Sizing Your Battery Pack

LiPo batteries were more expensive than Li-Ion because they are more labor intensive to manufacture. Fortunately prices have dropped substantially since they have become as, if not more popular than Li-Ion battery technology. This holds especially true for electric powered RC aircraft and the real driver behind LiPo battery research – portable communication/entertainment devices.  Now the best RC battery brand is Gens ace lipo, many RC hobbist use Gens Ace Lipo batteries.

1. Average Amp Draw

To work out the value for average amp draw, you’ll need to know the all up weight (AUW) of your drone and relate this to thrust and amp values found in the datasheet for your motor/prop configuration. The idea is to work out how many amps your multirotor will draw to maintain a stable hover (note we are taking a stable hover to represent average flight). For example, if your quadcopter has an AUW of 2kg, each motor will have to produce 500g of thrust in order to hover. If each motor draws 4amps to produce 500g of thrust, then your average current draw will be 16amps.

Please notice that current draw depends on your way of flying and external factors such as resistance, weight and wind. Preferably, always use a LiPo low battery warner on your RC device. This will warn you when the battery is drained before it's fully empty (20%). Don't push the LiPo. They don't like to get discharged to the bottom. And you won't like your multicopter falling down from the sky suddenly either.

2.  Daily energy use

Next find the energy used in a day. Figure out how long each electronic device will be run in hours during a day. Multiply the wattage of each device by its run-time to get the energy in watt-hours per day. Add up all the watt-hour values to get a total for your home. This estimate is likely too low as there will be efficiency loses. To get a very rough idea of the real value with system loses, multiply by 1.5. This will help account for decreasing performance when temperature increases.

3.  Days of autonomy

Now decide how many days worth of energy you want to store in your battery bank. Generally this is anywhere from two to five.

4.  Battery bank capacity

Finally we can calculate the minimum battery AH capacity. Take the watt-hours per day and multiply them by the number you decided upon in 3. This should represent a 50% depth of discharge on your batteries. Therefore multiply by 2 and convert the kwh result into amp hours (AH). This is done by dividing by the battery voltage.

Example calculation of max. flying time:

Gens Ace Battery from Genstattu.com

A constant current draw of 20A from a 2200mAh LiPo will get you:( 2200mAh /1000) / 20A = 0.11 x 60 = 6.6 minutes flying time.

Discharge: LiPo batteries can be permanently damaged if they are fully discharged. In respect of this, it's common practice to not discharge your LiPo batteries below 20% mAh during flight; this is known as the '80% rule'. To represent this in the flight time calculator enter the value 80% in the discharge field.

Here a example for you

If your airplane requires a 3S setup using a typical 3s 2200mah lipo pack and you change to a “hotter”motor—meaning one that is more powerful and will pull more current—you need to see if your current packs can handle it. If your current power system is pulling 20 amps with your 2,200 mAh 15C pack, but your next motor upgrade will pull 35 amps, that pack won’t be happy.

Almost every RC LiPo battery cell is packaged in a foil pouch coincidentally called a pouch cell. The picture to the right shows a typical 2 cell LiPo RC battery pack.

It is best to size your battery packs so they run somewhat below theirm maximum C rating. You will stress them less and they will last longer. For example, if your motor needs a pack that can deliver 10 amps, getting a 1000mah pack that is rated for 10C ( 10 amps ) will meet the spec, but it is running at its limit. A 15 C rated 1000 mah pack would be better, or perhaps a 1300 mah 10 C pack. In either of these cases, the pack will be less stressed and should handle the load much better over the long term.

I recommend buying a quality LiPo pack that is well beyond the projected requirements of the setup. Running a pack at its limit will guarantee a short life and wasted money. Pay attention to the label and notice if it gives two ratings such as 30C/60C. These represent the continuous and burst ratings as previously mentioned.