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#RC Airplane Series – 5
In this article, we are going to learn about the correct battery selection for our RC Airplane Project. This step is very crucial as the battery is going to be serving as the powerhouse for the entire set of electronic components on the plane.
I will be covering some examples as well for your clarity. Let’s directly dive into the topic !
Topics Covered :
- Why choose Lipo ?
- Battery specifications (Voltage, Capacity and ‘C’ rating/discharge rate)
- Flight time calculations
- Examples (to summarize)
1. Why choose Lipo Battery?
Among so many batteries, when it comes to the electronic projects (especially RC aircrafts), we directly choose Lipo over others.
LiPo stands for Lithium Polymer batteries. These are known for their ‘High Energy Density‘.
By comparing the definitions, we can see that, if more mass is accumulated in lesser volume, we term that substance as highly dense substance. Similarly, if given energy can be stored in a lesser volume, we term the substance as the one having ‘high energy density’ !
2. Battery Specifications :
When it comes to LiPo batteries, we have 3 main parameters which we need to check in order to select the right LiPo for our project. Those are :
- Number of cells (Voltage of the battery (in volts))
- Capacity (in mAh)
- ‘C’ rating/discharge rate
Voltage of LiPo battery :
A battery is a combination of cells. So basically to calculate the total voltage of the battery, we need to know the voltage of a single cell.
Thumbrule :
- The volatge of the cell in Lipo battery should not go below 3.3V and also should not cross 4V as in both cases it might damage the cell and hence the battery
- So, we decide a term called ‘Nominal Voltage‘. This is basically an approx. average value of the max and min voltages. In case of LiPo, we take it as 3.7 V. For batteries, we always consider the nominal voltage
S -> represents the number of cells
Based on the S number, we calculate the total voltage of battery.
For example, the above battery is ‘3S’ which implies that it has 3 cells in it. Therefore,
Suppose, we select a model for our BLDC motor :
DYS D2826-10 1400KV Outrunner Brushless Motor
Now, we need to correctly choose our motor based on the motor suggested specifications or check datasheet
from Robu.in
Now, selecting 2s LiPo or 3s LiPo depends on your model and requirements. The more voltage you apply, the more RPM you will get for the same given motor by the relation :
Capacity of Battery :
The Capacity of Battery gives you an idea of the time in which the battery will get drained off. The unit of Capacity for LiPo is ‘mAh’. It stands for milli-Amp hours.
We try to understand the same with an example. Suppose I have a battery of 4200 mAh.
- It implies that my battery will get drained off completely if I keep drawing 4.2 A from the battery continuously for 1hr.
- Now, from the same battery, if I draw only 2.1 A (less than 4.2 A), then the battery will drain off after 2 hrs ; giving me more usage time.
- Pretty Obvious that if you draw lesser current, the battery will allow more usage time !!
For RC Airplanes, Capacity plays an important role for determining the flight time (time for which the plane will fly).
We will discuss about Flight time in coming section below
'C' Rating / Discharge Rate :
This thing is nothing but a simple multiplier. It is used to know the actual strength of our battery. This value helps us to calculate the maximum current (continuous and burst) which the battery can provide safely !!
Formulation :
Some times, we need some more current than the maximum continuous current value as well. So in that case, the burst continuous current comes into picture. It shows that the battery can provide some extra amount of current as well if required though only for a short interval of time.
Usually, on batteries, only continuous discharge rate is given. For burst rate, we need to check the battery specifications on websites
For example,
My battery has specifications 2200 mAh, 11.1V and I want to decide the appropriate C rating to be chosen. I also know that the maximum current requirement for all my electronic components (majorly motor) is 20 A.
Solution :
So, I need a battery which must have the strength/ability to continuously providing 20A (though its not always needed). By the above formula,
We get, 20 = 2.2 * (C rating)
C rating = 9.1 C
So now, anything more than 9.1 C (like 10C, 15C, etc) is absolutely fine BUT less than 9.1 C is NOT OK !!
3. Flight Time Calculations :
A very important concept and is crucial especially for competitions were time constraints are there. As discussed in Capacity sub-section above, this concept is a lot dependent on the Capacity of battery.
Note the step-by-step procedure :
After completing all the steps, we get a flight time value. But once this flight time is completed for an RC plane, the battery will be completely drained off !! And WE DON’T WANT THIS !!
- It is always advisable for LiPo batteries to keep atleast 25 % remaining (i.e. use only 75 % of the battery). So if only 75 % of the battery is to be used, then we will obviously get only 75 % of our calculated flight time. This will be our min. actual flight time. It can be more than this but not less, since we calculated this value based on the ‘Max.’ current from motor.
4. Example :
The below document is a short example designed to get you more clarity on the thoery part. Do go through the document once you go through the article completely. Keep both, this article and the example side-by-side and then learn and analyze how it’s done !!
RC Airplane Series – All Articles (You are at Part – 5 !)
- Part-6 (Next) : ESC Calculations and Electronics Connection
- Part-4 (Previous): Motor and Propeller Selection
- Part-3 : Designing your Airplane !!
- Part-2 : Understanding the control surfaces !
- Part-1 : What’s in the Wings ?
Conclusion :
From this article, we got to learn about the procedure to select the correct LiPo battery for our project. In the next upcoming articles, we will cover the ESC calculations and also learn about the thrust test. Till then, Enjoy Learning !!
All the Best !!
