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Sep 18, 2018, 01:48 PM
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battery voltage, amp hour vs watt hour, flight time.


I'm sure this topic has been covered a lot... But my search ability is letting me down.
A lot of websites on this topic seem to say mah is all that matters for flight time. Some of the online flight time calculators don't even take voltage into consideration. (I was using one w/ a slider, and moving voltage did nothing to the results).

My question is which will give me longer flights? Isn't watt hour more important than amp hours?

So, in my example, 2s x950mah is 1900 watt hours. 3sx650 is 1950 watt hours.
Lets assume they are the same weight.

If I'm mostly hovering, would both batteries give me about the same flight time?
I could understand if I'm flying full throttle, the 3s will go faster and last a shorter amount of time. But if I'm flying the same speed w/ both batteries, would my flight time be about the same?

-Edited to say, motors and props and everything else stays the same. My motors are rated for both 2s and 3s, at similar efficiencies.

Thanks!
Last edited by wicked1; Sep 18, 2018 at 02:07 PM.
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Sep 19, 2018, 01:31 AM
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A lot of websites on this topic seem to say mah is all that matters for flight time.

Wrong !

Some of the online flight time calculators don't even take voltage into consideration. (I was using one w/ a slider, and moving voltage did nothing to the results).

Plain stupid !

My question is which will give me longer flights? Isn't watt hour more important than amp hours?

Right, Wh is the real measure of energy stored on board, as fuel litres in your tank, but more important is the ratio between weights, battery and total.
For any platform size, this ratio is the main factor that influence flight time.

So, in my example, 2s x950mah is 1900 watt hours. 3sx650 is 1950 watt hours.
Lets assume they are the same weight.


If same C, they must be very close weight, else some of them is fake.

If I'm mostly hovering, would both batteries give me about the same flight time?
...
-Edited to say, motors and props and everything else stays the same. My motors are rated for both 2s and 3s, at similar efficiencies.

You can't stay the same with same prop when switching from 2s to 3s... this is the main mistake in your rationale above.
The prop must be changed according to square root of voltages ratio.
3/2 = 1.5, sqrt is 1.22... so if you are using 8" with 3s, you must switch to 10" with 2s.

I doubt your motor has same efficiency at different voltage and right propeller chosen for same thrust.
There should be a difference of about 22%, same as the prop diameter.
That is the reason people are using big props for longer time, not for more thrust, or other reasons. Propulsion efficiency is the second factor that influence flight time, and depends first on prop size.
Sep 19, 2018, 09:38 AM
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Also, the amperage or "C" rating of a battery is important. Maximum Discharge Current = C-Rating * Capacity. So if you have a 1300mah battery with a C rating of 100, in theory that battery can discharge 130 amps safely. The C rating is a stupid unit of measurement and is over rated by almost every Chinese battery manufacture. Also the "pulse" rating is useless. If a battery says 100c/200c, ignore the 200c. Without a given variable in time for the pulse, it is a useless number. Always ignore the second C rating on a battery unless it gives an amount of time that you can pulse it for, and never use it as a reference to your power needs.

Batteries with higher C ratings tend to last longer, as they have lower internal resistance, they won't swell as fast. Don't go cheap on batteries and you'll be fine.
Sep 19, 2018, 10:37 AM
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For flight time C should be of no interest, not as the definition above.
Discharging a battery at 10C means a lot more info than 13A figure, for me the C factor stopped long time ago meaning something related to Amps... the first bell that ring when I hear 10C discharge is: this is a short flight ! Because 10C means 1/10 of an hour = 6 minutes !

Second meaning of the C factor should be: high C = low internal resistance, but is is not unfortunately... as fake as is C rating, same is IR... hth can they rate as 40C a battery with 16 mO per cell when new ?! is beyond my understanding...
To better understand the absurdity beyond this rating, 16 mO means that at 6Amps draw you have 0.1V voltage sag, per cell ! which is 20% of total capacity !!!
6 Ams draw for an 1300 is less than 5C discharge, it's a crime to label such junk with 40C !
Last edited by renatoa; Sep 19, 2018 at 10:53 AM.


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