Selecting Multirotor Batteries

by | May 6, 2014 | design, theory | 0 comments

Multirotor aircraft are powered by Lithium Polymer batteries. Before we get to the process of choosing a battery, let’s take a moment to talk about what a Lithium Polymer battery is and why this particular battery technology is used in multirotors.

About Lithium Polymer Batteries

Lithium Polymer (or LiPo) batteries are just one type of battery using one of many common battery chemistries. You are probably familiar with several others: lead-acid batteries are used in cars, lithium-ion batteries are commonly used in laptops, alkaline batteries are the most common and come in sizes like AA, AAA, and C. LiPo batteries have several advantages over these other battery types which make them well-suited for use in multirotors:

  • Lithium Polymer batteries offer the best capacity-to-weight ratio available from commercially-available battery types.
  • Lithium Polymer batteries are capable of very high discharge rates, which is an important requirement for use in multirotor aircraft since the motors require a large amount of current to generate the lift required to fly.
  • Lithium Polymer batteries sustain a more consistent voltage under load that other battery types.

Choosing Multirotor Batteries

To pick a battery for your multirotor, you will need to know two pieces of information: the total current draw of your multirotor’s power system, and the desired flight time. Let’s calculate the total current draw first.

While there are several components on a quadcopter that draw current from the battery, including the flight controller, ESCs, motors, any lights on the craft, et cetera, the motors are responsible for the vast majority of the multirotor’s total energy usage. Therefore, we will just ignore the other components and calculate the maximum total current draw of the quadcopter based on the current required by the four motors when they are at full throttle.

Start by taking a look at the documentation that came with your motors, either from the motor seller or the manufacturer. You should be able to find the maximum current draw for the motors in amps. This is the current the motor will draw at full throttle. To estimate the maximum current draw for the multirotor’s entire power system, multiply the motors’ maximum current draw by four (since there are four motors on a quadcopter).

So now that we know the total current draw of the motor, we will need to decide on our desired flight time. A longer flight time will require a larger capacity battery. This number is completely up to you but there are two things to keep in mind. First, flight times seem longer than you think they will. Five minutes of flying might not sound like a lot, but when you are in the field flying, five minutes will feel much longer. Plus, if you are shooting video during the five minute flight, you will have almost a gigabyte of footage. Second, as battery capacity increases, the weight and cost of LiPo batteries increases extremely quickly.

With the quadcopter maximum total current draw and desired flight time in mind, we can calculate the required battery capacity. First, translate the flight time into hours by dividing the flight time in minutes by sixty. For example, a five minute flight time is equal to a 0.083 hour flight time. Second, multiply the flight time in hours by the total current draw of the quadcopter in amps. This will give you the required battery capacity in amp-hours and it will probably be some number between three and ten. However, when shopping for batteries, you will see the battery capacity advertized in miliamp-hours. To translate from amp-hours to miliamp-hours, multiply the number you calculated earlier by 1000.

Finally, you should have the required capacity of the battery in miliamp-hours. There is just one more note I want to make. In order to keep the weight and cost of your quadcopter down, you might want to consider purchasing two smaller batteries rather than one large one. When you go out in the field, you can simply switch out the batteries and keep flying.