 75 amps of current.
What do these Specifications Mean?
This is the exact question, what significance do these specs hold in terms of determining how much power this motor can draw?
A clear and concise answer to this: None.
Despite these specifications, we are clueless as to how much power our brushless motor is going to draw.
Not that it is completely impossible to determine how much power brushless motors can draw. We can check that with a dynamometer for measuring mechanical power generated. We can gauge the amount of power that the motors are generating, the current being drawn and the amount of electrical or mechanical power being consumed or given out with the dyno.
With these values, we can depict the amount of current that the motor drew for a specific load of the motor.
Doesn’t this sound so easy? Nope, not at all.
Load on a Brushless Motor
The load refers to the amount of current drawn and is arguably an important part of the system. The actual current that the motor will pull comes from the Io value. (no-load value) Essentially the current drawn at zero load. Simply put, the Io value is a waste of energy since you won’t get any mechanical torque from the motor.
Interestingly, once the motor is loaded, (using a propeller) the current value starts increasing past the Io value. Which is about 2-3 amps for the motor used here. Then we increase the load further by placing a larger propeller onto our motor and skyrocketing the current up to 30-40 amps. Remember that the bigger load is represented by the bigger propeller used.
The point we are trying to make here is that as you increase the load on your motor, without interrupting anything else, the torque output of the motor must increase. The load is quantified as mechanical torque required by the motor. The translation from mechanical to electrical is torque being equal to current drawn. As torque output of the motor increases, the motor will draw an increasing amount of current.
The Difficulty in Determining Torque / Current for RC Cars
If life were simple, we would be easily able to predict the torque output of a motor. But, that’s not how it goes. There are so many factors or variables to consider that make up the torque requirement and thus current draw.
How about an online calculator? Although there are online calculators for RC airplanes or EDF jets, there is no such calculator for an RC car.
Why?
Let’s Discuss Influencing Factors
Because there are more factors involved in this type of situation than one can imagine. For instance, consider placing a brushless motor in an RC car and drive it at constant speed. There would be a significant difference in the power required to drive at constant speed vs accelerating up to that speed. Even if the RC enthusiast were to accelerate lightly vs aggressively, there would be a considerable difference in the amount of current being pulled[RT1] .
Similarly, if you opt for a larger tire, the same effect would be noticed. When we alter the RC car, by increasing load, we are asking the motor do more work. Even if we don’t alter the RC car, and drive it on grass, we would notice that the grass would cause the motor to load up. This is due to the friction between the tires and surface, causing the torque to rise.
Another example that can influence torque required is a strong headwind or tailwind. The greater the headwind speeds, the greater the amount of power required to penetrate the wind. The amount of current drawn is very difficult to estimate.
After all of this where do we go from here?
There are a couple means of determining how much current your system will draw. The best way is to place a datalogger within your RC Car and record the amount of current your system pulls. The big down side to this, is of course, it’s not a prediction. You at this point would already have all the components.
To make the best prediction, we recommend doing a bit of research on the internet. There are more than likely many RC enthusiasts out there with the same RC vehicle as you and based on their power system and results, you can gather data and expectations. If you can’t find someone who has the same power system as you, choose the next best thing. That is the most similar power system you can find.
Key Takeaway
First, the specifications of your RC car’s motor won’t tell you anything about how much current it will pull.
Secondly, the load that you place on your motor, determines how much power or current the motor will essentially draw. Thirdly, the load equates to the amount of mechanical torque required by the RC car.
Lastly, it is possible to predict the current drawn by utilizing someone else’s experience. Find another system out there that most closely matches the one you plan to use.
We hope this was an informative guide for you to understand these challenges and how to better overcome them. Be sure to check out the YouTube video here for more detail.