How-To

How-To Select the proper battery.

Once you’ve got your Frame, motors, ESCs and propellers selected, it’s time to select a battery.

Once again this means we have to do some math; we’ll need to find the amp draw of your system and then select a battery accordingly.

First you’ll need to understand the ratings that LiPo batteries use. These batteries have three important specifications that you need to pay attention to and they are capacity, current rating and voltage.
Capacity is listed in mAh (milliamps/hour), current rating is listed as a C rating, and voltage is listed as a cell number.

We’re going to refer to a Lumenier 4S 1500mAh 75C battery for our example.
Capacity = 1500mAh
Current Rating = 75C Continuous /150C Burst
Voltage  = 4S(4 cells) which is 14.8v nominal, 16.8v full charge.

To calculate the current output we’ll use the following formula:
Current Output = (Capacity/1000) x Current Rating
Current Output = (1500/1000)75
Which means the current output for this battery is 112.5A.
Now that you’ve got your current output we can match this with your system.
If you’ve been following our How-To’s then you’re already familiar with thrust charts. If not you can learn about thrust charts in our How to pick your Propeller article.

Using the thrust chart for your motors, find the current draw for the propellers you’re using.
Now that you’ve got your current draw, you multiply your current draw by the number of motors and this will be your current draw for your system. Keep in mind you will not be flying at max throttle all the time, you may want to use the current measurements for the 50-70% throttle range your battery selection calculations. Just be sure the max current is well within your burst C rating and ideally it should be only slightly over your continuous C rating if at all.

Ideally your system should only use about 80-90% of your battery’s available current during flight.

For Example, Lets say we’re building a QAV-R (5″) using the Lumenier Rx2204-14 2300KV motors and Gemfan 5×4.5 Glass Fiber props.

The Max current draw is 17.37A per motor and we have four motors.
This means our Max current for our system will be about 70A.
If we use a 1300mAh 4S 60C we’re going to right within range.
(1300/1000)60=78A
70A is almost 90% of  78A

Which means the Lumenier 1300mAh 4S 60C will be a great option for this setup since the max is below the continuous output rating.

For information about selecting your propellers, click here.
For information about selecting your motors and ESCs, click here.