Nowadays the vast majority of electric-powered RC devices operate with LiPo batteries. This technique, familiar from eg. mobile phones, has revolutionized the RC car's, plane's and copter's operating times as well as the power that they can provide.
LiPo batteries are made up of individual battery cells, which are connected in series or paraller depending on the application. The rated standard voltage of one single cell of a battery is 3.7 volts. In this case the term is 1S battery (one in series). The battery in the photo is a 3S battery, so it has three battery cells connected in series to each other, and because voltage increases when connecting power sources in series, the rated voltage of this battery is 3 x 3.7 volt = 11.1 volts. This is the battery voltage and type that is most commonly used in multi copters. The suitable battery for your chopper is determined by the specs of the motors. This value must not be exceeded!
When selecting the battery the next important thing is capacity. The example battery have a marking of 2.2Ah, in other words it is a 2200 mAh battery. This means that with one ampere load the battery would keep a steady voltage for bit over of two hours. For multicopters, the flight times are usually about 13-15 minutes and as it happens, the 2200mAh battery is a very good (and propably the most common) choice for a normal sized tri- or quadcopter.
The mathematics didn’t end just yet. When dealing with LiPo batteries, there are couple of different C values that you need to understand. These are the listed technical specifications of our example battery:
- Minimum Capacity: 2200mAh
- Configuration: 3S1P / 11.1v / 3Cell
- Constant Discharge: 40C
- Peak Discharge (10sec): 50C
- Pack Weight: 204g
- Pack Size: 104 x 27 x 35mm
- Charge Plug: JST-XH
- Discharge plug: XT60
Constant Discharge is reported to be 40C. This number tells you how much power the battery is able to give out when the motor requests it. This is the maximum current the battery is able to provide continuously without suffering damage.
It is easiest to look at the C value and think of it as a multiplier. The electical values are measured usually in amperes, volts or wattage. C-number can be converted to amperes by multiplying the battery capacity with the given C-number. So, with our battery pack the maximum continuous amout of amperes that you should draw from the battery is 40 x 2.2 A = 88 Amperes
Now it would be nice to know how much current our copter will be demanding from the battery when in flight. Earlier we discovered that our example engine have the maximum current draw of 18 amps. You simple take this value and multiply it with desired amount of engines, such as four, and this makes the "worst case current draw" of the copter to a total of 72 Amps (4 motors times 18 amps). Our example battery would be enough in this situation, but it is always safer to take a battery with a higher C value than the calculated one.
Many batteries report the C-number for charging as well. If the number is missing, the battery is safe to recharge only with 1 C’s current. (charging current is the C-number x battery capacity, for eg. 2.2A * 1 = 2.2 Amperes).You must not override the charging current stated by the manufacturer as this will damage the battery pack and in worst case set it on fire.
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