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Jun 19, 2015, 02:29 PM
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LiPo Battery Calculations

You need to know the wattage or amp draw for your motors at 50% hover and what the maximum rating is as well. (the 50% number will change depending on weight load and props used)(A watt meter may be useful here)

If your using 4 motors that draw 22 watts each at 50% throttle.
This would be 88watts draw continuous at hover.

I you then divide the watts by the nominal voltage of your pack you get the amp draw of your motors.
88Watt / 11.1Volts = 7.9Amps draw.

You then calculate what the battery your looking at can supply continuous.

In this case we will look at a 5200 10C cheap battery first.
5200mAh / 1000 = 5.2A x 10C = 52A
This battery can do 52A continuously.

You then do the following calculation to see what percentage of this your copter is using. I rounded 7.9 up to 8
8Amps / 52 amps = 15%
So if your copter is hovering at 8A your using 15% of what the battery can do. This is really good but not the full picture.

What happens when your not at hoover and your using 85% or more of throttle? Lets calculate what would happen with the same setup pushed to 100% use.

The motors in my example would go from 22 watts each to 90 watts each.
So the same calculations would give you this.

90W x 4 = 360W total
360W / 11.1V = 32.4A draw at 100% load.
5200mAh / 1000 = 5.2A x 10C = 52A (Same battery)
32 amps / 52 amps = 62% of what the battery can do.
Would it work? Yes, but this would hammer the battery and likely lead to major voltage sag. NOT GOOD! The magic number here is to try and stay under 25 to 30% of what the battery can do continuous and it will stay happy. (Your wires stay cooler as well.)

Now lets see what happens if we do the same calculations with a 30C version of this battery?

360W / 11.1V = 32.4A draw at 100% load.
5200mAh / 1000 = 5.2A x 30C = 156A continuous (30C version)
32 amps / 156 amps = 20%

20% falls under the 25 to 30% of the max load of the battery and should work out beautifully!
Last edited by Mowee; Jul 10, 2015 at 05:52 PM.
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Jun 19, 2015, 03:21 PM
Research Junkie
Great information for helping to choose and the importance of C rating. I was just reading today about how someone was using 5000mah 10c batteries and they were coming down hot.

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